[static private] Use explicit descriptors instead of an object (#8620)

This is similar to ec69b4bb1256c061ac76f53dfed09c4283ec6a31, which
was about private instance fields.

Private properties can be non-writable (thanks to decorators), or have
get/set accessors. If we stored this information on the `privateClass`
object, we would need to always use `Object.getOwnPropertyDescriptor`
before reading or writing a property because accessors need to be called
with the correct `this` context (it should be the actual class, not the
object hat stores the private properties). This commit simplifies that
operation a bit by removing the container object.

It also have another advantage, which instance fields already have
thanks to the use of separate weakmaps: unused private static fields
can be tree-shaken away or garbage-collected, while properties of an
object can't. Also, they can be easilier minified.

packages/babel-helpers/src/helpers.js

@@ -1055,24 +1055,26 @@ helpers.classPrivateFieldSet = helper("7.0.0-beta.0")`
 `;
 
 helpers.classStaticPrivateFieldSpecGet = helper("7.0.1")`
-  export default function _classStaticPrivateFieldSpecGet(
+  export default function _classStaticPrivateFieldSpecGet(receiver, classConstructor, descriptor) {
-    receiver, classConstructor, privateClass, privateId
-  ) {
     if (receiver !== classConstructor) {
       throw new TypeError("Private static access of wrong provenance");
     }
-    return privateClass[privateId];
+    return descriptor.value;
   }
 `;
 
 helpers.classStaticPrivateFieldSpecSet = helper("7.0.1")`
-  export default function _classStaticPrivateFieldSpecSet(
+  export default function _classStaticPrivateFieldSpecSet(receiver, classConstructor, descriptor, value) {
-    receiver, classConstructor, privateClass, privateId, value
-  ) {
     if (receiver !== classConstructor) {
       throw new TypeError("Private static access of wrong provenance");
     }
-    privateClass[privateId] = value;
+    if (!descriptor.writable) {
+      // This should only throw in strict mode, but class bodies are
+      // always strict and private fields can only be used inside
+      // class bodies.
+      throw new TypeError("attempted to set read only private field");
+    }
+    descriptor.value = value;
     return value;
   }
 `;

packages/babel-plugin-proposal-class-properties/src/index.js

@@ -164,40 +164,27 @@ export default declare((api, options) => {
     ...privateNameHandlerSpec,
 
     get(member) {
-      const { file, name, privateClassId, classRef } = this;
+      const { file, privateId, classRef } = this;
 
       return t.callExpression(
         file.addHelper("classStaticPrivateFieldSpecGet"),
-        [
+        [this.receiver(member), t.cloneNode(classRef), t.cloneNode(privateId)],
-          this.receiver(member),
-          classRef,
-          privateClassId,
-          t.stringLiteral(name),
-        ],
       );
     },
 
     set(member, value) {
-      const { file, name, privateClassId, classRef } = this;
+      const { file, privateId, classRef } = this;
 
       return t.callExpression(
         file.addHelper("classStaticPrivateFieldSpecSet"),
         [
           this.receiver(member),
-          classRef,
+          t.cloneNode(classRef),
-          privateClassId,
+          t.cloneNode(privateId),
-          t.stringLiteral(name),
           value,
         ],
       );
     },
-
-    call(member, args) {
-      // The first access (the get) should do the memo assignment.
-      this.memoise(member, 1);
-
-      return optimiseCall(this.get(member), this.receiver(member), args);
-    },
   };
 
   function buildClassPropertySpec(ref, path, state) {
@@ -308,50 +295,32 @@ export default declare((api, options) => {
       state,
     );
 
-    const staticNodesToAdd = [keyDecl, buildInit()];
+    return [keyDecl, buildInit()];
-    return [staticNodesToAdd];
   }
 
-  function buildClassStaticPrivatePropertySpec(
+  function buildClassStaticPrivatePropertySpec(ref, path, state) {
-    ref,
+    const { parentPath, scope } = path;
-    path,
+    const { name } = path.node.key.id;
-    state,
-    privateClassId,
-  ) {
-    const { scope, parentPath } = path;
-    const { key, value } = path.node;
-    const { name } = key.id;
-    const staticNodesToAdd = [];
-
-    if (!privateClassId) {
-      // Create a private static "host" object if it does not exist
-      privateClassId = path.scope.generateUidIdentifier(ref.name + "Statics");
-      staticNodesToAdd.push(
-        template.statement`const PRIVATE_CLASS_ID = Object.create(null);`({
-          PRIVATE_CLASS_ID: privateClassId,
-        }),
-      );
-    }
 
+    const privateId = scope.generateUidIdentifier(name);
     memberExpressionToFunctions(parentPath, privateNameVisitor, {
       name,
-      privateClassId,
+      privateId,
       classRef: ref,
       file: state,
       ...staticPrivatePropertyHandlerSpec,
     });
 
-    staticNodesToAdd.push(
+    return [
-      t.expressionStatement(
+      template.statement.ast`
-        t.callExpression(state.addHelper("defineProperty"), [
+        var ${privateId} = {
-          privateClassId,
+          // configurable is always false for private elements
-          t.stringLiteral(name),
+          // enumerable is always false for private elements
-          value || scope.buildUndefinedNode(),
+          writable: true,
-        ]),
+          value: ${path.node.value || scope.buildUndefinedNode()}
-      ),
+        }
-    );
+      `,
-
+    ];
-    return [staticNodesToAdd, privateClassId];
   }
 
   const buildClassProperty = loose
@@ -461,21 +430,16 @@ export default declare((api, options) => {
           }
         }
         let p = 0;
-        let privateClassId;
         for (const prop of props) {
           if (prop.node.static) {
             if (prop.isPrivate()) {
-              let staticNodesToAdd;
+              staticNodes.push(
-              [
+                ...buildClassStaticPrivateProperty(
-                staticNodesToAdd,
+                  t.cloneNode(ref),
-                privateClassId,
+                  prop,
-              ] = buildClassStaticPrivateProperty(
+                  state,
-                t.cloneNode(ref),
+                ),
-                prop,
-                state,
-                privateClassId,
               );
-              staticNodes.push(...staticNodesToAdd);
             } else {
               staticNodes.push(
                 buildClassProperty(t.cloneNode(ref), prop, state),

packages/babel-plugin-proposal-class-properties/test/fixtures/private/native-classes/output.js

@@ -7,7 +7,7 @@ class Foo {
   }
 
   static test() {
-    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _FooStatics, "foo");
+    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _foo);
   }
 
   test() {
@@ -16,8 +16,9 @@ class Foo {
 
 }
 
-var _FooStatics = Object.create(null);
+var _foo = {
-
+  writable: true,
-babelHelpers.defineProperty(_FooStatics, "foo", "foo");
+  value: "foo"
+};
 
 var _bar = new WeakMap();

packages/babel-plugin-proposal-class-properties/test/fixtures/private/non-block-arrow-func/output.mjs

@@ -1,20 +1,16 @@
 export default (param => {
-  var _class, _temp;
+  var _class, _temp, _props;
 
-  return function () {
+  return _temp = _class = class App {
-    _temp = _class = class App {
+    getParam() {
-      getParam() {
+      return param;
-        return param;
+    }
-      }
 
-    };
+  }, _props = {
-
+    writable: true,
-    var _classStatics = Object.create(null);
+    value: {
-
-    babelHelpers.defineProperty(_classStatics, "props", {
       prop1: 'prop1',
       prop2: 'prop2'
-    });
+    }
-    return _temp;
+  }, _temp;
-  }();
 });

packages/babel-plugin-proposal-class-properties/test/fixtures/private/reevaluated/output.js

@@ -1,38 +1,32 @@
 function classFactory() {
-  var _class, _temp;
+  var _class, _temp, _foo, _bar;
 
-  return function () {
+  return _temp = _class = class Foo {
-    _temp = _class = class Foo {
+    constructor() {
-      constructor() {
+      _foo.set(this, {
-        _foo.set(this, {
+        writable: true,
-          writable: true,
+        value: "foo"
-          value: "foo"
+      });
-        });
+    }
-      }
 
-      instance() {
+    instance() {
-        return babelHelpers.classPrivateFieldGet(this, _foo);
+      return babelHelpers.classPrivateFieldGet(this, _foo);
-      }
+    }
 
-      static() {
+    static() {
-        return babelHelpers.classStaticPrivateFieldSpecGet(Foo, _class, _classStatics, "bar");
+      return babelHelpers.classStaticPrivateFieldSpecGet(Foo, _class, _bar);
-      }
+    }
 
-      static instance(inst) {
+    static instance(inst) {
-        return babelHelpers.classPrivateFieldGet(inst, _foo);
+      return babelHelpers.classPrivateFieldGet(inst, _foo);
-      }
+    }
 
-      static static() {
+    static static() {
-        return babelHelpers.classStaticPrivateFieldSpecGet(Foo, _class, _classStatics, "bar");
+      return babelHelpers.classStaticPrivateFieldSpecGet(Foo, _class, _bar);
-      }
+    }
 
-    };
+  }, _foo = new WeakMap(), _bar = {
-
+    writable: true,
-    var _foo = new WeakMap();
+    value: "bar"
-
+  }, _temp;
-    var _classStatics = Object.create(null);
-
-    babelHelpers.defineProperty(_classStatics, "bar", "bar");
-    return _temp;
-  }();
 }

packages/babel-plugin-proposal-class-properties/test/fixtures/private/regression-T2983/output.mjs

@@ -1,16 +1,12 @@
-var _class, _temp;
+var _class, _temp, _test;
 
-call(function () {
+call((_temp = _class = class {}, _test = {
-  _temp = _class = class {};
+  writable: true,
-
+  value: true
-  var _classStatics = Object.create(null);
+}, _temp));
-
-  babelHelpers.defineProperty(_classStatics, "test", true);
-  return _temp;
-}());
 export default class _class2 {}
-
+var _test2 = {
-var _class2Statics = Object.create(null);
+  writable: true,
-
+  value: true
-babelHelpers.defineProperty(_class2Statics, "test", true);
+};
 ;

packages/babel-plugin-proposal-class-properties/test/fixtures/private/regression-T6719/output.js

@@ -1,18 +1,14 @@
 function withContext(ComposedComponent) {
-  var _class, _temp;
+  var _class, _temp, _propTypes;
 
-  return function () {
+  return _temp = _class = class WithContext extends Component {}, _propTypes = {
-    _temp = _class = class WithContext extends Component {};
+    writable: true,
-
+    value: {
-    var _classStatics = Object.create(null);
-
-    babelHelpers.defineProperty(_classStatics, "propTypes", {
       context: PropTypes.shape({
         addCss: PropTypes.func,
         setTitle: PropTypes.func,
         setMeta: PropTypes.func
       })
-    });
+    }
-    return _temp;
+  }, _temp;
-  }();
 }

packages/babel-plugin-proposal-class-properties/test/fixtures/private/static-call/output.js

@@ -10,14 +10,15 @@ function () {
   babelHelpers.createClass(Foo, [{
     key: "test",
     value: function test(x) {
-      return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _FooStatics, "foo").call(Foo, x);
+      return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _foo).call(Foo, x);
     }
   }]);
   return Foo;
 }();
 
-var _FooStatics = Object.create(null);
+var _foo = {
-
+  writable: true,
-babelHelpers.defineProperty(_FooStatics, "foo", function (x) {
+  value: function (x) {
-  return x;
+    return x;
-});
+  }
+};

packages/babel-plugin-proposal-class-properties/test/fixtures/private/static-export/output.mjs

@@ -1,10 +1,10 @@
 export class MyClass {}
-
+var _property = {
-var _MyClassStatics = Object.create(null);
+  writable: true,
-
+  value: value
-babelHelpers.defineProperty(_MyClassStatics, "property", value);
+};
 export default class MyClass2 {}
-
+var _property2 = {
-var _MyClass2Statics = Object.create(null);
+  writable: true,
-
+  value: value
-babelHelpers.defineProperty(_MyClass2Statics, "property", value);
+};

packages/babel-plugin-proposal-class-properties/test/fixtures/private/static-infer-name/output.js

@@ -1,10 +1,6 @@
-var _class, _temp;
+var _class, _temp, _num;
 
-var Foo = function () {
+var Foo = (_temp = _class = class Foo {}, _num = {
-  _temp = _class = class Foo {};
+  writable: true,
-
+  value: 0
-  var _classStatics = Object.create(null);
+}, _temp);
-
-  babelHelpers.defineProperty(_classStatics, "num", 0);
-  return _temp;
-}();

packages/babel-plugin-proposal-class-properties/test/fixtures/private/static-inherited/output.js

@@ -1,35 +1,37 @@
 class Base {
   static getThis() {
-    return babelHelpers.classStaticPrivateFieldSpecGet(this, Base, _BaseStatics, "foo");
+    return babelHelpers.classStaticPrivateFieldSpecGet(this, Base, _foo);
   }
 
   static updateThis(val) {
-    return babelHelpers.classStaticPrivateFieldSpecSet(this, Base, _BaseStatics, "foo", val);
+    return babelHelpers.classStaticPrivateFieldSpecSet(this, Base, _foo, val);
   }
 
   static getClass() {
-    return babelHelpers.classStaticPrivateFieldSpecGet(Base, Base, _BaseStatics, "foo");
+    return babelHelpers.classStaticPrivateFieldSpecGet(Base, Base, _foo);
   }
 
   static updateClass(val) {
-    return babelHelpers.classStaticPrivateFieldSpecSet(Base, Base, _BaseStatics, "foo", val);
+    return babelHelpers.classStaticPrivateFieldSpecSet(Base, Base, _foo, val);
   }
 
 }
 
-var _BaseStatics = Object.create(null);
+var _foo = {
-
+  writable: true,
-babelHelpers.defineProperty(_BaseStatics, "foo", 1);
+  value: 1
+};
 
 class Sub1 extends Base {
   static update(val) {
-    return babelHelpers.classStaticPrivateFieldSpecSet(this, Sub1, _Sub1Statics, "foo", val);
+    return babelHelpers.classStaticPrivateFieldSpecSet(this, Sub1, _foo2, val);
   }
 
 }
 
-var _Sub1Statics = Object.create(null);
+var _foo2 = {
-
+  writable: true,
-babelHelpers.defineProperty(_Sub1Statics, "foo", 2);
+  value: 2
+};
 
 class Sub2 extends Base {}

packages/babel-plugin-proposal-class-properties/test/fixtures/private/static-undefined/output.js

@@ -1,14 +1,15 @@
 class Foo {
   static test() {
-    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _FooStatics, "bar");
+    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _bar);
   }
 
   test() {
-    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _FooStatics, "bar");
+    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _bar);
   }
 
 }
 
-var _FooStatics = Object.create(null);
+var _bar = {
-
+  writable: true,
-babelHelpers.defineProperty(_FooStatics, "bar", void 0);
+  value: void 0
+};

packages/babel-plugin-proposal-class-properties/test/fixtures/private/static/output.js

@@ -1,17 +1,18 @@
 class Foo {
   static test() {
-    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _FooStatics, "bar");
+    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _bar);
   }
 
   test() {
-    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _FooStatics, "bar");
+    return babelHelpers.classStaticPrivateFieldSpecGet(Foo, Foo, _bar);
   }
 
 }
 
-var _FooStatics = Object.create(null);
+var _bar = {
-
+  writable: true,
-babelHelpers.defineProperty(_FooStatics, "bar", "foo");
+  value: "foo"
+};
 expect("bar" in Foo).toBe(false);
 expect(Foo.test()).toBe("foo");
 expect(Foo.test()).toBe("foo");