Class emit: cached repeat prototype repetitions in a variable

If two or more class members would assign to the class `.prototype`, that'll be stored in a temporary variable named like `proto`.

Author: Josh Goldberg

src/compiler/core.ts

@@ -1240,6 +1240,25 @@ namespace ts {
         return array[array.length - 1];
     }
 
+    /**
+     * Returns whether an array has exactly one member that matches a filter.
+     */
+    export function containsExactlyOne<T>(array: ReadonlyArray<T>, filter: (item: T) => boolean) {
+        let found = false;
+
+        for (const item of array) {
+            if (filter(item)) {
+                if (found) {
+                    return false;
+                }
+
+                found = true;
+            }
+        }
+
+        return found;
+    }
+
     /**
      * Returns the only element of an array if it contains only one element, `undefined` otherwise.
      */

src/compiler/transformers/es2015.ts

@@ -1543,21 +1543,28 @@ namespace ts {
          * @param node The ClassExpression or ClassDeclaration node.
          */
         function addClassMembers(statements: Statement[], node: ClassExpression | ClassDeclaration): void {
+            if (!node.members.length) {
+                return;
+            }
+
+            const prototypeStoragePlacement = statements.length;
+            let prototypeStorageName: Identifier | PropertyAccessExpression;
+
             for (const member of node.members) {
                 switch (member.kind) {
                     case SyntaxKind.SemicolonClassElement:
                         statements.push(transformSemicolonClassElementToStatement(<SemicolonClassElement>member));
                         break;
 
                     case SyntaxKind.MethodDeclaration:
-                        statements.push(transformClassMethodDeclarationToStatement(getClassMemberPrefix(node, member), <MethodDeclaration>member, node));
+                        statements.push(transformClassMethodDeclarationToStatement(getClassMemberPrefix(node, member, getPrototypeStorageName), <MethodDeclaration>member, node));
                         break;
 
                     case SyntaxKind.GetAccessor:
                     case SyntaxKind.SetAccessor:
                         const accessors = getAllAccessorDeclarations(node.members, <AccessorDeclaration>member);
                         if (member === accessors.firstAccessor) {
-                            statements.push(transformAccessorsToStatement(getClassMemberPrefix(node, member), accessors, node));
+                            statements.push(transformAccessorsToStatement(getClassMemberPrefix(node, member, getPrototypeStorageName), accessors, node));
                         }
 
                         break;
@@ -1571,6 +1578,51 @@ namespace ts {
                         break;
                 }
             }
+
+            /**
+             * Lazily creates the way class members will access the class prototype. 
+             */
+            function getPrototypeStorageName() {
+                if (prototypeStorageName) {
+                    return prototypeStorageName;
+                }
+
+                const originalPrototypeAccess = createPropertyAccess(getInternalName(node), "prototype");
+    
+                // If the class has exactly one non-static member, it'll access prototype members on itself:
+                //   ClassName.prototype.member = ...
+                if (containsExactlyOne(node.members, classMemberAssignsToPrototype)) {
+                    prototypeStorageName = originalPrototypeAccess;
+                    return prototypeStorageName;
+                }
+
+                // Since it has multiple non-static members, it'll store that prototype as a variable that can be minified:
+                //  var proto = ClassName.prototype;
+                //  proto.memberOne = ...
+                //  proto.memberTwo = ...
+                prototypeStorageName = createUniqueName("proto");
+
+                statements.splice(
+                    prototypeStoragePlacement,
+                    0,
+                    createVariableStatement(
+                        /*modifiers*/ undefined,
+                        createVariableDeclarationList([
+                            createVariableDeclaration(
+                                prototypeStorageName,
+                                /*type*/ undefined,
+                                originalPrototypeAccess,
+                            )
+                        ])
+                    )
+                );
+
+                return prototypeStorageName;
+            }
+        }
+
+        function classMemberAssignsToPrototype(node: ClassElement) {
+            return !hasModifier(node, ModifierFlags.Static) && !isConstructorDeclaration(node);
         }
 
         /**
@@ -4299,10 +4351,10 @@ namespace ts {
             return node;
         }
 
-        function getClassMemberPrefix(node: ClassExpression | ClassDeclaration, member: ClassElement) {
-            return hasModifier(member, ModifierFlags.Static)
+        function getClassMemberPrefix(node: ClassExpression | ClassDeclaration, member: ClassElement, getPrototypeStorageName: () => LeftHandSideExpression) {
+            return member && hasModifier(member, ModifierFlags.Static)
                 ? getInternalName(node)
-                : createPropertyAccess(getInternalName(node), "prototype");
+                : getPrototypeStorageName();
         }
 
         function hasSynthesizedDefaultSuperCall(constructor: ConstructorDeclaration | undefined, hasExtendsClause: boolean) {

tests/baselines/reference/ES5For-ofTypeCheck10.js

@@ -19,13 +19,14 @@ for (var v of new StringIterator) { }
 var StringIterator = /** @class */ (function () {
     function StringIterator() {
     }
-    StringIterator.prototype.next = function () {
+    var proto_1 = StringIterator.prototype;
+    proto_1.next = function () {
         return {
             done: true,
             value: ""
         };
     };
-    StringIterator.prototype[Symbol.iterator] = function () {
+    proto_1[Symbol.iterator] = function () {
         return this;
     };
     return StringIterator;

tests/baselines/reference/abstractProperty.js

@@ -48,12 +48,13 @@ var C = /** @class */ (function (_super) {
         _this.ro = "readonly please";
         return _this;
     }
-    Object.defineProperty(C.prototype, "prop", {
+    var proto_1 = C.prototype;
+    Object.defineProperty(proto_1, "prop", {
         get: function () { return "foo"; },
         set: function (v) { },
         enumerable: true,
         configurable: true
     });
-    C.prototype.m = function () { };
+    proto_1.m = function () { };
     return C;
 }(B));

tests/baselines/reference/abstractPropertyNegative.js

@@ -121,13 +121,14 @@ var WrongTypeAccessorImpl2 = /** @class */ (function (_super) {
 var AbstractAccessorMismatch = /** @class */ (function () {
     function AbstractAccessorMismatch() {
     }
-    Object.defineProperty(AbstractAccessorMismatch.prototype, "p1", {
+    var proto_1 = AbstractAccessorMismatch.prototype;
+    Object.defineProperty(proto_1, "p1", {
         set: function (val) { },
         enumerable: true,
         configurable: true
     });
     ;
-    Object.defineProperty(AbstractAccessorMismatch.prototype, "p2", {
+    Object.defineProperty(proto_1, "p2", {
         get: function () { return "should work"; },
         enumerable: true,
         configurable: true

tests/baselines/reference/accessibilityModifiers.js

@@ -127,13 +127,14 @@ var D = /** @class */ (function () {
 var E = /** @class */ (function () {
     function E() {
     }
-    E.prototype.method = function () { };
-    Object.defineProperty(E.prototype, "getter", {
+    var proto_1 = E.prototype;
+    proto_1.method = function () { };
+    Object.defineProperty(proto_1, "getter", {
         get: function () { return 0; },
         enumerable: true,
         configurable: true
     });
-    Object.defineProperty(E.prototype, "setter", {
+    Object.defineProperty(proto_1, "setter", {
         set: function (a) { },
         enumerable: true,
         configurable: true

tests/baselines/reference/accessorWithLineTerminator.js

@@ -11,7 +11,8 @@ class C {
 var C = /** @class */ (function () {
     function C() {
     }
-    Object.defineProperty(C.prototype, "x", {
+    var proto_1 = C.prototype;
+    Object.defineProperty(proto_1, "x", {
         get: function () { return 1; },
         set: function (v) { },
         enumerable: true,

tests/baselines/reference/accessorWithMismatchedAccessibilityModifiers.js

@@ -35,7 +35,8 @@ class F {
 var C = /** @class */ (function () {
     function C() {
     }
-    Object.defineProperty(C.prototype, "x", {
+    var proto_1 = C.prototype;
+    Object.defineProperty(proto_1, "x", {
         get: function () {
             return 1;
         },
@@ -49,7 +50,8 @@ var C = /** @class */ (function () {
 var D = /** @class */ (function () {
     function D() {
     }
-    Object.defineProperty(D.prototype, "x", {
+    var proto_2 = D.prototype;
+    Object.defineProperty(proto_2, "x", {
         get: function () {
             return 1;
         },
@@ -63,7 +65,8 @@ var D = /** @class */ (function () {
 var E = /** @class */ (function () {
     function E() {
     }
-    Object.defineProperty(E.prototype, "x", {
+    var proto_3 = E.prototype;
+    Object.defineProperty(proto_3, "x", {
         get: function () {
             return 1;
         },

tests/baselines/reference/accessorsAreNotContextuallyTyped.js

@@ -18,7 +18,8 @@ var r = c.x(''); // string
 var C = /** @class */ (function () {
     function C() {
     }
-    Object.defineProperty(C.prototype, "x", {
+    var proto_1 = C.prototype;
+    Object.defineProperty(proto_1, "x", {
         get: function () {
             return function (x) { return ""; };
         },

tests/baselines/reference/accessors_spec_section-4.5_error-cases.js

@@ -17,25 +17,26 @@ class LanguageSpec_section_4_5_error_cases {
 var LanguageSpec_section_4_5_error_cases = /** @class */ (function () {
     function LanguageSpec_section_4_5_error_cases() {
     }
-    Object.defineProperty(LanguageSpec_section_4_5_error_cases.prototype, "AnnotatedSetter_SetterFirst", {
+    var proto_1 = LanguageSpec_section_4_5_error_cases.prototype;
+    Object.defineProperty(proto_1, "AnnotatedSetter_SetterFirst", {
         get: function () { return ""; },
         set: function (a) { },
         enumerable: true,
         configurable: true
     });
-    Object.defineProperty(LanguageSpec_section_4_5_error_cases.prototype, "AnnotatedSetter_SetterLast", {
+    Object.defineProperty(proto_1, "AnnotatedSetter_SetterLast", {
         get: function () { return ""; },
         set: function (a) { },
         enumerable: true,
         configurable: true
     });
-    Object.defineProperty(LanguageSpec_section_4_5_error_cases.prototype, "AnnotatedGetter_GetterFirst", {
+    Object.defineProperty(proto_1, "AnnotatedGetter_GetterFirst", {
         get: function () { return ""; },
         set: function (aStr) { aStr = 0; },
         enumerable: true,
         configurable: true
     });
-    Object.defineProperty(LanguageSpec_section_4_5_error_cases.prototype, "AnnotatedGetter_GetterLast", {
+    Object.defineProperty(proto_1, "AnnotatedGetter_GetterLast", {
         get: function () { return ""; },
         set: function (aStr) { aStr = 0; },
         enumerable: true,

tests/baselines/reference/accessors_spec_section-4.5_inference.js

@@ -53,37 +53,38 @@ var B = /** @class */ (function (_super) {
 var LanguageSpec_section_4_5_inference = /** @class */ (function () {
     function LanguageSpec_section_4_5_inference() {
     }
-    Object.defineProperty(LanguageSpec_section_4_5_inference.prototype, "InferredGetterFromSetterAnnotation", {
+    var proto_1 = LanguageSpec_section_4_5_inference.prototype;
+    Object.defineProperty(proto_1, "InferredGetterFromSetterAnnotation", {
         get: function () { return new B(); },
         set: function (a) { },
         enumerable: true,
         configurable: true
     });
-    Object.defineProperty(LanguageSpec_section_4_5_inference.prototype, "InferredGetterFromSetterAnnotation_GetterFirst", {
+    Object.defineProperty(proto_1, "InferredGetterFromSetterAnnotation_GetterFirst", {
         get: function () { return new B(); },
         set: function (a) { },
         enumerable: true,
         configurable: true
     });
-    Object.defineProperty(LanguageSpec_section_4_5_inference.prototype, "InferredFromGetter", {
+    Object.defineProperty(proto_1, "InferredFromGetter", {
         get: function () { return new B(); },
         set: function (a) { },
         enumerable: true,
         configurable: true
     });
-    Object.defineProperty(LanguageSpec_section_4_5_inference.prototype, "InferredFromGetter_SetterFirst", {
+    Object.defineProperty(proto_1, "InferredFromGetter_SetterFirst", {
         get: function () { return new B(); },
         set: function (a) { },
         enumerable: true,
         configurable: true
     });
-    Object.defineProperty(LanguageSpec_section_4_5_inference.prototype, "InferredSetterFromGetterAnnotation", {
+    Object.defineProperty(proto_1, "InferredSetterFromGetterAnnotation", {
         get: function () { return new B(); },
         set: function (a) { },
         enumerable: true,
         configurable: true
     });
-    Object.defineProperty(LanguageSpec_section_4_5_inference.prototype, "InferredSetterFromGetterAnnotation_GetterFirst", {
+    Object.defineProperty(proto_1, "InferredSetterFromGetterAnnotation_GetterFirst", {
         get: function () { return new B(); },
         set: function (a) { },
         enumerable: true,

tests/baselines/reference/aliasUsageInAccessorsOfClass.js

@@ -68,7 +68,8 @@ var moduleA = require("./aliasUsage1_moduleA");
 var C2 = /** @class */ (function () {
     function C2() {
     }
-    Object.defineProperty(C2.prototype, "A", {
+    var proto_1 = C2.prototype;
+    Object.defineProperty(proto_1, "A", {
         get: function () {
             return this.x;
         },

tests/baselines/reference/ambiguousCallsWhereReturnTypesAgree.js

@@ -32,20 +32,22 @@ class TestClass2 {
 var TestClass = /** @class */ (function () {
     function TestClass() {
     }
-    TestClass.prototype.bar = function (x) {
+    var proto_1 = TestClass.prototype;
+    proto_1.bar = function (x) {
     };
-    TestClass.prototype.foo = function (x) {
+    proto_1.foo = function (x) {
         this.bar(x); // should not error
     };
     return TestClass;
 }());
 var TestClass2 = /** @class */ (function () {
     function TestClass2() {
     }
-    TestClass2.prototype.bar = function (x) {
+    var proto_2 = TestClass2.prototype;
+    proto_2.bar = function (x) {
         return 0;
     };
-    TestClass2.prototype.foo = function (x) {
+    proto_2.foo = function (x) {
         return this.bar(x); // should not error
     };
     return TestClass2;

tests/baselines/reference/anyIdenticalToItself.js

@@ -17,7 +17,8 @@ function foo(x, y) { }
 var C = /** @class */ (function () {
     function C() {
     }
-    Object.defineProperty(C.prototype, "X", {
+    var proto_1 = C.prototype;
+    Object.defineProperty(proto_1, "X", {
         get: function () {
             var y;
             return y;

tests/baselines/reference/arrayAssignmentTest1.js

@@ -102,8 +102,9 @@ var __extends = (this && this.__extends) || (function () {
 var C1 = /** @class */ (function () {
     function C1() {
     }
-    C1.prototype.IM1 = function () { return null; };
-    C1.prototype.C1M1 = function () { return null; };
+    var proto_1 = C1.prototype;
+    proto_1.IM1 = function () { return null; };
+    proto_1.C1M1 = function () { return null; };
     return C1;
 }());
 var C2 = /** @class */ (function (_super) {

tests/baselines/reference/arrayAssignmentTest2.js

@@ -76,8 +76,9 @@ var __extends = (this && this.__extends) || (function () {
 var C1 = /** @class */ (function () {
     function C1() {
     }
-    C1.prototype.IM1 = function () { return null; };
-    C1.prototype.C1M1 = function () { return null; };
+    var proto_1 = C1.prototype;
+    proto_1.IM1 = function () { return null; };
+    proto_1.C1M1 = function () { return null; };
     return C1;
 }());
 var C2 = /** @class */ (function (_super) {