diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 081f99f01f82c..c91bd63ece362 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -2689,7 +2689,7 @@ namespace ts {
         }
 
         function bindSourceFileAsExternalModule() {
-            bindAnonymousDeclaration(file, SymbolFlags.ValueModule, `"${removeFileExtension(file.fileName)}"` as __String);
+            bindAnonymousDeclaration(file, SymbolFlags.ValueModule, `"${removeFileExtension(file.fileName, options)}"` as __String);
         }
 
         function bindExportAssignment(node: ExportAssignment) {
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 4fe8e264e559b..53ccae2f67d72 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -3052,8 +3052,18 @@ namespace ts {
                 else {
                     const tsExtension = tryExtractTSExtension(moduleReference);
                     if (tsExtension) {
-                        const diag = Diagnostics.An_import_path_cannot_end_with_a_0_extension_Consider_importing_1_instead;
-                        error(errorNode, diag, tsExtension, removeExtension(moduleReference, tsExtension));
+                        if (compilerOptions.emitExtension === tsExtension) {
+                            error(errorNode, moduleNotFoundError, moduleReference);
+                        }
+                        else {
+                            const diag = Diagnostics.An_import_path_cannot_end_with_a_0_extension_Consider_importing_1_instead;
+                            let rightPath = removeExtension(moduleReference, tsExtension);
+                            if (compilerOptions.emitExtension) {
+                                // e.g.: Consider importing '{1}.mjs' instead.
+                                rightPath += compilerOptions.emitExtension;
+                            }
+                            error(errorNode, diag, tsExtension, rightPath);
+                        }
                     }
                     else if (!compilerOptions.resolveJsonModule &&
                         fileExtensionIs(moduleReference, Extension.Json) &&
@@ -6452,7 +6462,7 @@ namespace ts {
                                         getCurrentDirectory: () => context.tracker.moduleResolverHost!.getCurrentDirectory(),
                                         getCommonSourceDirectory: () => context.tracker.moduleResolverHost!.getCommonSourceDirectory()
                                     };
-                                    const newName = getResolvedExternalModuleName(resolverHost, targetFile);
+                                    const newName = getResolvedExternalModuleName(resolverHost, compilerOptions, targetFile);
                                     return createLiteral(newName);
                                 }
                             }
@@ -33713,16 +33723,26 @@ namespace ts {
                     Diagnostics.Import_declarations_in_a_namespace_cannot_reference_a_module);
                 return false;
             }
-            if (inAmbientExternalModule && isExternalModuleNameRelative(moduleName.text)) {
-                // we have already reported errors on top level imports\exports in external module augmentations in checkModuleDeclaration
-                // no need to do this again.
-                if (!isTopLevelInExternalModuleAugmentation(node)) {
-                    // TypeScript 1.0 spec (April 2013): 12.1.6
-                    // An ExternalImportDeclaration in an AmbientExternalModuleDeclaration may reference
-                    // other external modules only through top - level external module names.
-                    // Relative external module names are not permitted.
-                    error(node, Diagnostics.Import_or_export_declaration_in_an_ambient_module_declaration_cannot_reference_module_through_relative_module_name);
-                    return false;
+            if (isExternalModuleNameRelative(moduleName.text)) {
+                if (inAmbientExternalModule) {
+                    // we have already reported errors on top level imports\exports in external module augmentations in checkModuleDeclaration
+                    // no need to do this again.
+                    if (!isTopLevelInExternalModuleAugmentation(node)) {
+                        // TypeScript 1.0 spec (April 2013): 12.1.6
+                        // An ExternalImportDeclaration in an AmbientExternalModuleDeclaration may reference
+                        // other external modules only through top - level external module names.
+                        // Relative external module names are not permitted.
+                        error(node, Diagnostics.Import_or_export_declaration_in_an_ambient_module_declaration_cannot_reference_module_through_relative_module_name);
+                        return false;
+                    }
+                }
+                // Skip noImplicitExtensionName check for files in node_modules
+                else if (compilerOptions.noImplicitExtensionName && !getSourceFileOfNode(node).fileName.includes("node_modules")) {
+                    const extensionLess = removeFileExtension(moduleName.text, compilerOptions);
+                    if (extensionLess === moduleName.text) {
+                        error(node, Diagnostics.Import_with_an_implicit_extension_name_is_not_allowed_Try_to_import_from_0_1_instead, extensionLess, compilerOptions.emitExtension || ".js");
+                        return false;
+                    }
                 }
             }
             return true;
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 6fd9803aaacf4..c58bb34b1e29e 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -997,6 +997,24 @@ namespace ts {
             },
             description: Diagnostics.List_of_language_service_plugins
         },
+        {
+            name: "emitExtension",
+            type: "string",
+            category: Diagnostics.Basic_Options,
+            // will effect noImplicitExtensionName
+            affectsSemanticDiagnostics: true,
+            description: Diagnostics.Specify_the_extension_name_of_the_emitted_files,
+            showInSimplifiedHelpView: true,
+            affectsEmit: true,
+        },
+        {
+            name: "noImplicitExtensionName",
+            type: "boolean",
+            affectsSemanticDiagnostics: true,
+            showInSimplifiedHelpView: true,
+            category: Diagnostics.Additional_Checks,
+            description: Diagnostics.Report_error_for_implicit_extension_name,
+        },
     ];
 
     /* @internal */
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 726d75b67660c..9df55bdd4df06 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -4527,6 +4527,34 @@
         "category": "Error",
         "code": 6504
     },
+    "Specify the extension name of the emitted files.": {
+        "category": "Message",
+        "code": 6505
+    },
+    "emitExtension must start with '.', but here has '{0}', try to replace it with '.{0}'.": {
+        "category": "Error",
+        "code": 6506
+    },
+    "emitExtension can only be \".jsx\" when JSX is set to \"preserve\"": {
+        "category": "Error",
+        "code": 6507
+    },
+    "emitExtension can not be \".d.ts\"": {
+        "category": "Error",
+        "code": 6508
+    },
+    "emitExtension contains invalid chars": {
+        "category": "Error",
+        "code": 6509
+    },
+    "Report error for implicit extension name.": {
+        "category": "Message",
+        "code": 6510
+    },
+    "Import with an implicit extension name is not allowed. Try to import from '{0}{1}' instead.": {
+        "category": "Error",
+        "code": 6511
+    },
 
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index 51d268a682681..782204af2fe35 100644
--- a/src/compiler/emitter.ts
+++ b/src/compiler/emitter.ts
@@ -58,11 +58,11 @@ namespace ts {
         const outPath = options.outFile || options.out;
         let buildInfoExtensionLess: string;
         if (outPath) {
-            buildInfoExtensionLess = removeFileExtension(outPath);
+            buildInfoExtensionLess = removeFileExtension(outPath, options);
         }
         else {
             if (!configFile) return undefined;
-            const configFileExtensionLess = removeFileExtension(configFile);
+            const configFileExtensionLess = removeFileExtension(configFile, options);
             buildInfoExtensionLess = options.outDir ?
                 options.rootDir ?
                     resolvePath(options.outDir, getRelativePathFromDirectory(options.rootDir, configFileExtensionLess, /*ignoreCase*/ true)) :
@@ -77,7 +77,7 @@ namespace ts {
         const outPath = options.outFile || options.out!;
         const jsFilePath = options.emitDeclarationOnly ? undefined : outPath;
         const sourceMapFilePath = jsFilePath && getSourceMapFilePath(jsFilePath, options);
-        const declarationFilePath = (forceDtsPaths || getEmitDeclarations(options)) ? removeFileExtension(outPath) + Extension.Dts : undefined;
+        const declarationFilePath = (forceDtsPaths || getEmitDeclarations(options)) ? removeFileExtension(outPath, options) + Extension.Dts : undefined;
         const declarationMapPath = declarationFilePath && getAreDeclarationMapsEnabled(options) ? declarationFilePath + ".map" : undefined;
         const buildInfoPath = getTsBuildInfoEmitOutputFilePath(options);
         return { jsFilePath, sourceMapFilePath, declarationFilePath, declarationMapPath, buildInfoPath };
@@ -107,15 +107,19 @@ namespace ts {
         return (options.sourceMap && !options.inlineSourceMap) ? jsFilePath + ".map" : undefined;
     }
 
-    // JavaScript files are always LanguageVariant.JSX, as JSX syntax is allowed in .js files also.
+    // JavaScript files are emitted as LanguageVariant.JSX, as JSX syntax is allowed in .js files also.
     // So for JavaScript files, '.jsx' is only emitted if the input was '.jsx', and JsxEmit.Preserve.
     // For TypeScript, the only time to emit with a '.jsx' extension, is on JSX input, and JsxEmit.Preserve
+    // If compilerOptions.emitExtension is set, respect the output extension.
     /* @internal */
-    export function getOutputExtension(sourceFile: SourceFile, options: CompilerOptions): Extension {
+    export function getOutputExtension(sourceFile: SourceFile, options: CompilerOptions): string {
         if (isJsonSourceFile(sourceFile)) {
             return Extension.Json;
         }
 
+        if (options.emitExtension) {
+            return options.emitExtension;
+        }
         if (options.jsx === JsxEmit.Preserve) {
             if (isSourceFileJS(sourceFile)) {
                 if (fileExtensionIs(sourceFile.fileName, Extension.Jsx)) {
diff --git a/src/compiler/factory.ts b/src/compiler/factory.ts
index 48646e5b89dc2..c7428a75b7f78 100644
--- a/src/compiler/factory.ts
+++ b/src/compiler/factory.ts
@@ -1594,7 +1594,7 @@ namespace ts {
             return createLiteral(file.moduleName);
         }
         if (!file.isDeclarationFile && (options.out || options.outFile)) {
-            return createLiteral(getExternalModuleNameFromPath(host, file.fileName));
+            return createLiteral(getExternalModuleNameFromPath(host, options, file.fileName));
         }
         return undefined;
     }
diff --git a/src/compiler/moduleNameResolver.ts b/src/compiler/moduleNameResolver.ts
index 82e39ecfad460..e5f4d12ab4330 100644
--- a/src/compiler/moduleNameResolver.ts
+++ b/src/compiler/moduleNameResolver.ts
@@ -1070,8 +1070,8 @@ namespace ts {
 
         // If that didn't work, try stripping a ".js" or ".jsx" extension and replacing it with a TypeScript one;
         // e.g. "./foo.js" can be matched by "./foo.ts" or "./foo.d.ts"
-        if (hasJSFileExtension(candidate)) {
-            const extensionless = removeFileExtension(candidate);
+        if (hasJSFileExtension(candidate, state.compilerOptions)) {
+            const extensionless = removeFileExtension(candidate, state.compilerOptions);
             if (state.traceEnabled) {
                 const extension = candidate.substring(extensionless.length);
                 trace(state.host, Diagnostics.File_name_0_has_a_1_extension_stripping_it, candidate, extension);
diff --git a/src/compiler/moduleSpecifiers.ts b/src/compiler/moduleSpecifiers.ts
index f0ea43ed68b41..50cac1d60c5ad 100644
--- a/src/compiler/moduleSpecifiers.ts
+++ b/src/compiler/moduleSpecifiers.ts
@@ -21,7 +21,7 @@ namespace ts.moduleSpecifiers {
                 case "minimal": return Ending.Minimal;
                 case "index": return Ending.Index;
                 case "js": return Ending.JsExtension;
-                default: return usesJsExtensionOnImports(importingSourceFile) ? Ending.JsExtension
+                default: return usesJsExtensionOnImports(importingSourceFile, compilerOptions) ? Ending.JsExtension
                     : getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.NodeJs ? Ending.Index : Ending.Minimal;
             }
         }
@@ -30,7 +30,7 @@ namespace ts.moduleSpecifiers {
     function getPreferencesForUpdate(compilerOptions: CompilerOptions, oldImportSpecifier: string): Preferences {
         return {
             relativePreference: isExternalModuleNameRelative(oldImportSpecifier) ? RelativePreference.Relative : RelativePreference.NonRelative,
-            ending: hasJSFileExtension(oldImportSpecifier) ?
+            ending: hasJSFileExtension(oldImportSpecifier, compilerOptions) ?
                 Ending.JsExtension :
                 getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.NodeJs || endsWith(oldImportSpecifier, "index") ? Ending.Index : Ending.Minimal,
         };
@@ -131,7 +131,7 @@ namespace ts.moduleSpecifiers {
         }
 
         const importRelativeToBaseUrl = removeExtensionAndIndexPostFix(relativeToBaseUrl, ending, compilerOptions);
-        const fromPaths = paths && tryGetModuleNameFromPaths(removeFileExtension(relativeToBaseUrl), importRelativeToBaseUrl, paths);
+        const fromPaths = paths && tryGetModuleNameFromPaths(removeFileExtension(relativeToBaseUrl, compilerOptions), importRelativeToBaseUrl, compilerOptions, paths);
         const nonRelative = fromPaths === undefined ? importRelativeToBaseUrl : fromPaths;
 
         if (relativePreference === RelativePreference.NonRelative) {
@@ -152,8 +152,8 @@ namespace ts.moduleSpecifiers {
         return count;
     }
 
-    function usesJsExtensionOnImports({ imports }: SourceFile): boolean {
-        return firstDefined(imports, ({ text }) => pathIsRelative(text) ? hasJSFileExtension(text) : undefined) || false;
+    function usesJsExtensionOnImports({ imports }: SourceFile, compilerOptions: CompilerOptions): boolean {
+        return firstDefined(imports, ({ text }) => pathIsRelative(text) ? hasJSFileExtension(text, compilerOptions) : undefined) || false;
     }
 
     function numberOfDirectorySeparators(str: string) {
@@ -273,10 +273,10 @@ namespace ts.moduleSpecifiers {
         }
     }
 
-    function tryGetModuleNameFromPaths(relativeToBaseUrlWithIndex: string, relativeToBaseUrl: string, paths: MapLike<readonly string[]>): string | undefined {
+    function tryGetModuleNameFromPaths(relativeToBaseUrlWithIndex: string, relativeToBaseUrl: string, options: CompilerOptions, paths: MapLike<readonly string[]>): string | undefined {
         for (const key in paths) {
             for (const patternText of paths[key]) {
-                const pattern = removeFileExtension(normalizePath(patternText));
+                const pattern = removeFileExtension(normalizePath(patternText), options);
                 const indexOfStar = pattern.indexOf("*");
                 if (indexOfStar !== -1) {
                     const prefix = pattern.substr(0, indexOfStar);
@@ -306,7 +306,7 @@ namespace ts.moduleSpecifiers {
         const relativePath = normalizedSourcePath !== undefined ? ensurePathIsNonModuleName(getRelativePathFromDirectory(normalizedSourcePath, normalizedTargetPath, getCanonicalFileName)) : normalizedTargetPath;
         return getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.NodeJs
             ? removeExtensionAndIndexPostFix(relativePath, ending, compilerOptions)
-            : removeFileExtension(relativePath);
+            : removeFileExtension(relativePath, compilerOptions);
     }
 
     function tryGetModuleNameAsNodeModule(moduleFileName: string, { getCanonicalFileName, sourceDirectory }: Info, host: ModuleSpecifierResolutionHost, options: CompilerOptions, packageNameOnly?: boolean): string | undefined {
@@ -331,8 +331,9 @@ namespace ts.moduleSpecifiers {
             if (versionPaths) {
                 const subModuleName = moduleFileName.slice(parts.packageRootIndex + 1);
                 const fromPaths = tryGetModuleNameFromPaths(
-                    removeFileExtension(subModuleName),
+                    removeFileExtension(subModuleName, options),
                     removeExtensionAndIndexPostFix(subModuleName, Ending.Minimal, options),
+                    options,
                     versionPaths.paths
                 );
                 if (fromPaths !== undefined) {
@@ -365,14 +366,14 @@ namespace ts.moduleSpecifiers {
                 const mainFileRelative = packageJsonContent.typings || packageJsonContent.types || packageJsonContent.main;
                 if (isString(mainFileRelative)) {
                     const mainExportFile = toPath(mainFileRelative, packageRootPath, getCanonicalFileName);
-                    if (removeFileExtension(mainExportFile) === removeFileExtension(getCanonicalFileName(path))) {
+                    if (removeFileExtension(mainExportFile, options) === removeFileExtension(getCanonicalFileName(path), options)) {
                         return packageRootPath;
                     }
                 }
             }
 
             // We still have a file name - remove the extension
-            const fullModulePathWithoutExtension = removeFileExtension(path);
+            const fullModulePathWithoutExtension = removeFileExtension(path, options);
 
             // If the file is /index, it can be imported by its directory name
             // IFF there is not _also_ a file by the same name
@@ -469,7 +470,7 @@ namespace ts.moduleSpecifiers {
 
     function removeExtensionAndIndexPostFix(fileName: string, ending: Ending, options: CompilerOptions): string {
         if (fileExtensionIs(fileName, Extension.Json)) return fileName;
-        const noExtension = removeFileExtension(fileName);
+        const noExtension = removeFileExtension(fileName, options);
         switch (ending) {
             case Ending.Minimal:
                 return removeSuffix(noExtension, "/index");
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index e3f3e27a8272f..cf031a856ae79 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -2220,12 +2220,13 @@ namespace ts {
 
         /** This should have similar behavior to 'processSourceFile' without diagnostics or mutation. */
         function getSourceFileFromReference(referencingFile: SourceFile | UnparsedSource, ref: FileReference): SourceFile | undefined {
-            return getSourceFileFromReferenceWorker(resolveTripleslashReference(ref.fileName, referencingFile.fileName), fileName => filesByName.get(toPath(fileName)) || undefined);
+            return getSourceFileFromReferenceWorker(resolveTripleslashReference(ref.fileName, referencingFile.fileName), fileName => filesByName.get(toPath(fileName)) || undefined, options);
         }
 
         function getSourceFileFromReferenceWorker(
             fileName: string,
             getSourceFile: (fileName: string) => SourceFile | undefined,
+            options: CompilerOptions,
             fail?: (diagnostic: DiagnosticMessage, ...argument: string[]) => void,
             refFile?: SourceFile): SourceFile | undefined {
 
@@ -2233,7 +2234,7 @@ namespace ts {
                 const canonicalFileName = host.getCanonicalFileName(fileName);
                 if (!options.allowNonTsExtensions && !forEach(supportedExtensionsWithJsonIfResolveJsonModule, extension => fileExtensionIs(canonicalFileName, extension))) {
                     if (fail) {
-                        if (hasJSFileExtension(canonicalFileName)) {
+                        if (hasJSFileExtension(canonicalFileName, options)) {
                             fail(Diagnostics.File_0_is_a_JavaScript_file_Did_you_mean_to_enable_the_allowJs_option, fileName);
                         }
                         else {
@@ -2280,6 +2281,7 @@ namespace ts {
             getSourceFileFromReferenceWorker(
                 fileName,
                 fileName => findSourceFile(fileName, toPath(fileName), isDefaultLib, ignoreNoDefaultLib, refFile, packageId), // TODO: GH#18217
+                options,
                 (diagnostic, ...args) => fileProcessingDiagnostics.add(
                     createRefFileDiagnostic(refFile, diagnostic, ...args)
                 ),
@@ -2963,6 +2965,27 @@ namespace ts {
         }
 
         function verifyCompilerOptions() {
+            if (options.emitExtension) {
+                if (!startsWith(options.emitExtension, ".")) {
+                    createOptionValueDiagnostic("emitExtension", Diagnostics.emitExtension_must_start_with_but_here_has_0_try_to_replace_it_with_0, options.emitExtension);
+                    options.noEmitOnError = true;
+                }
+                if (options.emitExtension !== Extension.Jsx && options.jsx === JsxEmit.Preserve) {
+                    createOptionValueDiagnostic("emitExtension", Diagnostics.emitExtension_can_only_be_jsx_when_JSX_is_set_to_preserve, options.emitExtension);
+                }
+                if (options.emitExtension === Extension.Dts) {
+                    createOptionValueDiagnostic("emitExtension", Diagnostics.emitExtension_can_not_be_d_ts, options.emitExtension);
+                    options.noEmitOnError = true;
+                }
+                // To keep it simple at the first time,
+                // just accept . - a-z A-Z 0-9
+                // If there is demand, can extend to any Unicode and ban chars like / \ < > :
+                if (!options.emitExtension.match(/^(\.|-|[a-zA-Z0-9])+$/g)) {
+                    createOptionValueDiagnostic("emitExtension", Diagnostics.emitExtension_contains_invalid_chars, options.emitExtension);
+                    // Don't emit any file with this error, or it will break the file system unconditionally
+                    options.noEmitOnError = true;
+                }
+            }
             if (options.strictPropertyInitialization && !getStrictOptionValue(options, "strictNullChecks")) {
                 createDiagnosticForOptionName(Diagnostics.Option_0_cannot_be_specified_without_specifying_option_1, "strictPropertyInitialization", "strictNullChecks");
             }
@@ -3421,7 +3444,7 @@ namespace ts {
             // If options have --outFile or --out just check that
             const out = options.outFile || options.out;
             if (out) {
-                return isSameFile(filePath, out) || isSameFile(filePath, removeFileExtension(out) + Extension.Dts);
+                return isSameFile(filePath, out) || isSameFile(filePath, removeFileExtension(out, options) + Extension.Dts);
             }
 
             // If declarationDir is specified, return if its a file in that directory
@@ -3436,7 +3459,7 @@ namespace ts {
 
             if (fileExtensionIsOneOf(filePath, supportedJSExtensions) || fileExtensionIs(filePath, Extension.Dts)) {
                 // Otherwise just check if sourceFile with the name exists
-                const filePathWithoutExtension = removeFileExtension(filePath);
+                const filePathWithoutExtension = removeFileExtension(filePath, options);
                 return !!getSourceFileByPath((filePathWithoutExtension + Extension.Ts) as Path) ||
                     !!getSourceFileByPath((filePathWithoutExtension + Extension.Tsx) as Path);
             }
diff --git a/src/compiler/transformers/declarations.ts b/src/compiler/transformers/declarations.ts
index 743aa1b476932..7d9addc054d50 100644
--- a/src/compiler/transformers/declarations.ts
+++ b/src/compiler/transformers/declarations.ts
@@ -237,7 +237,7 @@ namespace ts {
                             const newFile = updateSourceFileNode(sourceFile, [createModuleDeclaration(
                                 [],
                                 [createModifier(SyntaxKind.DeclareKeyword)],
-                                createLiteral(getResolvedExternalModuleName(context.getEmitHost(), sourceFile)),
+                                createLiteral(getResolvedExternalModuleName(context.getEmitHost(), options, sourceFile)),
                                 createModuleBlock(setTextRange(createNodeArray(transformAndReplaceLatePaintedStatements(statements)), sourceFile.statements))
                             )], /*isDeclarationFile*/ true, /*referencedFiles*/ [], /*typeReferences*/ [], /*hasNoDefaultLib*/ false, /*libReferences*/ []);
                             return newFile;
@@ -632,7 +632,7 @@ namespace ts {
             resultHasExternalModuleIndicator = resultHasExternalModuleIndicator || (parent.kind !== SyntaxKind.ModuleDeclaration && parent.kind !== SyntaxKind.ImportType);
             if (isStringLiteralLike(input)) {
                 if (isBundledEmit) {
-                    const newName = getExternalModuleNameFromDeclaration(context.getEmitHost(), resolver, parent);
+                    const newName = getExternalModuleNameFromDeclaration(context.getEmitHost(), resolver, parent, options);
                     if (newName) {
                         return createLiteral(newName);
                     }
diff --git a/src/compiler/tsbuildPublic.ts b/src/compiler/tsbuildPublic.ts
index 3eb793b7599bb..adeca69acb32d 100644
--- a/src/compiler/tsbuildPublic.ts
+++ b/src/compiler/tsbuildPublic.ts
@@ -1800,7 +1800,7 @@ namespace ts {
 
         // If options have --outFile or --out, check if its that
         const out = configFile.options.outFile || configFile.options.out;
-        if (out && (isSameFile(state, fileName, out) || isSameFile(state, fileName, removeFileExtension(out) + Extension.Dts))) {
+        if (out && (isSameFile(state, fileName, out) || isSameFile(state, fileName, removeFileExtension(out, state.projectCompilerOptions) + Extension.Dts))) {
             return true;
         }
 
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index cf083fac5e897..0b64051a7f533 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -5134,6 +5134,7 @@ namespace ts {
         declaration?: boolean;
         declarationMap?: boolean;
         emitDeclarationOnly?: boolean;
+        emitExtension?: string;
         declarationDir?: string;
         /* @internal */ diagnostics?: boolean;
         /* @internal */ extendedDiagnostics?: boolean;
@@ -5172,6 +5173,7 @@ namespace ts {
         noErrorTruncation?: boolean;
         noFallthroughCasesInSwitch?: boolean;
         noImplicitAny?: boolean;  // Always combine with strict property
+        noImplicitExtensionName?: boolean
         noImplicitReturns?: boolean;
         noImplicitThis?: boolean;  // Always combine with strict property
         noStrictGenericChecks?: boolean;
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 2acaa3200fb89..55b1e140a8937 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -3767,27 +3767,27 @@ namespace ts {
         getCurrentDirectory(): string;
     }
 
-    export function getResolvedExternalModuleName(host: ResolveModuleNameResolutionHost, file: SourceFile, referenceFile?: SourceFile): string {
-        return file.moduleName || getExternalModuleNameFromPath(host, file.fileName, referenceFile && referenceFile.fileName);
+    export function getResolvedExternalModuleName(host: ResolveModuleNameResolutionHost, options: CompilerOptions, file: SourceFile, referenceFile?: SourceFile): string {
+        return file.moduleName || getExternalModuleNameFromPath(host, options, file.fileName, referenceFile && referenceFile.fileName);
     }
 
-    export function getExternalModuleNameFromDeclaration(host: ResolveModuleNameResolutionHost, resolver: EmitResolver, declaration: ImportEqualsDeclaration | ImportDeclaration | ExportDeclaration | ModuleDeclaration | ImportTypeNode): string | undefined {
+    export function getExternalModuleNameFromDeclaration(host: ResolveModuleNameResolutionHost, resolver: EmitResolver, declaration: ImportEqualsDeclaration | ImportDeclaration | ExportDeclaration | ModuleDeclaration | ImportTypeNode, compilerOptions: CompilerOptions): string | undefined {
         const file = resolver.getExternalModuleFileFromDeclaration(declaration);
         if (!file || file.isDeclarationFile) {
             return undefined;
         }
-        return getResolvedExternalModuleName(host, file);
+        return getResolvedExternalModuleName(host, compilerOptions, file);
     }
 
     /**
      * Resolves a local path to a path which is absolute to the base of the emit
      */
-    export function getExternalModuleNameFromPath(host: ResolveModuleNameResolutionHost, fileName: string, referencePath?: string): string {
+    export function getExternalModuleNameFromPath(host: ResolveModuleNameResolutionHost, options: CompilerOptions, fileName: string, referencePath?: string): string {
         const getCanonicalFileName = (f: string) => host.getCanonicalFileName(f);
         const dir = toPath(referencePath ? getDirectoryPath(referencePath) : host.getCommonSourceDirectory(), host.getCurrentDirectory(), getCanonicalFileName);
         const filePath = getNormalizedAbsolutePath(fileName, host.getCurrentDirectory());
         const relativePath = getRelativePathToDirectoryOrUrl(dir, filePath, dir, getCanonicalFileName, /*isAbsolutePathAnUrl*/ false);
-        const extensionless = removeFileExtension(relativePath);
+        const extensionless = removeFileExtension(relativePath, options);
         return referencePath ? ensurePathIsNonModuleName(extensionless) : extensionless;
     }
 
@@ -3795,10 +3795,10 @@ namespace ts {
         const compilerOptions = host.getCompilerOptions();
         let emitOutputFilePathWithoutExtension: string;
         if (compilerOptions.outDir) {
-            emitOutputFilePathWithoutExtension = removeFileExtension(getSourceFilePathInNewDir(fileName, host, compilerOptions.outDir));
+            emitOutputFilePathWithoutExtension = removeFileExtension(getSourceFilePathInNewDir(fileName, host, compilerOptions.outDir), compilerOptions);
         }
         else {
-            emitOutputFilePathWithoutExtension = removeFileExtension(fileName);
+            emitOutputFilePathWithoutExtension = removeFileExtension(fileName, compilerOptions);
         }
 
         return emitOutputFilePathWithoutExtension + extension;
@@ -3814,7 +3814,7 @@ namespace ts {
         const path = outputDir
             ? getSourceFilePathInNewDirWorker(fileName, outputDir, currentDirectory, commonSourceDirectory, getCanonicalFileName)
             : fileName;
-        return removeFileExtension(path) + Extension.Dts;
+        return removeFileExtension(path, options) + Extension.Dts;
     }
 
     export interface EmitFileNames {
@@ -5390,7 +5390,7 @@ namespace ts {
         return compilerOptions.target || ScriptTarget.ES3;
     }
 
-    export function getEmitModuleKind(compilerOptions: {module?: CompilerOptions["module"], target?: CompilerOptions["target"]}) {
+    export function getEmitModuleKind(compilerOptions: { module?: CompilerOptions["module"], target?: CompilerOptions["target"] }) {
         return typeof compilerOptions.module === "number" ?
             compilerOptions.module :
             getEmitScriptTarget(compilerOptions) >= ScriptTarget.ES2015 ? ModuleKind.ES2015 : ModuleKind.CommonJS;
@@ -5900,7 +5900,13 @@ namespace ts {
         return scriptKind === ScriptKind.JS || scriptKind === ScriptKind.JSX;
     }
 
-    export function hasJSFileExtension(fileName: string): boolean {
+    export function hasJSFileExtension(fileName: string, compilerOptions: CompilerOptions): boolean {
+        if (compilerOptions && compilerOptions.emitExtension) {
+            return some(
+                [compilerOptions.emitExtension].concat(supportedJSExtensions),
+                extension => fileExtensionIs(fileName, extension)
+            );
+        }
         return some(supportedJSExtensions, extension => fileExtensionIs(fileName, extension));
     }
 
@@ -5973,8 +5979,10 @@ namespace ts {
     }
 
     const extensionsToRemove = [Extension.Dts, Extension.Ts, Extension.Js, Extension.Tsx, Extension.Jsx, Extension.Json];
-    export function removeFileExtension(path: string): string {
-        for (const ext of extensionsToRemove) {
+    export function removeFileExtension(path: string, options: CompilerOptions): string {
+        const extra = options && options.emitExtension;
+        const remove: string[] = extra ? (extensionsToRemove as string[]).concat(extra) : extensionsToRemove;
+        for (const ext of remove) {
             const extensionless = tryRemoveExtension(path, ext);
             if (extensionless !== undefined) {
                 return extensionless;
diff --git a/src/harness/compilerImpl.ts b/src/harness/compilerImpl.ts
index 19d6545c999fa..0418238249ba5 100644
--- a/src/harness/compilerImpl.ts
+++ b/src/harness/compilerImpl.ts
@@ -70,7 +70,7 @@ namespace compiler {
             const dts = this.dts = new collections.SortedMap<string, documents.TextDocument>({ comparer: this.vfs.stringComparer, sort: "insertion" });
             const maps = this.maps = new collections.SortedMap<string, documents.TextDocument>({ comparer: this.vfs.stringComparer, sort: "insertion" });
             for (const document of this.host.outputs) {
-                if (vpath.isJavaScript(document.file) || ts.fileExtensionIs(document.file, ts.Extension.Json)) {
+                if (vpath.isJavaScript(document.file, options) || ts.fileExtensionIs(document.file, ts.Extension.Json)) {
                     js.set(document.file, document);
                 }
                 else if (vpath.isDeclaration(document.file)) {
diff --git a/src/harness/harnessIO.ts b/src/harness/harnessIO.ts
index 7aebbb1cd2ce5..678f6f1024925 100644
--- a/src/harness/harnessIO.ts
+++ b/src/harness/harnessIO.ts
@@ -470,7 +470,7 @@ namespace Harness {
                 if (vpath.isDeclaration(file.unitName) || vpath.isJson(file.unitName)) {
                     dtsFiles.push(file);
                 }
-                else if (vpath.isTypeScript(file.unitName) || (vpath.isJavaScript(file.unitName) && options.allowJs)) {
+                else if (vpath.isTypeScript(file.unitName) || (vpath.isJavaScript(file.unitName, options) && options.allowJs)) {
                     const declFile = findResultCodeFile(file.unitName);
                     if (declFile && !findUnit(declFile.file, declInputFiles) && !findUnit(declFile.file, declOtherFiles)) {
                         dtsFiles.push({ unitName: declFile.file, content: Utils.removeByteOrderMark(declFile.text) });
@@ -499,7 +499,7 @@ namespace Harness {
                     sourceFileName = outFile;
                 }
 
-                const dTsFileName = ts.removeFileExtension(sourceFileName) + ts.Extension.Dts;
+                const dTsFileName = ts.removeFileExtension(sourceFileName, options) + ts.Extension.Dts;
                 return result.dts.get(dTsFileName);
             }
 
diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index fbaf9ba545d0a..5aa3bb4005e4a 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -291,7 +291,7 @@ namespace Harness.LanguageService {
         getHost(): LanguageServiceAdapterHost { return this.host; }
         getLanguageService(): ts.LanguageService { return ts.createLanguageService(this.host); }
         getClassifier(): ts.Classifier { return ts.createClassifier(); }
-        getPreProcessedFileInfo(fileName: string, fileContents: string): ts.PreProcessedFileInfo { return ts.preProcessFile(fileContents, /* readImportFiles */ true, ts.hasJSFileExtension(fileName)); }
+        getPreProcessedFileInfo(fileName: string, fileContents: string): ts.PreProcessedFileInfo { return ts.preProcessFile(fileContents, /* readImportFiles */ true, ts.hasJSFileExtension(fileName, this.host.getCompilationSettings())); }
     }
 
     /// Shim adapter
diff --git a/src/jsTyping/jsTyping.ts b/src/jsTyping/jsTyping.ts
index 70efac4aee1e5..182b937a8fc1d 100644
--- a/src/jsTyping/jsTyping.ts
+++ b/src/jsTyping/jsTyping.ts
@@ -123,7 +123,7 @@ namespace ts.JsTyping {
         // Only infer typings for .js and .jsx files
         fileNames = mapDefined(fileNames, fileName => {
             const path = normalizePath(fileName);
-            if (hasJSFileExtension(path)) {
+            if (hasJSFileExtension(path, {})) {
                 return path;
             }
         });
@@ -219,9 +219,9 @@ namespace ts.JsTyping {
          */
         function getTypingNamesFromSourceFileNames(fileNames: string[]) {
             const fromFileNames = mapDefined(fileNames, j => {
-                if (!hasJSFileExtension(j)) return undefined;
+                if (!hasJSFileExtension(j, {})) return undefined;
 
-                const inferredTypingName = removeFileExtension(getBaseFileName(j.toLowerCase()));
+                const inferredTypingName = removeFileExtension(getBaseFileName(j.toLowerCase()), {});
                 const cleanedTypingName = removeMinAndVersionNumbers(inferredTypingName);
                 return safeList.get(cleanedTypingName);
             });
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index 24f311d20bf72..0416649c9bad3 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -3492,7 +3492,7 @@ namespace ts.server {
                     if (typeAcquisition.enable || typeAcquisition.enableAutoDiscovery) {
                         const baseName = getBaseFileName(toFileNameLowerCase(normalizedNames[i]));
                         if (fileExtensionIs(baseName, "js")) {
-                            const inferredTypingName = removeFileExtension(baseName);
+                            const inferredTypingName = removeFileExtension(baseName, {});
                             const cleanedTypingName = removeMinAndVersionNumbers(inferredTypingName);
                             const typeName = this.legacySafelist.get(cleanedTypingName);
                             if (typeName !== undefined) {
diff --git a/src/server/project.ts b/src/server/project.ts
index ffcc55791c9df..42827f6ca0a9d 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -1024,7 +1024,7 @@ namespace ts.server {
                     if (isGeneratedFileWatcher(this.generatedFilesMap)) {
                         // --out
                         if (!outPath || !this.isValidGeneratedFileWatcher(
-                            removeFileExtension(outPath) + Extension.Dts,
+                            removeFileExtension(outPath, this.compilerOptions) + Extension.Dts,
                             this.generatedFilesMap,
                         )) {
                             this.clearGeneratedFileWatch();
diff --git a/src/services/codefixes/helpers.ts b/src/services/codefixes/helpers.ts
index 48f2ee9858496..f7acaa94df89e 100644
--- a/src/services/codefixes/helpers.ts
+++ b/src/services/codefixes/helpers.ts
@@ -37,7 +37,7 @@ namespace ts.codefix {
             return undefined;
         }
         const checker = context.program.getTypeChecker();
-        const scriptTarget = getEmitScriptTarget(context.program.getCompilerOptions());
+        const compilerOptions = context.program.getCompilerOptions();
         const declaration = declarations[0];
         const name = getSynthesizedDeepClone(getNameOfDeclaration(declaration), /*includeTrivia*/ false) as PropertyName;
         const visibilityModifier = createVisibilityModifier(getModifierFlags(declaration));
@@ -52,7 +52,7 @@ namespace ts.codefix {
                 const flags = preferences.quotePreference === "single" ? NodeBuilderFlags.UseSingleQuotesForStringLiteralType : undefined;
                 let typeNode = checker.typeToTypeNode(type, enclosingDeclaration, flags, getNoopSymbolTrackerWithResolver(context));
                 if (importAdder) {
-                    const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeNode, type, scriptTarget);
+                    const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeNode, type, compilerOptions);
                     if (importableReference) {
                         typeNode = importableReference.typeReference;
                         importSymbols(importAdder, importableReference.symbols);
@@ -74,7 +74,7 @@ namespace ts.codefix {
                     ? [allAccessors.firstAccessor, allAccessors.secondAccessor]
                     : [allAccessors.firstAccessor];
                 if (importAdder) {
-                    const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeNode, type, scriptTarget);
+                    const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeNode, type, compilerOptions);
                     if (importableReference) {
                         typeNode = importableReference.typeReference;
                         importSymbols(importAdder, importableReference.symbols);
@@ -161,7 +161,7 @@ namespace ts.codefix {
     ): MethodDeclaration | undefined {
         const program = context.program;
         const checker = program.getTypeChecker();
-        const scriptTarget = getEmitScriptTarget(program.getCompilerOptions());
+        const compilerOptions = program.getCompilerOptions();
         const signatureDeclaration = <MethodDeclaration>checker.signatureToSignatureDeclaration(signature, SyntaxKind.MethodDeclaration, enclosingDeclaration, NodeBuilderFlags.NoTruncation | NodeBuilderFlags.SuppressAnyReturnType, getNoopSymbolTrackerWithResolver(context));
         if (!signatureDeclaration) {
             return undefined;
@@ -172,14 +172,14 @@ namespace ts.codefix {
                 forEach(signatureDeclaration.typeParameters, (typeParameterDecl, i) => {
                     const typeParameter = signature.typeParameters![i];
                     if (typeParameterDecl.constraint) {
-                        const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeParameterDecl.constraint, typeParameter.constraint, scriptTarget);
+                        const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeParameterDecl.constraint, typeParameter.constraint, compilerOptions);
                         if (importableReference) {
                             typeParameterDecl.constraint = importableReference.typeReference;
                             importSymbols(importAdder, importableReference.symbols);
                         }
                     }
                     if (typeParameterDecl.default) {
-                        const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeParameterDecl.default, typeParameter.default, scriptTarget);
+                        const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeParameterDecl.default, typeParameter.default, compilerOptions);
                         if (importableReference) {
                             typeParameterDecl.default = importableReference.typeReference;
                             importSymbols(importAdder, importableReference.symbols);
@@ -189,14 +189,14 @@ namespace ts.codefix {
             }
             forEach(signatureDeclaration.parameters, (parameterDecl, i) => {
                 const parameter = signature.parameters[i];
-                const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(parameterDecl.type, checker.getTypeAtLocation(parameter.valueDeclaration), scriptTarget);
+                const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(parameterDecl.type, checker.getTypeAtLocation(parameter.valueDeclaration), compilerOptions);
                 if (importableReference) {
                     parameterDecl.type = importableReference.typeReference;
                     importSymbols(importAdder, importableReference.symbols);
                 }
             });
             if (signatureDeclaration.type) {
-                const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(signatureDeclaration.type, signature.resolvedReturnType, scriptTarget);
+                const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(signatureDeclaration.type, signature.resolvedReturnType, compilerOptions);
                 if (importableReference) {
                     signatureDeclaration.type = importableReference.typeReference;
                     importSymbols(importAdder, importableReference.symbols);
@@ -408,11 +408,11 @@ namespace ts.codefix {
      * returns an equivalent type reference node with any nested ImportTypeNodes also replaced
      * with type references, and a list of symbols that must be imported to use the type reference.
      */
-    export function tryGetAutoImportableReferenceFromImportTypeNode(importTypeNode: TypeNode | undefined, type: Type | undefined, scriptTarget: ScriptTarget) {
+    export function tryGetAutoImportableReferenceFromImportTypeNode(importTypeNode: TypeNode | undefined, type: Type | undefined, compilerOptions: CompilerOptions) {
         if (importTypeNode && isLiteralImportTypeNode(importTypeNode) && importTypeNode.qualifier && (!type || type.symbol)) {
             // Symbol for the left-most thing after the dot
             const firstIdentifier = getFirstIdentifier(importTypeNode.qualifier);
-            const name = getNameForExportedSymbol(firstIdentifier.symbol, scriptTarget);
+            const name = getNameForExportedSymbol(firstIdentifier.symbol, compilerOptions);
             const qualifier = name !== firstIdentifier.text
                 ? replaceFirstIdentifierOfEntityName(importTypeNode.qualifier, createIdentifier(name))
                 : importTypeNode.qualifier;
@@ -421,7 +421,7 @@ namespace ts.codefix {
             const typeArguments: TypeNode[] = [];
             if (importTypeNode.typeArguments) {
                 importTypeNode.typeArguments.forEach(arg => {
-                    const ref = tryGetAutoImportableReferenceFromImportTypeNode(arg, /*undefined*/ type, scriptTarget);
+                    const ref = tryGetAutoImportableReferenceFromImportTypeNode(arg, /*undefined*/ type, compilerOptions);
                     if (ref) {
                         symbols.push(...ref.symbols);
                         typeArguments.push(ref.typeReference);
diff --git a/src/services/codefixes/importFixes.ts b/src/services/codefixes/importFixes.ts
index 57fd8c1dc6a79..944ae8ad35fc6 100644
--- a/src/services/codefixes/importFixes.ts
+++ b/src/services/codefixes/importFixes.ts
@@ -56,7 +56,7 @@ namespace ts.codefix {
 
         function addImportFromExportedSymbol(exportedSymbol: Symbol, usageIsTypeOnly?: boolean) {
             const moduleSymbol = Debug.checkDefined(exportedSymbol.parent);
-            const symbolName = getNameForExportedSymbol(exportedSymbol, getEmitScriptTarget(compilerOptions));
+            const symbolName = getNameForExportedSymbol(exportedSymbol, compilerOptions);
             const checker = program.getTypeChecker();
             const symbol = checker.getMergedSymbol(skipAlias(exportedSymbol, checker));
             const exportInfos = getAllReExportingModules(sourceFile, symbol, moduleSymbol, symbolName, sourceFile, compilerOptions, checker, program.getSourceFiles());
@@ -591,7 +591,7 @@ namespace ts.codefix {
             defaultExport.escapedName !== InternalSymbolName.ExportEquals) {
             return { symbolForMeaning: defaultExport, name: defaultExport.getName() };
         }
-        return { symbolForMeaning: defaultExport, name: moduleSymbolToValidIdentifier(moduleSymbol, compilerOptions.target!) };
+        return { symbolForMeaning: defaultExport, name: moduleSymbolToValidIdentifier(moduleSymbol, compilerOptions) };
     }
 
     function getNameForExportDefault(symbol: Symbol): string | undefined {
@@ -861,8 +861,8 @@ namespace ts.codefix {
             || (!!globalCachePath && startsWith(getCanonicalFileName(globalCachePath), toNodeModulesParent));
     }
 
-    export function moduleSymbolToValidIdentifier(moduleSymbol: Symbol, target: ScriptTarget): string {
-        return moduleSpecifierToValidIdentifier(removeFileExtension(stripQuotes(moduleSymbol.name)), target);
+    export function moduleSymbolToValidIdentifier(moduleSymbol: Symbol, compilerOptions: CompilerOptions): string {
+        return moduleSpecifierToValidIdentifier(removeFileExtension(stripQuotes(moduleSymbol.name), compilerOptions), getEmitScriptTarget(compilerOptions));
     }
 
     export function moduleSpecifierToValidIdentifier(moduleSpecifier: string, target: ScriptTarget): string {
diff --git a/src/services/codefixes/inferFromUsage.ts b/src/services/codefixes/inferFromUsage.ts
index 0af71bbbb9ca0..4ac3a9c299b2e 100644
--- a/src/services/codefixes/inferFromUsage.ts
+++ b/src/services/codefixes/inferFromUsage.ts
@@ -310,7 +310,7 @@ namespace ts.codefix {
                 const typeTag = isGetAccessorDeclaration(declaration) ? createJSDocReturnTag(typeExpression, "") : createJSDocTypeTag(typeExpression, "");
                 addJSDocTags(changes, sourceFile, parent, [typeTag]);
             }
-            else if (!tryReplaceImportTypeNodeWithAutoImport(typeNode, declaration, type, sourceFile, changes, importAdder, getEmitScriptTarget(program.getCompilerOptions()))) {
+            else if (!tryReplaceImportTypeNodeWithAutoImport(typeNode, declaration, type, sourceFile, changes, importAdder, program.getCompilerOptions())) {
                 changes.tryInsertTypeAnnotation(sourceFile, declaration, typeNode);
             }
         }
@@ -323,9 +323,9 @@ namespace ts.codefix {
         sourceFile: SourceFile,
         changes: textChanges.ChangeTracker,
         importAdder: ImportAdder,
-        scriptTarget: ScriptTarget
+        options: CompilerOptions
     ): boolean {
-        const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeNode, type, scriptTarget);
+        const importableReference = tryGetAutoImportableReferenceFromImportTypeNode(typeNode, type, options);
         if (importableReference && changes.tryInsertTypeAnnotation(sourceFile, declaration, importableReference.typeReference)) {
             forEach(importableReference.symbols, s => importAdder.addImportFromExportedSymbol(s, /*usageIsTypeOnly*/ true));
             return true;
diff --git a/src/services/completions.ts b/src/services/completions.ts
index 11f16ccf8fe2f..7a2f3c4c2cc53 100644
--- a/src/services/completions.ts
+++ b/src/services/completions.ts
@@ -227,7 +227,7 @@ namespace ts.Completions {
                 location,
                 sourceFile,
                 typeChecker,
-                compilerOptions.target!,
+                compilerOptions,
                 log,
                 completionKind,
                 preferences,
@@ -250,7 +250,7 @@ namespace ts.Completions {
                 location,
                 sourceFile,
                 typeChecker,
-                compilerOptions.target!,
+                compilerOptions,
                 log,
                 completionKind,
                 preferences,
@@ -436,7 +436,7 @@ namespace ts.Completions {
         location: Node | undefined,
         sourceFile: SourceFile,
         typeChecker: TypeChecker,
-        target: ScriptTarget,
+        options: CompilerOptions,
         log: Log,
         kind: CompletionKind,
         preferences: UserPreferences,
@@ -454,7 +454,7 @@ namespace ts.Completions {
         const uniques = createMap<true>();
         for (const symbol of symbols) {
             const origin = symbolToOriginInfoMap ? symbolToOriginInfoMap[getSymbolId(symbol)] : undefined;
-            const info = getCompletionEntryDisplayNameForSymbol(symbol, target, origin, kind);
+            const info = getCompletionEntryDisplayNameForSymbol(symbol, options, origin, kind);
             if (!info) {
                 continue;
             }
@@ -564,7 +564,7 @@ namespace ts.Completions {
         // completion entry.
         return firstDefined(symbols, (symbol): SymbolCompletion | undefined => {
             const origin = symbolToOriginInfoMap[getSymbolId(symbol)];
-            const info = getCompletionEntryDisplayNameForSymbol(symbol, compilerOptions.target!, origin, completionKind);
+            const info = getCompletionEntryDisplayNameForSymbol(symbol, compilerOptions, origin, completionKind);
             return info && info.name === entryId.name && getSourceFromOrigin(origin) === entryId.source
                 ? { type: "symbol" as const, symbol, location, symbolToOriginInfoMap, previousToken, isJsxInitializer, isTypeOnlyLocation }
                 : undefined;
@@ -673,7 +673,7 @@ namespace ts.Completions {
             exportedSymbol,
             moduleSymbol,
             sourceFile,
-            getNameForExportedSymbol(symbol, compilerOptions.target!),
+            getNameForExportedSymbol(symbol, compilerOptions),
             host,
             program,
             formatContext,
@@ -1360,7 +1360,7 @@ namespace ts.Completions {
 
             if (shouldOfferImportCompletions()) {
                 const lowerCaseTokenText = previousToken && isIdentifier(previousToken) ? previousToken.text.toLowerCase() : "";
-                const autoImportSuggestions = getSymbolsFromOtherSourceFileExports(program.getCompilerOptions().target!, host);
+                const autoImportSuggestions = getSymbolsFromOtherSourceFileExports(program.getCompilerOptions(), host);
                 if (!detailsEntryId && importSuggestionsCache) {
                     importSuggestionsCache.set(sourceFile.fileName, autoImportSuggestions, host.getProjectVersion && host.getProjectVersion());
                 }
@@ -1541,7 +1541,7 @@ namespace ts.Completions {
          *    occur for that symbol---that is, the original symbol is not in Bucket A, so we should include the alias. Move
          *    everything from Bucket C to Bucket A.
          */
-        function getSymbolsFromOtherSourceFileExports(target: ScriptTarget, host: LanguageServiceHost): readonly AutoImportSuggestion[] {
+        function getSymbolsFromOtherSourceFileExports(compilerOptions: CompilerOptions, host: LanguageServiceHost): readonly AutoImportSuggestion[] {
             const cached = importSuggestionsCache && importSuggestionsCache.get(
                 sourceFile.fileName,
                 typeChecker,
@@ -1640,7 +1640,7 @@ namespace ts.Completions {
                 const origin: SymbolOriginInfoExport = { kind: SymbolOriginInfoKind.Export, moduleSymbol, isDefaultExport };
                 results.push({
                     symbol,
-                    symbolName: getNameForExportedSymbol(symbol, target),
+                    symbolName: getNameForExportedSymbol(symbol, compilerOptions),
                     origin,
                     skipFilter,
                 });
@@ -2396,11 +2396,11 @@ namespace ts.Completions {
     }
     function getCompletionEntryDisplayNameForSymbol(
         symbol: Symbol,
-        target: ScriptTarget,
+        options: CompilerOptions,
         origin: SymbolOriginInfo | undefined,
         kind: CompletionKind,
     ): CompletionEntryDisplayNameForSymbol | undefined {
-        const name = originIsExport(origin) ? getNameForExportedSymbol(symbol, target) : symbol.name;
+        const name = originIsExport(origin) ? getNameForExportedSymbol(symbol, options) : symbol.name;
         if (name === undefined
             // If the symbol is external module, don't show it in the completion list
             // (i.e declare module "http" { const x; } | // <= request completion here, "http" should not be there)
@@ -2411,7 +2411,7 @@ namespace ts.Completions {
         }
 
         const validNameResult: CompletionEntryDisplayNameForSymbol = { name, needsConvertPropertyAccess: false };
-        if (isIdentifierText(name, target) || symbol.valueDeclaration && isPrivateIdentifierPropertyDeclaration(symbol.valueDeclaration)) {
+        if (isIdentifierText(name, options.target) || symbol.valueDeclaration && isPrivateIdentifierPropertyDeclaration(symbol.valueDeclaration)) {
             return validNameResult;
         }
         switch (kind) {
diff --git a/src/services/jsDoc.ts b/src/services/jsDoc.ts
index 7118fe8b76181..0e13028b945f2 100644
--- a/src/services/jsDoc.ts
+++ b/src/services/jsDoc.ts
@@ -294,7 +294,7 @@ namespace ts.JsDoc {
         const preamble = "/**" + newLine + indentationStr + " * ";
         const result =
             preamble + newLine +
-            parameterDocComments(parameters, hasJSFileExtension(sourceFile.fileName), indentationStr, newLine) +
+            parameterDocComments(parameters, hasJSFileExtension(sourceFile.fileName, {}), indentationStr, newLine) +
             indentationStr + " */" +
             (tokenStart === position ? newLine + indentationStr : "");
 
diff --git a/src/services/navigationBar.ts b/src/services/navigationBar.ts
index 38faf25da7f99..7c42a9a71a1d7 100644
--- a/src/services/navigationBar.ts
+++ b/src/services/navigationBar.ts
@@ -680,7 +680,7 @@ namespace ts.NavigationBar {
             case SyntaxKind.SourceFile:
                 const sourceFile = <SourceFile>node;
                 return isExternalModule(sourceFile)
-                    ? `"${escapeString(getBaseFileName(removeFileExtension(normalizePath(sourceFile.fileName))))}"`
+                    ? `"${escapeString(getBaseFileName(removeFileExtension(normalizePath(sourceFile.fileName), {})))}"`
                     : "<global>";
             case SyntaxKind.ExportAssignment:
                 return isExportAssignment(node) && node.isExportEquals ? InternalSymbolName.ExportEquals : InternalSymbolName.Default;
diff --git a/src/services/refactors/moveToNewFile.ts b/src/services/refactors/moveToNewFile.ts
index a9b8171cbc98b..a4cab4615294d 100644
--- a/src/services/refactors/moveToNewFile.ts
+++ b/src/services/refactors/moveToNewFile.ts
@@ -130,7 +130,7 @@ namespace ts.refactor {
         updateImportsInOtherFiles(changes, program, oldFile, usage.movedSymbols, newModuleName);
 
         return [
-            ...getNewFileImportsAndAddExportInOldFile(oldFile, usage.oldImportsNeededByNewFile, usage.newFileImportsFromOldFile, changes, checker, useEs6ModuleSyntax, quotePreference),
+            ...getNewFileImportsAndAddExportInOldFile(oldFile, usage.oldImportsNeededByNewFile, usage.newFileImportsFromOldFile, changes, checker, useEs6ModuleSyntax, quotePreference, program.getCompilerOptions()),
             ...addExports(oldFile, toMove.all, usage.oldFileImportsFromNewFile, useEs6ModuleSyntax),
         ];
     }
@@ -398,6 +398,7 @@ namespace ts.refactor {
         checker: TypeChecker,
         useEs6ModuleSyntax: boolean,
         quotePreference: QuotePreference,
+        compilerOptions: CompilerOptions,
     ): readonly SupportedImportStatement[] {
         const copiedOldImports: SupportedImportStatement[] = [];
         for (const oldStatement of oldFile.statements) {
@@ -429,7 +430,7 @@ namespace ts.refactor {
             }
         });
 
-        append(copiedOldImports, makeImportOrRequire(oldFileDefault, oldFileNamedImports, removeFileExtension(getBaseFileName(oldFile.fileName)), useEs6ModuleSyntax, quotePreference));
+        append(copiedOldImports, makeImportOrRequire(oldFileDefault, oldFileNamedImports, removeFileExtension(getBaseFileName(oldFile.fileName), compilerOptions), useEs6ModuleSyntax, quotePreference));
         return copiedOldImports;
     }
 
diff --git a/src/services/rename.ts b/src/services/rename.ts
index 4cdb4f14dc857..8932c51ba0054 100644
--- a/src/services/rename.ts
+++ b/src/services/rename.ts
@@ -3,7 +3,7 @@ namespace ts.Rename {
     export function getRenameInfo(program: Program, sourceFile: SourceFile, position: number, options?: RenameInfoOptions): RenameInfo {
         const node = getAdjustedRenameLocation(getTouchingPropertyName(sourceFile, position));
         if (nodeIsEligibleForRename(node)) {
-            const renameInfo = getRenameInfoForNode(node, program.getTypeChecker(), sourceFile, declaration => program.isSourceFileDefaultLibrary(declaration.getSourceFile()), options);
+            const renameInfo = getRenameInfoForNode(node, program.getTypeChecker(), sourceFile, declaration => program.isSourceFileDefaultLibrary(declaration.getSourceFile()), program.getCompilerOptions(), options);
             if (renameInfo) {
                 return renameInfo;
             }
@@ -11,7 +11,7 @@ namespace ts.Rename {
         return getRenameInfoError(Diagnostics.You_cannot_rename_this_element);
     }
 
-    function getRenameInfoForNode(node: Node, typeChecker: TypeChecker, sourceFile: SourceFile, isDefinedInLibraryFile: (declaration: Node) => boolean, options?: RenameInfoOptions): RenameInfo | undefined {
+    function getRenameInfoForNode(node: Node, typeChecker: TypeChecker, sourceFile: SourceFile, isDefinedInLibraryFile: (declaration: Node) => boolean, compilerOptions: CompilerOptions, options?: RenameInfoOptions): RenameInfo | undefined {
         const symbol = typeChecker.getSymbolAtLocation(node);
         if (!symbol) return;
         // Only allow a symbol to be renamed if it actually has at least one declaration.
@@ -29,7 +29,7 @@ namespace ts.Rename {
         }
 
         if (isStringLiteralLike(node) && tryGetImportFromModuleSpecifier(node)) {
-            return options && options.allowRenameOfImportPath ? getRenameInfoForModule(node, sourceFile, symbol) : undefined;
+            return options && options.allowRenameOfImportPath ? getRenameInfoForModule(node, sourceFile, symbol, compilerOptions) : undefined;
         }
 
         const kind = SymbolDisplay.getSymbolKind(typeChecker, symbol, node);
@@ -41,14 +41,14 @@ namespace ts.Rename {
         return getRenameInfoSuccess(displayName, fullDisplayName, kind, SymbolDisplay.getSymbolModifiers(symbol), node, sourceFile);
     }
 
-    function getRenameInfoForModule(node: StringLiteralLike, sourceFile: SourceFile, moduleSymbol: Symbol): RenameInfo | undefined {
+    function getRenameInfoForModule(node: StringLiteralLike, sourceFile: SourceFile, moduleSymbol: Symbol, compilerOptions: CompilerOptions): RenameInfo | undefined {
         if (!isExternalModuleNameRelative(node.text)) {
             return getRenameInfoError(Diagnostics.You_cannot_rename_a_module_via_a_global_import);
         }
 
         const moduleSourceFile = find(moduleSymbol.declarations, isSourceFile);
         if (!moduleSourceFile) return undefined;
-        const withoutIndex = endsWith(node.text, "/index") || endsWith(node.text, "/index.js") ? undefined : tryRemoveSuffix(removeFileExtension(moduleSourceFile.fileName), "/index");
+        const withoutIndex = endsWith(node.text, "/index") || endsWith(node.text, "/index.js") ? undefined : tryRemoveSuffix(removeFileExtension(moduleSourceFile.fileName, compilerOptions), "/index");
         const name = withoutIndex === undefined ? moduleSourceFile.fileName : withoutIndex;
         const kind = withoutIndex === undefined ? ScriptElementKind.moduleElement : ScriptElementKind.directory;
         const indexAfterLastSlash = node.text.lastIndexOf("/") + 1;
diff --git a/src/services/sourcemaps.ts b/src/services/sourcemaps.ts
index b582ba0065d86..435e82244070d 100644
--- a/src/services/sourcemaps.ts
+++ b/src/services/sourcemaps.ts
@@ -79,7 +79,7 @@ namespace ts {
             const outPath = options.outFile || options.out;
 
             const declarationPath = outPath ?
-                removeFileExtension(outPath) + Extension.Dts :
+                removeFileExtension(outPath, options) + Extension.Dts :
                 getDeclarationEmitOutputFilePathWorker(info.fileName, program.getCompilerOptions(), currentDirectory, program.getCommonSourceDirectory(), getCanonicalFileName);
             if (declarationPath === undefined) return undefined;
 
diff --git a/src/services/stringCompletions.ts b/src/services/stringCompletions.ts
index 313b37430e2ed..703c9ff656893 100644
--- a/src/services/stringCompletions.ts
+++ b/src/services/stringCompletions.ts
@@ -8,11 +8,11 @@ namespace ts.Completions.StringCompletions {
         if (isInString(sourceFile, position, contextToken)) {
             if (!contextToken || !isStringLiteralLike(contextToken)) return undefined;
             const entries = getStringLiteralCompletionEntries(sourceFile, contextToken, position, checker, options, host);
-            return convertStringLiteralCompletions(entries, sourceFile, checker, log, preferences);
+            return convertStringLiteralCompletions(entries, sourceFile, checker, log, preferences, options);
         }
     }
 
-    function convertStringLiteralCompletions(completion: StringLiteralCompletion | undefined, sourceFile: SourceFile, checker: TypeChecker, log: Log, preferences: UserPreferences): CompletionInfo | undefined {
+    function convertStringLiteralCompletions(completion: StringLiteralCompletion | undefined, sourceFile: SourceFile, checker: TypeChecker, log: Log, preferences: UserPreferences, compilerOptions: CompilerOptions): CompletionInfo | undefined {
         if (completion === undefined) {
             return undefined;
         }
@@ -27,7 +27,7 @@ namespace ts.Completions.StringCompletions {
                     sourceFile,
                     sourceFile,
                     checker,
-                    ScriptTarget.ESNext,
+                    compilerOptions,
                     log,
                     CompletionKind.String,
                     preferences
@@ -363,7 +363,7 @@ namespace ts.Completions.StringCompletions {
                     continue;
                 }
 
-                const foundFileName = includeExtensions || fileExtensionIs(filePath, Extension.Json) ? getBaseFileName(filePath) : removeFileExtension(getBaseFileName(filePath));
+                const foundFileName = includeExtensions || fileExtensionIs(filePath, Extension.Json) ? getBaseFileName(filePath) : removeFileExtension(getBaseFileName(filePath), host.getCompilationSettings());
                 foundFiles.set(foundFileName, tryGetExtensionFromPath(filePath));
             }
 
@@ -528,7 +528,7 @@ namespace ts.Completions.StringCompletions {
         const matches = mapDefined(tryReadDirectory(host, baseDirectory, fileExtensions, /*exclude*/ undefined, [includeGlob]), match => {
             const extension = tryGetExtensionFromPath(match);
             const name = trimPrefixAndSuffix(match);
-            return name === undefined ? undefined : nameAndKind(removeFileExtension(name), ScriptElementKind.scriptElement, extension);
+            return name === undefined ? undefined : nameAndKind(removeFileExtension(name, host.getCompilationSettings()), ScriptElementKind.scriptElement, extension);
         });
         const directories = mapDefined(tryGetDirectories(host, baseDirectory).map(d => combinePaths(baseDirectory, d)), dir => {
             const name = trimPrefixAndSuffix(dir);
diff --git a/src/services/utilities.ts b/src/services/utilities.ts
index 58a39726589a9..8de613aa1822e 100644
--- a/src/services/utilities.ts
+++ b/src/services/utilities.ts
@@ -2760,11 +2760,11 @@ namespace ts {
         return isArray(valueOrArray) ? first(valueOrArray) : valueOrArray;
     }
 
-    export function getNameForExportedSymbol(symbol: Symbol, scriptTarget: ScriptTarget) {
+    export function getNameForExportedSymbol(symbol: Symbol, compilerOptions: CompilerOptions) {
         if (symbol.escapedName === InternalSymbolName.ExportEquals || symbol.escapedName === InternalSymbolName.Default) {
             // Name of "export default foo;" is "foo". Name of "export default 0" is the filename converted to camelCase.
             return firstDefined(symbol.declarations, d => isExportAssignment(d) && isIdentifier(d.expression) ? d.expression.text : undefined)
-                || codefix.moduleSymbolToValidIdentifier(Debug.checkDefined(symbol.parent), scriptTarget);
+                || codefix.moduleSymbolToValidIdentifier(Debug.checkDefined(symbol.parent), compilerOptions);
         }
         return symbol.name;
     }
diff --git a/src/testRunner/projectsRunner.ts b/src/testRunner/projectsRunner.ts
index d90972fe09a76..430cda43d0910 100644
--- a/src/testRunner/projectsRunner.ts
+++ b/src/testRunner/projectsRunner.ts
@@ -357,10 +357,10 @@ namespace project {
                     if (compilerOptions.outDir) {
                         let sourceFilePath = ts.getNormalizedAbsolutePath(sourceFile.fileName, compilerResult.program!.getCurrentDirectory());
                         sourceFilePath = sourceFilePath.replace(compilerResult.program!.getCommonSourceDirectory(), "");
-                        emitOutputFilePathWithoutExtension = ts.removeFileExtension(ts.combinePaths(compilerOptions.outDir, sourceFilePath));
+                        emitOutputFilePathWithoutExtension = ts.removeFileExtension(ts.combinePaths(compilerOptions.outDir, sourceFilePath), compilerOptions);
                     }
                     else {
-                        emitOutputFilePathWithoutExtension = ts.removeFileExtension(sourceFile.fileName);
+                        emitOutputFilePathWithoutExtension = ts.removeFileExtension(sourceFile.fileName, compilerOptions);
                     }
 
                     const outputDtsFileName = emitOutputFilePathWithoutExtension + ts.Extension.Dts;
@@ -371,7 +371,7 @@ namespace project {
                     }
                 }
                 else {
-                    const outputDtsFileName = ts.removeFileExtension(compilerOptions.outFile || compilerOptions.out!) + ts.Extension.Dts;
+                    const outputDtsFileName = ts.removeFileExtension(compilerOptions.outFile || compilerOptions.out!, compilerOptions) + ts.Extension.Dts;
                     const outputDtsFile = findOutputDtsFile(outputDtsFileName)!;
                     if (!ts.contains(allInputFiles, outputDtsFile)) {
                         allInputFiles.unshift(outputDtsFile);
diff --git a/src/testRunner/test262Runner.ts b/src/testRunner/test262Runner.ts
index 728d3fb322790..a43b5e34e3328 100644
--- a/src/testRunner/test262Runner.ts
+++ b/src/testRunner/test262Runner.ts
@@ -34,7 +34,7 @@ namespace Harness {
 
                 before(() => {
                     const content = IO.readFile(filePath)!;
-                    const testFilename = ts.removeFileExtension(filePath).replace(/\//g, "_") + ".test";
+                    const testFilename = ts.removeFileExtension(filePath, {}).replace(/\//g, "_") + ".test";
                     const testCaseContent = TestCaseParser.makeUnitsFromTest(content, testFilename);
 
                     const inputFiles: Compiler.TestFile[] = testCaseContent.testUnitData.map(unit => {
@@ -51,7 +51,7 @@ namespace Harness {
 
                     testState.compilerResult = Compiler.compileFiles(
                         [Test262BaselineRunner.helperFile].concat(inputFiles),
-                        /*otherFiles*/ [],
+                        /*otherFiles*/[],
                         /* harnessOptions */ undefined,
                         Test262BaselineRunner.options,
                         /* currentDirectory */ undefined);
@@ -107,4 +107,4 @@ namespace Harness {
             }
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/testRunner/unittests/tsserver/resolutionCache.ts b/src/testRunner/unittests/tsserver/resolutionCache.ts
index d89a739c8d1ca..e0ade5f39f1af 100644
--- a/src/testRunner/unittests/tsserver/resolutionCache.ts
+++ b/src/testRunner/unittests/tsserver/resolutionCache.ts
@@ -530,7 +530,7 @@ namespace ts.projectSystem {
 
         function getExpectedRelativeModuleResolutionTrace(host: TestServerHost, file: File, module: File, moduleName: string, expectedTrace: string[] = []) {
             getExpectedResolutionTraceHeader(expectedTrace, file, moduleName);
-            expectedTrace.push(`Loading module as file / folder, candidate module location '${removeFileExtension(module.path)}', target file type 'TypeScript'.`);
+            expectedTrace.push(`Loading module as file / folder, candidate module location '${removeFileExtension(module.path, {})}', target file type 'TypeScript'.`);
             getExpectedMissedLocationResolutionTrace(host, expectedTrace, getDirectoryPath(normalizePath(combinePaths(getDirectoryPath(file.path), moduleName))), module, moduleName.substring(moduleName.lastIndexOf("/") + 1), /*useNodeModules*/ false);
             getExpectedResolutionTraceFooter(expectedTrace, module, moduleName, /*addRealPathTrace*/ false);
             return expectedTrace;
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 5c0326d63ca24..f715b9336228d 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -2657,6 +2657,7 @@ declare namespace ts {
         declaration?: boolean;
         declarationMap?: boolean;
         emitDeclarationOnly?: boolean;
+        emitExtension?: string;
         declarationDir?: string;
         disableSizeLimit?: boolean;
         disableSourceOfProjectReferenceRedirect?: boolean;
@@ -2686,6 +2687,7 @@ declare namespace ts {
         noErrorTruncation?: boolean;
         noFallthroughCasesInSwitch?: boolean;
         noImplicitAny?: boolean;
+        noImplicitExtensionName?: boolean;
         noImplicitReturns?: boolean;
         noImplicitThis?: boolean;
         noStrictGenericChecks?: boolean;
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index 5bd7da4518176..dc114da2eeacd 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -2657,6 +2657,7 @@ declare namespace ts {
         declaration?: boolean;
         declarationMap?: boolean;
         emitDeclarationOnly?: boolean;
+        emitExtension?: string;
         declarationDir?: string;
         disableSizeLimit?: boolean;
         disableSourceOfProjectReferenceRedirect?: boolean;
@@ -2686,6 +2687,7 @@ declare namespace ts {
         noErrorTruncation?: boolean;
         noFallthroughCasesInSwitch?: boolean;
         noImplicitAny?: boolean;
+        noImplicitExtensionName?: boolean;
         noImplicitReturns?: boolean;
         noImplicitThis?: boolean;
         noStrictGenericChecks?: boolean;
diff --git a/tests/baselines/reference/emitExtensionDiag2691.errors.txt b/tests/baselines/reference/emitExtensionDiag2691.errors.txt
new file mode 100644
index 0000000000000..013bfd1eff326
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionDiag2691.errors.txt
@@ -0,0 +1,13 @@
+tests/cases/compiler/1.ts(1,17): error TS2691: An import path cannot end with a '.ts' extension. Consider importing './0.mjs' instead.
+
+
+==== tests/cases/compiler/0.ts (0 errors) ====
+    // For the deno case
+    export default 0
+    
+==== tests/cases/compiler/1.ts (1 errors) ====
+    import num from './0.ts'
+                    ~~~~~~~~
+!!! error TS2691: An import path cannot end with a '.ts' extension. Consider importing './0.mjs' instead.
+    num + 1
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionDiag2691.js b/tests/baselines/reference/emitExtensionDiag2691.js
new file mode 100644
index 0000000000000..7240b61f2bec7
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionDiag2691.js
@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/emitExtensionDiag2691.ts] ////
+
+//// [0.ts]
+// For the deno case
+export default 0
+
+//// [1.ts]
+import num from './0.ts'
+num + 1
+
+
+//// [0.mjs]
+"use strict";
+exports.__esModule = true;
+// For the deno case
+exports["default"] = 0;
+//// [1.mjs]
+"use strict";
+exports.__esModule = true;
+var _0_ts_1 = require("./0.ts");
+_0_ts_1["default"] + 1;
diff --git a/tests/baselines/reference/emitExtensionDiag2691.symbols b/tests/baselines/reference/emitExtensionDiag2691.symbols
new file mode 100644
index 0000000000000..d69465f43b5bf
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionDiag2691.symbols
@@ -0,0 +1,11 @@
+=== tests/cases/compiler/0.ts ===
+// For the deno case
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.ts'
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
+num + 1
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
diff --git a/tests/baselines/reference/emitExtensionDiag2691.types b/tests/baselines/reference/emitExtensionDiag2691.types
new file mode 100644
index 0000000000000..462864414fd02
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionDiag2691.types
@@ -0,0 +1,13 @@
+=== tests/cases/compiler/0.ts ===
+// For the deno case
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.ts'
+>num : any
+
+num + 1
+>num + 1 : any
+>num : any
+>1 : 1
+
diff --git a/tests/baselines/reference/emitExtensionEmitCustomExtname.js b/tests/baselines/reference/emitExtensionEmitCustomExtname.js
new file mode 100644
index 0000000000000..b1cc654f268b9
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionEmitCustomExtname.js
@@ -0,0 +1,19 @@
+//// [tests/cases/compiler/emitExtensionEmitCustomExtname.ts] ////
+
+//// [0.ts]
+export default 1
+
+//// [1.ts]
+import x from './0'
+x + 1
+
+
+//// [0.cpp]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 1;
+//// [1.cpp]
+"use strict";
+exports.__esModule = true;
+var _0_1 = require("./0");
+_0_1["default"] + 1;
diff --git a/tests/baselines/reference/emitExtensionEmitCustomExtname.symbols b/tests/baselines/reference/emitExtensionEmitCustomExtname.symbols
new file mode 100644
index 0000000000000..6aa09396d3d29
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionEmitCustomExtname.symbols
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import x from './0'
+>x : Symbol(x, Decl(1.ts, 0, 6))
+
+x + 1
+>x : Symbol(x, Decl(1.ts, 0, 6))
+
diff --git a/tests/baselines/reference/emitExtensionEmitCustomExtname.types b/tests/baselines/reference/emitExtensionEmitCustomExtname.types
new file mode 100644
index 0000000000000..a00a4fa80ddc6
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionEmitCustomExtname.types
@@ -0,0 +1,12 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import x from './0'
+>x : 1
+
+x + 1
+>x + 1 : number
+>x : 1
+>1 : 1
+
diff --git a/tests/baselines/reference/emitExtensionEmitCustomExtnamePathEscape.errors.txt b/tests/baselines/reference/emitExtensionEmitCustomExtnamePathEscape.errors.txt
new file mode 100644
index 0000000000000..c8155b54709db
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionEmitCustomExtnamePathEscape.errors.txt
@@ -0,0 +1,7 @@
+error TS6509: emitExtension contains invalid chars
+
+
+!!! error TS6509: emitExtension contains invalid chars
+==== tests/cases/compiler/0.ts (0 errors) ====
+    export default 1
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionEmitCustomExtnamePathEscape.symbols b/tests/baselines/reference/emitExtensionEmitCustomExtnamePathEscape.symbols
new file mode 100644
index 0000000000000..2e9d59769cb1a
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionEmitCustomExtnamePathEscape.symbols
@@ -0,0 +1,4 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionEmitCustomExtnamePathEscape.types b/tests/baselines/reference/emitExtensionEmitCustomExtnamePathEscape.types
new file mode 100644
index 0000000000000..2e9d59769cb1a
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionEmitCustomExtnamePathEscape.types
@@ -0,0 +1,4 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionEmitCustomExtnameWithDot.js b/tests/baselines/reference/emitExtensionEmitCustomExtnameWithDot.js
new file mode 100644
index 0000000000000..f77b3e5bebe22
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionEmitCustomExtnameWithDot.js
@@ -0,0 +1,19 @@
+//// [tests/cases/compiler/emitExtensionEmitCustomExtnameWithDot.ts] ////
+
+//// [0.ts]
+export default 1
+
+//// [1.ts]
+import x from './0'
+x + 1
+
+
+//// [0.strange.js]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 1;
+//// [1.strange.js]
+"use strict";
+exports.__esModule = true;
+var _0_1 = require("./0");
+_0_1["default"] + 1;
diff --git a/tests/baselines/reference/emitExtensionEmitCustomExtnameWithDot.symbols b/tests/baselines/reference/emitExtensionEmitCustomExtnameWithDot.symbols
new file mode 100644
index 0000000000000..6aa09396d3d29
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionEmitCustomExtnameWithDot.symbols
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import x from './0'
+>x : Symbol(x, Decl(1.ts, 0, 6))
+
+x + 1
+>x : Symbol(x, Decl(1.ts, 0, 6))
+
diff --git a/tests/baselines/reference/emitExtensionEmitCustomExtnameWithDot.types b/tests/baselines/reference/emitExtensionEmitCustomExtnameWithDot.types
new file mode 100644
index 0000000000000..a00a4fa80ddc6
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionEmitCustomExtnameWithDot.types
@@ -0,0 +1,12 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import x from './0'
+>x : 1
+
+x + 1
+>x + 1 : number
+>x : 1
+>1 : 1
+
diff --git a/tests/baselines/reference/emitExtensionOptionsDTS.errors.txt b/tests/baselines/reference/emitExtensionOptionsDTS.errors.txt
new file mode 100644
index 0000000000000..7a9dceec4e7f4
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsDTS.errors.txt
@@ -0,0 +1,9 @@
+error TS6508: emitExtension can not be ".d.ts"
+
+
+!!! error TS6508: emitExtension can not be ".d.ts"
+==== tests/cases/compiler/emitExtensionOptionsDTS.ts (0 errors) ====
+    // @Filename 1.ts
+    // emitExtension can not be ".d.ts"
+    export default 0
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsDTS.symbols b/tests/baselines/reference/emitExtensionOptionsDTS.symbols
new file mode 100644
index 0000000000000..ec7e3ce26a257
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsDTS.symbols
@@ -0,0 +1,6 @@
+=== tests/cases/compiler/emitExtensionOptionsDTS.ts ===
+// @Filename 1.ts
+No type information for this code.// emitExtension can not be ".d.ts"
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsDTS.types b/tests/baselines/reference/emitExtensionOptionsDTS.types
new file mode 100644
index 0000000000000..ec7e3ce26a257
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsDTS.types
@@ -0,0 +1,6 @@
+=== tests/cases/compiler/emitExtensionOptionsDTS.ts ===
+// @Filename 1.ts
+No type information for this code.// emitExtension can not be ".d.ts"
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsInvalidChars.errors.txt b/tests/baselines/reference/emitExtensionOptionsInvalidChars.errors.txt
new file mode 100644
index 0000000000000..7fc6947d78253
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsInvalidChars.errors.txt
@@ -0,0 +1,11 @@
+error TS6509: emitExtension contains invalid chars
+
+
+!!! error TS6509: emitExtension contains invalid chars
+==== tests/cases/compiler/0.ts (0 errors) ====
+    export default 1
+    
+==== tests/cases/compiler/1.ts (0 errors) ====
+    import x from './0'
+    x + 1
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsInvalidChars.symbols b/tests/baselines/reference/emitExtensionOptionsInvalidChars.symbols
new file mode 100644
index 0000000000000..6aa09396d3d29
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsInvalidChars.symbols
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import x from './0'
+>x : Symbol(x, Decl(1.ts, 0, 6))
+
+x + 1
+>x : Symbol(x, Decl(1.ts, 0, 6))
+
diff --git a/tests/baselines/reference/emitExtensionOptionsInvalidChars.types b/tests/baselines/reference/emitExtensionOptionsInvalidChars.types
new file mode 100644
index 0000000000000..a00a4fa80ddc6
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsInvalidChars.types
@@ -0,0 +1,12 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import x from './0'
+>x : 1
+
+x + 1
+>x + 1 : number
+>x : 1
+>1 : 1
+
diff --git a/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.errors.txt b/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.errors.txt
new file mode 100644
index 0000000000000..c89d9197ca2d1
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.errors.txt
@@ -0,0 +1,8 @@
+error TS6507: emitExtension can only be ".jsx" when JSX is set to "preserve"
+
+
+!!! error TS6507: emitExtension can only be ".jsx" when JSX is set to "preserve"
+==== tests/cases/compiler/0.ts (0 errors) ====
+    // emitExtension can only be ".jsx" when JSX is set to "preserve"
+    export default 1
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.js b/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.js
new file mode 100644
index 0000000000000..a348d020db894
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.js
@@ -0,0 +1,10 @@
+//// [0.ts]
+// emitExtension can only be ".jsx" when JSX is set to "preserve"
+export default 1
+
+
+//// [0.mjs]
+"use strict";
+exports.__esModule = true;
+// emitExtension can only be ".jsx" when JSX is set to "preserve"
+exports["default"] = 1;
diff --git a/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.symbols b/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.symbols
new file mode 100644
index 0000000000000..6cf028860bb10
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.symbols
@@ -0,0 +1,5 @@
+=== tests/cases/compiler/0.ts ===
+// emitExtension can only be ".jsx" when JSX is set to "preserve"
+No type information for this code.export default 1
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.types b/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.types
new file mode 100644
index 0000000000000..6cf028860bb10
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsJSXPreserveBad.types
@@ -0,0 +1,5 @@
+=== tests/cases/compiler/0.ts ===
+// emitExtension can only be ".jsx" when JSX is set to "preserve"
+No type information for this code.export default 1
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsJSXPreserveNormal.js b/tests/baselines/reference/emitExtensionOptionsJSXPreserveNormal.js
new file mode 100644
index 0000000000000..e70dfbafba0c4
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsJSXPreserveNormal.js
@@ -0,0 +1,8 @@
+//// [0.ts]
+export default 1
+
+
+//// [0.jsx]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 1;
diff --git a/tests/baselines/reference/emitExtensionOptionsJSXPreserveNormal.symbols b/tests/baselines/reference/emitExtensionOptionsJSXPreserveNormal.symbols
new file mode 100644
index 0000000000000..2e9d59769cb1a
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsJSXPreserveNormal.symbols
@@ -0,0 +1,4 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsJSXPreserveNormal.types b/tests/baselines/reference/emitExtensionOptionsJSXPreserveNormal.types
new file mode 100644
index 0000000000000..2e9d59769cb1a
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsJSXPreserveNormal.types
@@ -0,0 +1,4 @@
+=== tests/cases/compiler/0.ts ===
+export default 1
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsNormal.js b/tests/baselines/reference/emitExtensionOptionsNormal.js
new file mode 100644
index 0000000000000..2d09dc08704df
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsNormal.js
@@ -0,0 +1,8 @@
+//// [0.ts]
+export default 0
+
+
+//// [0.mjs]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 0;
diff --git a/tests/baselines/reference/emitExtensionOptionsNormal.symbols b/tests/baselines/reference/emitExtensionOptionsNormal.symbols
new file mode 100644
index 0000000000000..4240767327d08
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsNormal.symbols
@@ -0,0 +1,4 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsNormal.types b/tests/baselines/reference/emitExtensionOptionsNormal.types
new file mode 100644
index 0000000000000..4240767327d08
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsNormal.types
@@ -0,0 +1,4 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsStartWithNonDot.errors.txt b/tests/baselines/reference/emitExtensionOptionsStartWithNonDot.errors.txt
new file mode 100644
index 0000000000000..3fd1276fdb81a
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsStartWithNonDot.errors.txt
@@ -0,0 +1,8 @@
+error TS6506: emitExtension must start with '.', but here has 'mjs', try to replace it with '.mjs'.
+
+
+!!! error TS6506: emitExtension must start with '.', but here has 'mjs', try to replace it with '.mjs'.
+==== tests/cases/compiler/0.ts (0 errors) ====
+    // emitExtension must start with '.'
+    export default 0
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsStartWithNonDot.symbols b/tests/baselines/reference/emitExtensionOptionsStartWithNonDot.symbols
new file mode 100644
index 0000000000000..8b32c60e43171
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsStartWithNonDot.symbols
@@ -0,0 +1,5 @@
+=== tests/cases/compiler/0.ts ===
+// emitExtension must start with '.'
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionOptionsStartWithNonDot.types b/tests/baselines/reference/emitExtensionOptionsStartWithNonDot.types
new file mode 100644
index 0000000000000..8b32c60e43171
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionOptionsStartWithNonDot.types
@@ -0,0 +1,5 @@
+=== tests/cases/compiler/0.ts ===
+// emitExtension must start with '.'
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.errors.txt b/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.errors.txt
new file mode 100644
index 0000000000000..5c2a135561407
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.errors.txt
@@ -0,0 +1,17 @@
+tests/cases/compiler/1.ts(1,17): error TS2691: An import path cannot end with a '.ts' extension. Consider importing './0.tsx' instead.
+
+
+==== tests/cases/compiler/0.ts (0 errors) ====
+    // For the deno case
+    export default 0
+    
+==== tests/cases/compiler/1.ts (1 errors) ====
+    import num from './0.ts'
+                    ~~~~~~~~
+!!! error TS2691: An import path cannot end with a '.ts' extension. Consider importing './0.tsx' instead.
+    import num2 from './2.tsx'
+    num + num2
+    
+==== tests/cases/compiler/2.tsx (0 errors) ====
+    export default 1
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.js b/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.js
new file mode 100644
index 0000000000000..fd53042178292
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.js
@@ -0,0 +1,30 @@
+//// [tests/cases/compiler/emitExtensionRejectMixedResolveTSFile.ts] ////
+
+//// [0.ts]
+// For the deno case
+export default 0
+
+//// [1.ts]
+import num from './0.ts'
+import num2 from './2.tsx'
+num + num2
+
+//// [2.tsx]
+export default 1
+
+
+//// [0.tsx]
+"use strict";
+exports.__esModule = true;
+// For the deno case
+exports["default"] = 0;
+//// [2.tsx]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 1;
+//// [1.tsx]
+"use strict";
+exports.__esModule = true;
+var _0_ts_1 = require("./0.ts");
+var _2_tsx_1 = require("./2.tsx");
+_0_ts_1["default"] + _2_tsx_1["default"];
diff --git a/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.symbols b/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.symbols
new file mode 100644
index 0000000000000..ec9427ca8b594
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.symbols
@@ -0,0 +1,19 @@
+=== tests/cases/compiler/0.ts ===
+// For the deno case
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.ts'
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
+import num2 from './2.tsx'
+>num2 : Symbol(num2, Decl(1.ts, 1, 6))
+
+num + num2
+>num : Symbol(num, Decl(1.ts, 0, 6))
+>num2 : Symbol(num2, Decl(1.ts, 1, 6))
+
+=== tests/cases/compiler/2.tsx ===
+export default 1
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.types b/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.types
new file mode 100644
index 0000000000000..dcf9317b46d49
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionRejectMixedResolveTSFile.types
@@ -0,0 +1,20 @@
+=== tests/cases/compiler/0.ts ===
+// For the deno case
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.ts'
+>num : any
+
+import num2 from './2.tsx'
+>num2 : 1
+
+num + num2
+>num + num2 : any
+>num : any
+>num2 : 1
+
+=== tests/cases/compiler/2.tsx ===
+export default 1
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionRejectResolveTSFile.errors.txt b/tests/baselines/reference/emitExtensionRejectResolveTSFile.errors.txt
new file mode 100644
index 0000000000000..5b7e101de3def
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionRejectResolveTSFile.errors.txt
@@ -0,0 +1,12 @@
+tests/cases/compiler/1.ts(1,17): error TS2691: An import path cannot end with a '.ts' extension. Consider importing './0.mjs' instead.
+
+
+==== tests/cases/compiler/0.ts (0 errors) ====
+    export default 0
+    
+==== tests/cases/compiler/1.ts (1 errors) ====
+    import num from './0.ts'
+                    ~~~~~~~~
+!!! error TS2691: An import path cannot end with a '.ts' extension. Consider importing './0.mjs' instead.
+    num + 1
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionRejectResolveTSFile.js b/tests/baselines/reference/emitExtensionRejectResolveTSFile.js
new file mode 100644
index 0000000000000..e95f89638d53e
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionRejectResolveTSFile.js
@@ -0,0 +1,19 @@
+//// [tests/cases/compiler/emitExtensionRejectResolveTSFile.ts] ////
+
+//// [0.ts]
+export default 0
+
+//// [1.ts]
+import num from './0.ts'
+num + 1
+
+
+//// [0.mjs]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 0;
+//// [1.mjs]
+"use strict";
+exports.__esModule = true;
+var _0_ts_1 = require("./0.ts");
+_0_ts_1["default"] + 1;
diff --git a/tests/baselines/reference/emitExtensionRejectResolveTSFile.symbols b/tests/baselines/reference/emitExtensionRejectResolveTSFile.symbols
new file mode 100644
index 0000000000000..c088de76473ed
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionRejectResolveTSFile.symbols
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.ts'
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
+num + 1
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
diff --git a/tests/baselines/reference/emitExtensionRejectResolveTSFile.types b/tests/baselines/reference/emitExtensionRejectResolveTSFile.types
new file mode 100644
index 0000000000000..608eded70d200
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionRejectResolveTSFile.types
@@ -0,0 +1,12 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.ts'
+>num : any
+
+num + 1
+>num + 1 : any
+>num : any
+>1 : 1
+
diff --git a/tests/baselines/reference/emitExtensionResolveExtToTSFile.js b/tests/baselines/reference/emitExtensionResolveExtToTSFile.js
new file mode 100644
index 0000000000000..bd5700a332e87
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionResolveExtToTSFile.js
@@ -0,0 +1,19 @@
+//// [tests/cases/compiler/emitExtensionResolveExtToTSFile.ts] ////
+
+//// [0.ts]
+export default 0
+
+//// [1.ts]
+import value from './0.mjs'
+value + 1
+
+
+//// [0.mjs]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 0;
+//// [1.mjs]
+"use strict";
+exports.__esModule = true;
+var _0_mjs_1 = require("./0.mjs");
+_0_mjs_1["default"] + 1;
diff --git a/tests/baselines/reference/emitExtensionResolveExtToTSFile.symbols b/tests/baselines/reference/emitExtensionResolveExtToTSFile.symbols
new file mode 100644
index 0000000000000..941e79f112989
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionResolveExtToTSFile.symbols
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import value from './0.mjs'
+>value : Symbol(value, Decl(1.ts, 0, 6))
+
+value + 1
+>value : Symbol(value, Decl(1.ts, 0, 6))
+
diff --git a/tests/baselines/reference/emitExtensionResolveExtToTSFile.types b/tests/baselines/reference/emitExtensionResolveExtToTSFile.types
new file mode 100644
index 0000000000000..58225b7dd313e
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionResolveExtToTSFile.types
@@ -0,0 +1,12 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import value from './0.mjs'
+>value : 0
+
+value + 1
+>value + 1 : number
+>value : 0
+>1 : 1
+
diff --git a/tests/baselines/reference/emitExtensionResolveTSFile.js b/tests/baselines/reference/emitExtensionResolveTSFile.js
new file mode 100644
index 0000000000000..40c7a96dc5049
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionResolveTSFile.js
@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/emitExtensionResolveTSFile.ts] ////
+
+//// [0.ts]
+// For the deno case
+export default 0
+
+//// [1.ts]
+import num from './0.ts'
+num + 1
+
+
+//// [0.ts]
+"use strict";
+exports.__esModule = true;
+// For the deno case
+exports["default"] = 0;
+//// [1.ts]
+"use strict";
+exports.__esModule = true;
+var _0_ts_1 = require("./0.ts");
+_0_ts_1["default"] + 1;
diff --git a/tests/baselines/reference/emitExtensionResolveTSFile.symbols b/tests/baselines/reference/emitExtensionResolveTSFile.symbols
new file mode 100644
index 0000000000000..d69465f43b5bf
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionResolveTSFile.symbols
@@ -0,0 +1,11 @@
+=== tests/cases/compiler/0.ts ===
+// For the deno case
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.ts'
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
+num + 1
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
diff --git a/tests/baselines/reference/emitExtensionResolveTSFile.types b/tests/baselines/reference/emitExtensionResolveTSFile.types
new file mode 100644
index 0000000000000..b851a65aeeded
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionResolveTSFile.types
@@ -0,0 +1,13 @@
+=== tests/cases/compiler/0.ts ===
+// For the deno case
+No type information for this code.export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.ts'
+>num : 0
+
+num + 1
+>num + 1 : number
+>num : 0
+>1 : 1
+
diff --git a/tests/baselines/reference/emitExtensionTSNotFound.errors.txt b/tests/baselines/reference/emitExtensionTSNotFound.errors.txt
new file mode 100644
index 0000000000000..12df1f73a57c4
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionTSNotFound.errors.txt
@@ -0,0 +1,10 @@
+tests/cases/compiler/0.ts(2,17): error TS2307: Cannot find module './1.ts'.
+
+
+==== tests/cases/compiler/0.ts (1 errors) ====
+    // should not emit error 2691 but 2307
+    import num from './1.ts'
+                    ~~~~~~~~
+!!! error TS2307: Cannot find module './1.ts'.
+    num + 1
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionTSNotFound.js b/tests/baselines/reference/emitExtensionTSNotFound.js
new file mode 100644
index 0000000000000..acd0168414aa2
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionTSNotFound.js
@@ -0,0 +1,12 @@
+//// [0.ts]
+// should not emit error 2691 but 2307
+import num from './1.ts'
+num + 1
+
+
+//// [0.ts]
+"use strict";
+exports.__esModule = true;
+// should not emit error 2691 but 2307
+var _1_ts_1 = require("./1.ts");
+_1_ts_1["default"] + 1;
diff --git a/tests/baselines/reference/emitExtensionTSNotFound.symbols b/tests/baselines/reference/emitExtensionTSNotFound.symbols
new file mode 100644
index 0000000000000..68a783d50c14b
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionTSNotFound.symbols
@@ -0,0 +1,8 @@
+=== tests/cases/compiler/0.ts ===
+// should not emit error 2691 but 2307
+import num from './1.ts'
+>num : Symbol(num, Decl(0.ts, 1, 6))
+
+num + 1
+>num : Symbol(num, Decl(0.ts, 1, 6))
+
diff --git a/tests/baselines/reference/emitExtensionTSNotFound.types b/tests/baselines/reference/emitExtensionTSNotFound.types
new file mode 100644
index 0000000000000..4a1c9e32c8bc5
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionTSNotFound.types
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/0.ts ===
+// should not emit error 2691 but 2307
+import num from './1.ts'
+>num : any
+
+num + 1
+>num + 1 : any
+>num : any
+>1 : 1
+
diff --git a/tests/baselines/reference/emitExtensionUnknownExtension.errors.txt b/tests/baselines/reference/emitExtensionUnknownExtension.errors.txt
new file mode 100644
index 0000000000000..ca921037ad4b3
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionUnknownExtension.errors.txt
@@ -0,0 +1,12 @@
+tests/cases/compiler/1.ts(1,17): error TS2307: Cannot find module './0.cjs'.
+
+
+==== tests/cases/compiler/0.ts (0 errors) ====
+    export default 0
+    
+==== tests/cases/compiler/1.ts (1 errors) ====
+    import num from './0.cjs'
+                    ~~~~~~~~~
+!!! error TS2307: Cannot find module './0.cjs'.
+    num + 1
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/emitExtensionUnknownExtension.js b/tests/baselines/reference/emitExtensionUnknownExtension.js
new file mode 100644
index 0000000000000..2aad2e7b842e6
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionUnknownExtension.js
@@ -0,0 +1,19 @@
+//// [tests/cases/compiler/emitExtensionUnknownExtension.ts] ////
+
+//// [0.ts]
+export default 0
+
+//// [1.ts]
+import num from './0.cjs'
+num + 1
+
+
+//// [0.mjs]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 0;
+//// [1.mjs]
+"use strict";
+exports.__esModule = true;
+var _0_cjs_1 = require("./0.cjs");
+_0_cjs_1["default"] + 1;
diff --git a/tests/baselines/reference/emitExtensionUnknownExtension.symbols b/tests/baselines/reference/emitExtensionUnknownExtension.symbols
new file mode 100644
index 0000000000000..ca2d67e4a4eaa
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionUnknownExtension.symbols
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.cjs'
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
+num + 1
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
diff --git a/tests/baselines/reference/emitExtensionUnknownExtension.types b/tests/baselines/reference/emitExtensionUnknownExtension.types
new file mode 100644
index 0000000000000..6dfc32fca4662
--- /dev/null
+++ b/tests/baselines/reference/emitExtensionUnknownExtension.types
@@ -0,0 +1,12 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.cjs'
+>num : any
+
+num + 1
+>num + 1 : any
+>num : any
+>1 : 1
+
diff --git a/tests/baselines/reference/noImplicitExtensionNameError.errors.txt b/tests/baselines/reference/noImplicitExtensionNameError.errors.txt
new file mode 100644
index 0000000000000..825dd6c8504b9
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameError.errors.txt
@@ -0,0 +1,15 @@
+tests/cases/compiler/1.ts(2,1): error TS6511: Import with an implicit extension name is not allowed. Try to import from './0.js' instead.
+
+
+==== tests/cases/compiler/0.ts (0 errors) ====
+    export default 0
+    
+==== tests/cases/compiler/1.ts (1 errors) ====
+    // Should be error
+    import num from './0'
+    ~~~~~~~~~~~~~~~~~~~~~
+!!! error TS6511: Import with an implicit extension name is not allowed. Try to import from './0.js' instead.
+    // Should be okay
+    import num2 from './0.js'
+    num + num2
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/noImplicitExtensionNameError.js b/tests/baselines/reference/noImplicitExtensionNameError.js
new file mode 100644
index 0000000000000..ce3deb1c2201c
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameError.js
@@ -0,0 +1,25 @@
+//// [tests/cases/compiler/noImplicitExtensionNameError.ts] ////
+
+//// [0.ts]
+export default 0
+
+//// [1.ts]
+// Should be error
+import num from './0'
+// Should be okay
+import num2 from './0.js'
+num + num2
+
+
+//// [0.js]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 0;
+//// [1.js]
+"use strict";
+exports.__esModule = true;
+// Should be error
+var _0_1 = require("./0");
+// Should be okay
+var _0_js_1 = require("./0.js");
+_0_1["default"] + _0_js_1["default"];
diff --git a/tests/baselines/reference/noImplicitExtensionNameError.symbols b/tests/baselines/reference/noImplicitExtensionNameError.symbols
new file mode 100644
index 0000000000000..b88cddd70441e
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameError.symbols
@@ -0,0 +1,16 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+// Should be error
+import num from './0'
+>num : Symbol(num, Decl(1.ts, 1, 6))
+
+// Should be okay
+import num2 from './0.js'
+>num2 : Symbol(num2, Decl(1.ts, 3, 6))
+
+num + num2
+>num : Symbol(num, Decl(1.ts, 1, 6))
+>num2 : Symbol(num2, Decl(1.ts, 3, 6))
+
diff --git a/tests/baselines/reference/noImplicitExtensionNameError.types b/tests/baselines/reference/noImplicitExtensionNameError.types
new file mode 100644
index 0000000000000..2be25dbee6de0
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameError.types
@@ -0,0 +1,17 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+// Should be error
+import num from './0'
+>num : 0
+
+// Should be okay
+import num2 from './0.js'
+>num2 : 0
+
+num + num2
+>num + num2 : number
+>num : 0
+>num2 : 0
+
diff --git a/tests/baselines/reference/noImplicitExtensionNameNonRelative.errors.txt b/tests/baselines/reference/noImplicitExtensionNameNonRelative.errors.txt
new file mode 100644
index 0000000000000..b8648fa5afd27
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameNonRelative.errors.txt
@@ -0,0 +1,19 @@
+tests/cases/compiler/1.ts(4,17): error TS2307: Cannot find module 'fs/my_file'.
+tests/cases/compiler/1.ts(5,18): error TS2307: Cannot find module 'fs/my_file.js'.
+
+
+==== tests/cases/compiler/fs/my_file.ts (0 errors) ====
+    export default 0
+    
+==== tests/cases/compiler/1.ts (2 errors) ====
+    // Should both be okay.
+    // This check is not applied to non-relative import.
+    // e.g. Node's ESModule support https://nodejs.org/api/esm.html#esm_package_exports
+    import num from 'fs/my_file'
+                    ~~~~~~~~~~~~
+!!! error TS2307: Cannot find module 'fs/my_file'.
+    import num2 from 'fs/my_file.js'
+                     ~~~~~~~~~~~~~~~
+!!! error TS2307: Cannot find module 'fs/my_file.js'.
+    num + num2
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/noImplicitExtensionNameNonRelative.js b/tests/baselines/reference/noImplicitExtensionNameNonRelative.js
new file mode 100644
index 0000000000000..f96038d32a0a1
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameNonRelative.js
@@ -0,0 +1,27 @@
+//// [tests/cases/compiler/noImplicitExtensionNameNonRelative.ts] ////
+
+//// [my_file.ts]
+export default 0
+
+//// [1.ts]
+// Should both be okay.
+// This check is not applied to non-relative import.
+// e.g. Node's ESModule support https://nodejs.org/api/esm.html#esm_package_exports
+import num from 'fs/my_file'
+import num2 from 'fs/my_file.js'
+num + num2
+
+
+//// [my_file.js]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 0;
+//// [1.js]
+"use strict";
+exports.__esModule = true;
+// Should both be okay.
+// This check is not applied to non-relative import.
+// e.g. Node's ESModule support https://nodejs.org/api/esm.html#esm_package_exports
+var my_file_1 = require("fs/my_file");
+var my_file_js_1 = require("fs/my_file.js");
+my_file_1["default"] + my_file_js_1["default"];
diff --git a/tests/baselines/reference/noImplicitExtensionNameNonRelative.symbols b/tests/baselines/reference/noImplicitExtensionNameNonRelative.symbols
new file mode 100644
index 0000000000000..83a7f9c2f2db8
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameNonRelative.symbols
@@ -0,0 +1,17 @@
+=== tests/cases/compiler/fs/my_file.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+// Should both be okay.
+// This check is not applied to non-relative import.
+// e.g. Node's ESModule support https://nodejs.org/api/esm.html#esm_package_exports
+import num from 'fs/my_file'
+>num : Symbol(num, Decl(1.ts, 3, 6))
+
+import num2 from 'fs/my_file.js'
+>num2 : Symbol(num2, Decl(1.ts, 4, 6))
+
+num + num2
+>num : Symbol(num, Decl(1.ts, 3, 6))
+>num2 : Symbol(num2, Decl(1.ts, 4, 6))
+
diff --git a/tests/baselines/reference/noImplicitExtensionNameNonRelative.types b/tests/baselines/reference/noImplicitExtensionNameNonRelative.types
new file mode 100644
index 0000000000000..6e5696caf7799
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameNonRelative.types
@@ -0,0 +1,18 @@
+=== tests/cases/compiler/fs/my_file.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+// Should both be okay.
+// This check is not applied to non-relative import.
+// e.g. Node's ESModule support https://nodejs.org/api/esm.html#esm_package_exports
+import num from 'fs/my_file'
+>num : any
+
+import num2 from 'fs/my_file.js'
+>num2 : any
+
+num + num2
+>num + num2 : any
+>num : any
+>num2 : any
+
diff --git a/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.errors.txt b/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.errors.txt
new file mode 100644
index 0000000000000..d45a8b1133ea3
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.errors.txt
@@ -0,0 +1,14 @@
+tests/cases/compiler/1.ts(3,1): error TS6511: Import with an implicit extension name is not allowed. Try to import from './0.mjs' instead.
+
+
+==== tests/cases/compiler/0.ts (0 errors) ====
+    export default 0
+    
+==== tests/cases/compiler/1.ts (1 errors) ====
+    import num from './0.mjs'
+    // should be error
+    import num2 from './0'
+    ~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS6511: Import with an implicit extension name is not allowed. Try to import from './0.mjs' instead.
+    num + num2
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.js b/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.js
new file mode 100644
index 0000000000000..6af322ece93b0
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.js
@@ -0,0 +1,23 @@
+//// [tests/cases/compiler/noImplicitExtensionNameWithEmitExtension.ts] ////
+
+//// [0.ts]
+export default 0
+
+//// [1.ts]
+import num from './0.mjs'
+// should be error
+import num2 from './0'
+num + num2
+
+
+//// [0.mjs]
+"use strict";
+exports.__esModule = true;
+exports["default"] = 0;
+//// [1.mjs]
+"use strict";
+exports.__esModule = true;
+var _0_mjs_1 = require("./0.mjs");
+// should be error
+var _0_1 = require("./0");
+_0_mjs_1["default"] + _0_1["default"];
diff --git a/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.symbols b/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.symbols
new file mode 100644
index 0000000000000..1af3b466f8b3d
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.symbols
@@ -0,0 +1,15 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.mjs'
+>num : Symbol(num, Decl(1.ts, 0, 6))
+
+// should be error
+import num2 from './0'
+>num2 : Symbol(num2, Decl(1.ts, 2, 6))
+
+num + num2
+>num : Symbol(num, Decl(1.ts, 0, 6))
+>num2 : Symbol(num2, Decl(1.ts, 2, 6))
+
diff --git a/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.types b/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.types
new file mode 100644
index 0000000000000..e108d98a7fe03
--- /dev/null
+++ b/tests/baselines/reference/noImplicitExtensionNameWithEmitExtension.types
@@ -0,0 +1,16 @@
+=== tests/cases/compiler/0.ts ===
+export default 0
+No type information for this code.
+No type information for this code.=== tests/cases/compiler/1.ts ===
+import num from './0.mjs'
+>num : 0
+
+// should be error
+import num2 from './0'
+>num2 : 0
+
+num + num2
+>num + num2 : number
+>num : 0
+>num2 : 0
+
diff --git a/tests/baselines/reference/showConfig/Shows tsconfig for single option/emitExtension/tsconfig.json b/tests/baselines/reference/showConfig/Shows tsconfig for single option/emitExtension/tsconfig.json
new file mode 100644
index 0000000000000..c4ecd585230f8
--- /dev/null
+++ b/tests/baselines/reference/showConfig/Shows tsconfig for single option/emitExtension/tsconfig.json	
@@ -0,0 +1,5 @@
+{
+    "compilerOptions": {
+        "emitExtension": "someString"
+    }
+}
diff --git a/tests/baselines/reference/showConfig/Shows tsconfig for single option/noImplicitExtensionName/tsconfig.json b/tests/baselines/reference/showConfig/Shows tsconfig for single option/noImplicitExtensionName/tsconfig.json
new file mode 100644
index 0000000000000..178987a00f2c9
--- /dev/null
+++ b/tests/baselines/reference/showConfig/Shows tsconfig for single option/noImplicitExtensionName/tsconfig.json	
@@ -0,0 +1,5 @@
+{
+    "compilerOptions": {
+        "noImplicitExtensionName": true
+    }
+}
diff --git a/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json b/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json
index 54d53fdf72db4..0e92eaf0eb34e 100644
--- a/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json	
@@ -21,6 +21,7 @@
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -37,6 +38,7 @@
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with advanced options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with advanced options/tsconfig.json
index 1ce4f0225001f..efc97d9a6844b 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with advanced options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with advanced options/tsconfig.json	
@@ -21,6 +21,7 @@
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -37,6 +38,7 @@
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json
index edbfb38c2e1a3..1cbe9031c5e95 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json	
@@ -21,6 +21,7 @@
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -37,6 +38,7 @@
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json
index aad370c1e6974..db5c681dc4c94 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json	
@@ -21,6 +21,7 @@
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -37,6 +38,7 @@
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json
index 4c1a8235295f9..ab4b3286eb96c 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json	
@@ -21,6 +21,7 @@
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -37,6 +38,7 @@
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json
index 965fb1e28b98c..86f78e4566661 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json	
@@ -21,6 +21,7 @@
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -37,6 +38,7 @@
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json
index 54d53fdf72db4..0e92eaf0eb34e 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json	
@@ -21,6 +21,7 @@
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -37,6 +38,7 @@
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json
index d6d044fa42b81..1a5dba930f3d7 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json	
@@ -21,6 +21,7 @@
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -37,6 +38,7 @@
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json
index 27af8c27249b1..d79d8e9c14ac2 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json	
@@ -21,6 +21,7 @@
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -37,6 +38,7 @@
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tsc/runWithoutArgs/initial-build/show-help-with-ExitStatus.DiagnosticsPresent_OutputsSkipped.js b/tests/baselines/reference/tsc/runWithoutArgs/initial-build/show-help-with-ExitStatus.DiagnosticsPresent_OutputsSkipped.js
index e9c781c0616d5..63e61057d2a9d 100644
--- a/tests/baselines/reference/tsc/runWithoutArgs/initial-build/show-help-with-ExitStatus.DiagnosticsPresent_OutputsSkipped.js
+++ b/tests/baselines/reference/tsc/runWithoutArgs/initial-build/show-help-with-ExitStatus.DiagnosticsPresent_OutputsSkipped.js
@@ -44,6 +44,8 @@ Options:
  --noFallthroughCasesInSwitch                       Report errors for fallthrough cases in switch statement.
  --types                                            Type declaration files to be included in compilation.
  --esModuleInterop                                  Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'.
+ --emitExtension                                    Specify the extension name of the emitted files.
+ --noImplicitExtensionName                          Report error for implicit extension name.
  @<file>                                            Insert command line options and files from a file.
 exitCode:: ExitStatus.DiagnosticsPresent_OutputsSkipped
 
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/declarationDir-is-specified.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/declarationDir-is-specified.js
index 36c9d0609c14f..ef1e5e7b04cef 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/declarationDir-is-specified.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/declarationDir-is-specified.js
@@ -42,6 +42,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -58,6 +59,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-and-declarationDir-is-specified.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-and-declarationDir-is-specified.js
index f755ab350ecce..ae5e11ddf7e9a 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-and-declarationDir-is-specified.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-and-declarationDir-is-specified.js
@@ -42,6 +42,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -58,6 +59,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-is-specified.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-is-specified.js
index 933745206f23a..bde47e262c9e7 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-is-specified.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-is-specified.js
@@ -42,6 +42,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -58,6 +59,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/with-outFile.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/with-outFile.js
index 9bb54d9380e23..7c90aef43367e 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/with-outFile.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/with-outFile.js
@@ -42,6 +42,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -58,6 +59,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified.js
index 158a39b488077..4e961e8541257 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified.js
@@ -42,6 +42,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
     // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
     // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */
+    // "emitExtension": "",                   /* Specify the extension name of the emitted files. */
 
     /* Strict Type-Checking Options */
     "strict": true,                           /* Enable all strict type-checking options. */
@@ -58,6 +59,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noUnusedParameters": true,            /* Report errors on unused parameters. */
     // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
     // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+    // "noImplicitExtensionName": true,       /* Report error for implicit extension name. */
 
     /* Module Resolution Options */
     // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
diff --git a/tests/cases/compiler/emitExtensionDiag2691.ts b/tests/cases/compiler/emitExtensionDiag2691.ts
new file mode 100644
index 0000000000000..fc99f1a03a926
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionDiag2691.ts
@@ -0,0 +1,9 @@
+// @emitExtension: .mjs
+
+// @Filename: 0.ts
+// For the deno case
+export default 0
+
+// @Filename: 1.ts
+import num from './0.ts'
+num + 1
diff --git a/tests/cases/compiler/emitExtensionEmitCustomExtname.ts b/tests/cases/compiler/emitExtensionEmitCustomExtname.ts
new file mode 100644
index 0000000000000..cc4405db27027
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionEmitCustomExtname.ts
@@ -0,0 +1,8 @@
+// @emitExtension: .cpp
+
+// @Filename: 0.ts
+export default 1
+
+// @Filename: 1.ts
+import x from './0'
+x + 1
diff --git a/tests/cases/compiler/emitExtensionEmitCustomExtnamePathEscape.ts b/tests/cases/compiler/emitExtensionEmitCustomExtnamePathEscape.ts
new file mode 100644
index 0000000000000..6a3e638c2eb4e
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionEmitCustomExtnamePathEscape.ts
@@ -0,0 +1,4 @@
+// @emitExtension: ../../../../index.js
+
+// @Filename: 0.ts
+export default 1
diff --git a/tests/cases/compiler/emitExtensionEmitCustomExtnameWithDot.ts b/tests/cases/compiler/emitExtensionEmitCustomExtnameWithDot.ts
new file mode 100644
index 0000000000000..0d417a411a70b
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionEmitCustomExtnameWithDot.ts
@@ -0,0 +1,8 @@
+// @emitExtension: .strange.js
+
+// @Filename: 0.ts
+export default 1
+
+// @Filename: 1.ts
+import x from './0'
+x + 1
diff --git a/tests/cases/compiler/emitExtensionOptionsDTS.ts b/tests/cases/compiler/emitExtensionOptionsDTS.ts
new file mode 100644
index 0000000000000..c2a1ba062574b
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionOptionsDTS.ts
@@ -0,0 +1,5 @@
+// @emitExtension: .d.ts
+
+// @Filename 1.ts
+// emitExtension can not be ".d.ts"
+export default 0
diff --git a/tests/cases/compiler/emitExtensionOptionsInvalidChars.ts b/tests/cases/compiler/emitExtensionOptionsInvalidChars.ts
new file mode 100644
index 0000000000000..eeb7b9dd00d3e
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionOptionsInvalidChars.ts
@@ -0,0 +1,8 @@
+// @emitExtension: .!<>/\.js
+
+// @Filename: 0.ts
+export default 1
+
+// @Filename: 1.ts
+import x from './0'
+x + 1
diff --git a/tests/cases/compiler/emitExtensionOptionsJSXPreserveBad.ts b/tests/cases/compiler/emitExtensionOptionsJSXPreserveBad.ts
new file mode 100644
index 0000000000000..26abd6682c735
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionOptionsJSXPreserveBad.ts
@@ -0,0 +1,6 @@
+// @emitExtension: .mjs
+// @jsx: preserve
+
+// @Filename: 0.ts
+// emitExtension can only be ".jsx" when JSX is set to "preserve"
+export default 1
diff --git a/tests/cases/compiler/emitExtensionOptionsJSXPreserveNormal.ts b/tests/cases/compiler/emitExtensionOptionsJSXPreserveNormal.ts
new file mode 100644
index 0000000000000..917103d7972fc
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionOptionsJSXPreserveNormal.ts
@@ -0,0 +1,5 @@
+// @emitExtension: .jsx
+// @jsx: preserve
+
+// @Filename: 0.ts
+export default 1
diff --git a/tests/cases/compiler/emitExtensionOptionsNormal.ts b/tests/cases/compiler/emitExtensionOptionsNormal.ts
new file mode 100644
index 0000000000000..4358a79ecb197
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionOptionsNormal.ts
@@ -0,0 +1,4 @@
+// @emitExtension: .mjs
+
+// @Filename: 0.ts
+export default 0
diff --git a/tests/cases/compiler/emitExtensionOptionsStartWithNonDot.ts b/tests/cases/compiler/emitExtensionOptionsStartWithNonDot.ts
new file mode 100644
index 0000000000000..482a602bc5c55
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionOptionsStartWithNonDot.ts
@@ -0,0 +1,5 @@
+// @emitExtension: mjs
+
+// @Filename: 0.ts
+// emitExtension must start with '.'
+export default 0
diff --git a/tests/cases/compiler/emitExtensionRejectMixedResolveTSFile.ts b/tests/cases/compiler/emitExtensionRejectMixedResolveTSFile.ts
new file mode 100644
index 0000000000000..70a88447c351f
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionRejectMixedResolveTSFile.ts
@@ -0,0 +1,15 @@
+// @emitExtension: .tsx
+// @outDir: ../out/
+// @jsx: react
+
+// @Filename: 0.ts
+// For the deno case
+export default 0
+
+// @Filename: 1.ts
+import num from './0.ts'
+import num2 from './2.tsx'
+num + num2
+
+// @Filename: 2.tsx
+export default 1
diff --git a/tests/cases/compiler/emitExtensionRejectResolveTSFile.ts b/tests/cases/compiler/emitExtensionRejectResolveTSFile.ts
new file mode 100644
index 0000000000000..096ea063dfda1
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionRejectResolveTSFile.ts
@@ -0,0 +1,9 @@
+// @emitExtension: .mjs
+// @outDir: ../out/
+
+// @Filename: 0.ts
+export default 0
+
+// @Filename: 1.ts
+import num from './0.ts'
+num + 1
diff --git a/tests/cases/compiler/emitExtensionResolveExtToTSFile.ts b/tests/cases/compiler/emitExtensionResolveExtToTSFile.ts
new file mode 100644
index 0000000000000..95c594e17df07
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionResolveExtToTSFile.ts
@@ -0,0 +1,8 @@
+// @emitExtension: .mjs
+
+// @Filename: 0.ts
+export default 0
+
+// @Filename: 1.ts
+import value from './0.mjs'
+value + 1
diff --git a/tests/cases/compiler/emitExtensionResolveTSFile.ts b/tests/cases/compiler/emitExtensionResolveTSFile.ts
new file mode 100644
index 0000000000000..529be1c28641d
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionResolveTSFile.ts
@@ -0,0 +1,10 @@
+// @emitExtension: .ts
+// @outDir: ../out/
+
+// @Filename: 0.ts
+// For the deno case
+export default 0
+
+// @Filename: 1.ts
+import num from './0.ts'
+num + 1
diff --git a/tests/cases/compiler/emitExtensionTSNotFound.ts b/tests/cases/compiler/emitExtensionTSNotFound.ts
new file mode 100644
index 0000000000000..fd69020767b30
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionTSNotFound.ts
@@ -0,0 +1,7 @@
+// @emitExtension: .ts
+// @outDir: ./out/
+
+// @Filename: 0.ts
+// should not emit error 2691 but 2307
+import num from './1.ts'
+num + 1
diff --git a/tests/cases/compiler/emitExtensionUnknownExtension.ts b/tests/cases/compiler/emitExtensionUnknownExtension.ts
new file mode 100644
index 0000000000000..eb52432bd5559
--- /dev/null
+++ b/tests/cases/compiler/emitExtensionUnknownExtension.ts
@@ -0,0 +1,8 @@
+// @emitExtension: .mjs
+
+// @Filename: 0.ts
+export default 0
+
+// @Filename: 1.ts
+import num from './0.cjs'
+num + 1
diff --git a/tests/cases/compiler/noImplicitExtensionNameError.ts b/tests/cases/compiler/noImplicitExtensionNameError.ts
new file mode 100644
index 0000000000000..074f1ee5cc412
--- /dev/null
+++ b/tests/cases/compiler/noImplicitExtensionNameError.ts
@@ -0,0 +1,11 @@
+// @noImplicitExtensionName: true
+
+// @Filename: 0.ts
+export default 0
+
+// @Filename: 1.ts
+// Should be error
+import num from './0'
+// Should be okay
+import num2 from './0.js'
+num + num2
diff --git a/tests/cases/compiler/noImplicitExtensionNameNonRelative.ts b/tests/cases/compiler/noImplicitExtensionNameNonRelative.ts
new file mode 100644
index 0000000000000..968d32487b8d2
--- /dev/null
+++ b/tests/cases/compiler/noImplicitExtensionNameNonRelative.ts
@@ -0,0 +1,12 @@
+// @noImplicitExtensionName: true
+
+// @Filename: fs/my_file.ts
+export default 0
+
+// @Filename: 1.ts
+// Should both be okay.
+// This check is not applied to non-relative import.
+// e.g. Node's ESModule support https://nodejs.org/api/esm.html#esm_package_exports
+import num from 'fs/my_file'
+import num2 from 'fs/my_file.js'
+num + num2
diff --git a/tests/cases/compiler/noImplicitExtensionNameWithEmitExtension.ts b/tests/cases/compiler/noImplicitExtensionNameWithEmitExtension.ts
new file mode 100644
index 0000000000000..c62101c909cc5
--- /dev/null
+++ b/tests/cases/compiler/noImplicitExtensionNameWithEmitExtension.ts
@@ -0,0 +1,11 @@
+// @noImplicitExtensionName: true
+// @emitExtension: .mjs
+
+// @Filename: 0.ts
+export default 0
+
+// @Filename: 1.ts
+import num from './0.mjs'
+// should be error
+import num2 from './0'
+num + num2