[fix] add svelte field when packaging (#2431)

Several heuristics in Kit/vite-plugin-svelte to tell Vite to mark Svelte packages rely on the "svelte" property. Vite/Rollup/Webpack plugin can all deal with it.

Fixes #1959

Co-authored-by: Bjorn Lu <[email protected]>

Author: dummdidumm

.changeset/friendly-chefs-draw.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Add "svelte" field to package.json when running package command

packages/kit/src/packaging/index.js

@@ -39,6 +39,7 @@ export async function make_package(config, cwd = process.cwd()) {
 
 	/** @type {Record<string, string>} */
 	const clashes = {};
+	let contains_svelte_files = false;
 
 	for (const file of files) {
 		const ext = path.extname(file);
@@ -66,6 +67,7 @@ export async function make_package(config, cwd = process.cwd()) {
 			out_contents = config.preprocess
 				? (await preprocess(source, config.preprocess, { filename })).code
 				: source;
+			contains_svelte_files = true;
 		} else if (ext === '.ts' && file.endsWith('.d.ts')) {
 			// TypeScript's declaration emit won't copy over the d.ts files, so we do it here
 			out_file = file;
@@ -104,6 +106,34 @@ export async function make_package(config, cwd = process.cwd()) {
 	}
 
 	pkg.exports = { ...generated, ...pkg.exports };
+
+	if (!pkg.svelte && contains_svelte_files) {
+		// Several heuristics in Kit/vite-plugin-svelte to tell Vite to mark Svelte packages
+		// rely on the "svelte" property. Vite/Rollup/Webpack plugin can all deal with it.
+		// See https://github.com/sveltejs/kit/issues/1959 for more info and related threads.
+		if (pkg.exports['.']) {
+			const svelte_export =
+				typeof pkg.exports['.'] === 'string'
+					? pkg.exports['.']
+					: pkg.exports['.'].import || pkg.exports['.'].default;
+			if (svelte_export) {
+				pkg.svelte = svelte_export;
+			} else {
+				console.warn(
+					'Cannot generate a "svelte" entry point because ' +
+						'the "." entry in "exports" is not a string. ' +
+						'If you set it by hand, please also set one of the options as a "svelte" entry point'
+				);
+			}
+		} else {
+			console.warn(
+				'Cannot generate a "svelte" entry point because ' +
+					'the "." entry in "exports" is missing. ' +
+					'Please specify one or set a "svelte" entry point yourself'
+			);
+		}
+	}
+
 	write(path.join(cwd, config.kit.package.dir, 'package.json'), JSON.stringify(pkg, null, '  '));
 
 	const whitelist = fs.readdirSync(cwd).filter((file) => {

packages/kit/src/packaging/test/fixtures/emitTypes-false/expected/package.json

@@ -8,5 +8,6 @@
 		"./Test.svelte": "./Test.svelte",
 		"./Test2.svelte": "./Test2.svelte",
 		".": "./index.js"
-	}
+	},
+	"svelte": "./index.js"
 }

packages/kit/src/packaging/test/fixtures/exports-merge/expected/package.json

@@ -16,5 +16,6 @@
 		"./Test": "./Test.svelte",
 		"./package.json": "./package.json"
 	},
-	"type": "module"
+	"type": "module",
+	"svelte": "./index.js"
 }

packages/kit/src/packaging/test/fixtures/exports-replace/expected/package.json

@@ -8,5 +8,6 @@
 			"import": "./index.js"
 		},
 		"./package.json": "./package.json"
-	}
+	},
+	"svelte": "./Test.svelte"
 }

packages/kit/src/packaging/test/fixtures/exports-replace/package.json

@@ -7,5 +7,6 @@
 		".": {
 			"import": "./index.js"
 		}
-	}
+	},
+	"svelte": "./Test.svelte"
 }

packages/kit/src/packaging/test/fixtures/files-exclude/expected/package.json

@@ -8,5 +8,6 @@
 		"./internal": "./internal/index.js",
 		"./Test.svelte": "./Test.svelte",
 		".": "./index.js"
-	}
+	},
+	"svelte": "./index.js"
 }

packages/kit/src/packaging/test/fixtures/javascript/expected/package.json

@@ -11,5 +11,6 @@
 		"./Test2.svelte": "./Test2.svelte",
 		"./utils": "./utils.js",
 		".": "./index.js"
-	}
+	},
+	"svelte": "./index.js"
 }

packages/kit/src/packaging/test/fixtures/typescript/expected/package.json

@@ -9,5 +9,6 @@
 		"./Test2.svelte": "./Test2.svelte",
 		"./utils": "./utils.js",
 		".": "./index.js"
-	}
+	},
+	"svelte": "./index.js"
 }