Private class methods stage 3 (#8654)

* Add private method syntax support

* Add private method spec support

* Add private method loose support

* Throw error if static private method is used

* Add more isStatic & isMethod checks

* Remove `writable:false` from private method inits

`writable` is false by default.

* Add private method func obj equality check

* Throw if private accessor is used

* Add check for fields === private method loose mode

* Throw buildCodeFrameErrors instead of Errors

* Move obj destructuring inside for loop

* Remove "computed" from ClassPrivateMethod type def

packages/babel-generator/src/generators/classes.js

@@ -140,6 +140,12 @@ export function ClassMethod(node: Object) {
   this.print(node.body, node);
 }
 
+export function ClassPrivateMethod(node: Object) {
+  this._classMethodHead(node);
+  this.space();
+  this.print(node.body, node);
+}
+
 export function _classMethodHead(node) {
   this.printJoin(node.decorators, node);
 

packages/babel-generator/test/fixtures/types/ClassBody-MethodDefinition/input.js

@@ -5,6 +5,15 @@ class Foo {
   get foo() {}
   set foo(bar) {}
 
+  async #foo() {}
+  #foo() {}
+  get #foo() {}
+  set #foo(bar) {}
+  * #foo() {}
+  async * #foo() {}
+  get #bar() {}
+  set #baz(taz) {}
+
   static async foo() {}
   static foo() {}
   static ["foo"]() {}

packages/babel-generator/test/fixtures/types/ClassBody-MethodDefinition/options.json

@@ -0,0 +1 @@
+{ "plugins": ["classPrivateMethods", "asyncGenerators"] }

packages/babel-generator/test/fixtures/types/ClassBody-MethodDefinition/output.js

@@ -9,6 +9,22 @@ class Foo {
 
   set foo(bar) {}
 
+  async #foo() {}
+
+  #foo() {}
+
+  get #foo() {}
+
+  set #foo(bar) {}
+
+  *#foo() {}
+
+  async *#foo() {}
+
+  get #bar() {}
+
+  set #baz(taz) {}
+
   static async foo() {}
 
   static foo() {}

packages/babel-helpers/src/helpers.js

@@ -1739,3 +1739,18 @@ helpers.decorate = helper("7.1.5")`
     return constructor;
   }
 `;
+
+helpers.classPrivateMethodGet = helper("7.1.6")`
+  export default function _classPrivateMethodGet(receiver, privateSet, fn) {
+    if (!privateSet.has(receiver)) {
+      throw new TypeError("attempted to get private field on non-instance");
+    }
+    return fn;
+  }
+`;
+
+helpers.classPrivateMethodSet = helper("7.1.6")`
+  export default function _classPrivateMethodSet() {
+    throw new TypeError("attempted to reassign private method");
+  }
+`;

packages/babel-plugin-class-features/src/features.js

@@ -39,19 +39,43 @@ export function isLoose(file, feature) {
 }
 
 export function verifyUsedFeatures(path, file) {
+  if (hasDecorators(path) && !hasFeature(file, FEATURES.decorators)) {
+    throw path.buildCodeFrameError("Decorators are not enabled.");
+  }
+
   if (hasFeature(file, FEATURES.decorators)) {
     throw new Error(
       "@babel/plugin-class-features doesn't support decorators yet.",
     );
   }
-  if (hasFeature(file, FEATURES.privateMethods)) {
-    throw new Error(
-      "@babel/plugin-class-features doesn't support private methods yet.",
-    );
+
+  if (path.isClassPrivateMethod()) {
+    if (!hasFeature(file, FEATURES.privateMethods)) {
+      throw path.buildCodeFrameError("Class private methods are not enabled.");
+    }
+
+    if (path.node.static) {
+      throw path.buildCodeFrameError(
+        "@babel/plugin-class-features doesn't support class static private methods yet.",
+      );
+    }
+
+    if (path.node.kind !== "method") {
+      throw path.buildCodeFrameError(
+        "@babel/plugin-class-features doesn't support class private accessors yet.",
+      );
+    }
   }
 
-  if (hasDecorators(path) && !hasFeature(file, FEATURES.decorators)) {
-    throw path.buildCodeFrameError("Decorators are not enabled.");
+  if (
+    hasFeature(file, FEATURES.privateMethods) &&
+    hasFeature(file, FEATURES.fields) &&
+    isLoose(file, FEATURES.privateMethods) !== isLoose(file, FEATURES.fields)
+  ) {
+    throw path.buildCodeFrameError(
+      "'loose' mode configuration must be the same for both @babel/plugin-proposal-class-properties " +
+        "and @babel/plugin-proposal-private-methods",
+    );
   }
 
   if (path.isProperty()) {

packages/babel-plugin-class-features/src/fields.js

@@ -11,6 +11,10 @@ export function buildPrivateNamesMap(props) {
       privateNamesMap.set(name, {
         id: prop.scope.generateUidIdentifier(name),
         static: !!prop.node.static,
+        method: prop.isClassPrivateMethod(),
+        methodId: prop.isClassPrivateMethod()
+          ? prop.scope.generateUidIdentifier(name)
+          : undefined,
       });
     }
   }
@@ -20,20 +24,22 @@ export function buildPrivateNamesMap(props) {
 export function buildPrivateNamesNodes(privateNamesMap, loose, state) {
   const initNodes = [];
 
-  for (const [name, { id, static: isStatic }] of privateNamesMap) {
-    // In loose mode, both static and instance fields hare transpiled using a
+  for (const [name, value] of privateNamesMap) {
+    // In loose mode, both static and instance fields are transpiled using a
     // secret non-enumerable property. Hence, we also need to generate that
-    // key (using the classPrivateFieldLooseKey helper) in loose mode.
+    // key (using the classPrivateFieldLooseKey helper).
     // In spec mode, only instance fields need a "private name" initializer
-    // (the WeakMap), becase static fields are directly assigned to a variable
-    // in the buildPrivateStaticFieldInitSpec function.
-
+    // because static fields are directly assigned to a variable in the
+    // buildPrivateStaticFieldInitSpec function.
+    const { id, static: isStatic, method: isMethod } = value;
     if (loose) {
       initNodes.push(
         template.statement.ast`
           var ${id} = ${state.addHelper("classPrivateFieldLooseKey")}("${name}")
         `,
       );
+    } else if (isMethod && !isStatic) {
+      initNodes.push(template.statement.ast`var ${id} = new WeakSet();`);
     } else if (!isStatic) {
       initNodes.push(template.statement.ast`var ${id} = new WeakMap();`);
     }
@@ -42,7 +48,7 @@ export function buildPrivateNamesNodes(privateNamesMap, loose, state) {
   return initNodes;
 }
 
-// Traverses the class scope, handling private name references.  If an inner
+// Traverses the class scope, handling private name references. If an inner
 // class redeclares the same private name, it will hand off traversal to the
 // restricted visitor (which doesn't traverse the inner class's inner scope).
 const privateNameVisitor = {
@@ -61,7 +67,9 @@ const privateNameVisitor = {
     const body = path.get("body.body");
 
     for (const prop of body) {
-      if (!prop.isClassPrivateProperty()) continue;
+      if (!prop.isPrivate()) {
+        continue;
+      }
       if (!privateNamesMap.has(prop.node.key.id.name)) continue;
 
       // This class redeclares the private name.
@@ -108,13 +116,24 @@ const privateNameHandlerSpec = {
   get(member) {
     const { classRef, privateNamesMap, file } = this;
     const { name } = member.node.property.id;
-    const { id, static: isStatic } = privateNamesMap.get(name);
+    const {
+      id,
+      static: isStatic,
+      method: isMethod,
+      methodId,
+    } = privateNamesMap.get(name);
 
-    if (isStatic) {
+    if (isStatic && !isMethod) {
       return t.callExpression(
         file.addHelper("classStaticPrivateFieldSpecGet"),
         [this.receiver(member), t.cloneNode(classRef), t.cloneNode(id)],
       );
+    } else if (isMethod) {
+      return t.callExpression(file.addHelper("classPrivateMethodGet"), [
+        this.receiver(member),
+        t.cloneNode(id),
+        t.cloneNode(methodId),
+      ]);
     } else {
       return t.callExpression(file.addHelper("classPrivateFieldGet"), [
         this.receiver(member),
@@ -126,13 +145,17 @@ const privateNameHandlerSpec = {
   set(member, value) {
     const { classRef, privateNamesMap, file } = this;
     const { name } = member.node.property.id;
-    const { id, static: isStatic } = privateNamesMap.get(name);
+    const { id, static: isStatic, method: isMethod } = privateNamesMap.get(
+      name,
+    );
 
-    if (isStatic) {
+    if (isStatic && !isMethod) {
       return t.callExpression(
         file.addHelper("classStaticPrivateFieldSpecSet"),
         [this.receiver(member), t.cloneNode(classRef), t.cloneNode(id), value],
       );
+    } else if (isMethod) {
+      return t.callExpression(file.addHelper("classPrivateMethodSet"), []);
     } else {
       return t.callExpression(file.addHelper("classPrivateFieldSet"), [
         this.receiver(member),
@@ -231,6 +254,25 @@ function buildPrivateStaticFieldInitSpec(prop, privateNamesMap) {
   `;
 }
 
+function buildPrivateMethodInitLoose(ref, prop, privateNamesMap) {
+  const { methodId, id } = privateNamesMap.get(prop.node.key.id.name);
+
+  return template.statement.ast`
+    Object.defineProperty(${ref}, ${id}, {
+      // configurable is false by default
+      // enumerable is false by default
+      // writable is false by default
+      value: ${methodId.name}
+    });
+  `;
+}
+
+function buildPrivateInstanceMethodInitSpec(ref, prop, privateNamesMap) {
+  const { id } = privateNamesMap.get(prop.node.key.id.name);
+
+  return template.statement.ast`${id}.add(${ref})`;
+}
+
 function buildPublicFieldInitLoose(ref, prop) {
   const { key, computed } = prop.node;
   const value = prop.node.value || prop.scope.buildUndefinedNode();
@@ -257,6 +299,16 @@ function buildPublicFieldInitSpec(ref, prop, state) {
   );
 }
 
+function buildPrivateInstanceMethodDeclaration(prop, privateNamesMap) {
+  const { methodId } = privateNamesMap.get(prop.node.key.id.name);
+  const { params, body } = prop.node;
+  const methodValue = t.functionExpression(methodId, params, body);
+
+  return t.variableDeclaration("var", [
+    t.variableDeclarator(methodId, methodValue),
+  ]);
+}
+
 export function buildFieldsInitNodes(
   ref,
   props,
@@ -269,34 +321,34 @@ export function buildFieldsInitNodes(
 
   for (const prop of props) {
     const isStatic = prop.node.static;
-    const isPrivate = prop.isPrivate();
+    const isPrivateField = prop.isClassPrivateProperty();
+    const isPrivateMethod = prop.isClassPrivateMethod();
 
-    // Pattern matching please
     switch (true) {
-      case isStatic && isPrivate && loose:
+      case isStatic && isPrivateField && loose:
         staticNodes.push(
           buildPrivateFieldInitLoose(t.cloneNode(ref), prop, privateNamesMap),
         );
         break;
-      case isStatic && isPrivate && !loose:
+      case isStatic && isPrivateField && !loose:
         staticNodes.push(
           buildPrivateStaticFieldInitSpec(prop, privateNamesMap),
         );
         break;
-      case isStatic && !isPrivate && loose:
+      case isStatic && !isPrivateField && loose:
         staticNodes.push(buildPublicFieldInitLoose(t.cloneNode(ref), prop));
         break;
-      case isStatic && !isPrivate && !loose:
+      case isStatic && !isPrivateField && !loose:
         staticNodes.push(
           buildPublicFieldInitSpec(t.cloneNode(ref), prop, state),
         );
         break;
-      case !isStatic && isPrivate && loose:
+      case !isStatic && isPrivateField && loose:
         instanceNodes.push(
           buildPrivateFieldInitLoose(t.thisExpression(), prop, privateNamesMap),
         );
         break;
-      case !isStatic && isPrivate && !loose:
+      case !isStatic && isPrivateField && !loose:
         instanceNodes.push(
           buildPrivateInstanceFieldInitSpec(
             t.thisExpression(),
@@ -305,10 +357,34 @@ export function buildFieldsInitNodes(
           ),
         );
         break;
-      case !isStatic && !isPrivate && loose:
+      case !isStatic && isPrivateMethod && loose:
+        instanceNodes.push(
+          buildPrivateMethodInitLoose(
+            t.thisExpression(),
+            prop,
+            privateNamesMap,
+          ),
+        );
+        staticNodes.push(
+          buildPrivateInstanceMethodDeclaration(prop, privateNamesMap),
+        );
+        break;
+      case !isStatic && isPrivateMethod && !loose:
+        instanceNodes.push(
+          buildPrivateInstanceMethodInitSpec(
+            t.thisExpression(),
+            prop,
+            privateNamesMap,
+          ),
+        );
+        staticNodes.push(
+          buildPrivateInstanceMethodDeclaration(prop, privateNamesMap),
+        );
+        break;
+      case !isStatic && !isPrivateField && loose:
         instanceNodes.push(buildPublicFieldInitLoose(t.thisExpression(), prop));
         break;
-      case !isStatic && !isPrivate && !loose:
+      case !isStatic && !isPrivateField && !loose:
         instanceNodes.push(
           buildPublicFieldInitSpec(t.thisExpression(), prop, state),
         );

packages/babel-plugin-class-features/src/index.js

@@ -75,7 +75,6 @@ export default declare((api, options) => {
         enableFeature(this.file, FEATURES.fields, fields.loose);
       }
       if (privateMethods.enabled) {
-        throw new Error("Private methods are not supported yet");
         enableFeature(this.file, FEATURES.privateMethods);
       }
       if (decorators.enabled) {
@@ -107,7 +106,7 @@ export default declare((api, options) => {
             computedPaths.push(path);
           }
 
-          if (path.isClassPrivateProperty()) {
+          if (path.isPrivate()) {
             const { name } = path.node.key.id;
 
             if (privateNames.has(name)) {
@@ -116,7 +115,7 @@ export default declare((api, options) => {
             privateNames.add(name);
           }
 
-          if (path.isProperty()) {
+          if (path.isProperty() || path.isClassPrivateMethod()) {
             props.push(path);
           } else if (path.isClassMethod({ kind: "constructor" })) {
             constructor = path;
@@ -131,7 +130,6 @@ export default declare((api, options) => {
           nameFunction(path);
           ref = path.scope.generateUidIdentifier("class");
         } else {
-          // path.isClassDeclaration() && path.node.id
           ref = path.node.id;
         }
 

packages/babel-plugin-proposal-private-methods/.npmignore

@@ -0,0 +1,3 @@
+src
+test
+*.log

packages/babel-plugin-proposal-private-methods/README.md

@@ -0,0 +1,19 @@
+# @babel/plugin-proposal-private-methods
+
+> This plugin transforms private class methods
+
+See our website [@babel/plugin-proposal-private-methods](https://babeljs.io/docs/en/next/babel-plugin-proposal-private-methods.html) for more information.
+
+## Install
+
+Using npm:
+
+```sh
+npm install --save-dev @babel/plugin-proposal-private-methods
+```
+
+or using yarn:
+
+```sh
+yarn add @babel/plugin-proposal-private-methods --dev
+```

packages/babel-plugin-proposal-private-methods/package.json

@@ -0,0 +1,25 @@
+{
+  "name": "@babel/plugin-proposal-private-methods",
+  "version": "7.1.0",
+  "description": "This plugin transforms private class methods",
+  "repository": "https://github.com/babel/babel/tree/master/packages/babel-plugin-proposal-private-methods",
+  "license": "MIT",
+  "publishConfig": {
+    "access": "public"
+  },
+  "main": "lib/index.js",
+  "keywords": [
+    "babel-plugin"
+  ],
+  "dependencies": {
+    "@babel/helper-plugin-utils": "^7.0.0",
+    "@babel/plugin-class-features": "^7.1.4"
+  },
+  "peerDependencies": {
+    "@babel/core": "^7.0.0-0"
+  },
+  "devDependencies": {
+    "@babel/core": "^7.0.0",
+    "@babel/helper-plugin-test-runner": "^7.0.0"
+  }
+}

packages/babel-plugin-proposal-private-methods/src/index.js

@@ -0,0 +1,25 @@
+import { declare } from "@babel/helper-plugin-utils";
+import pluginClassFeatures, {
+  enableFeature,
+  FEATURES,
+} from "@babel/plugin-class-features";
+
+export default declare((api, options) => {
+  api.assertVersion(7);
+
+  const { loose } = options;
+
+  return {
+    name: "proposal-private-methods",
+
+    inherits: pluginClassFeatures,
+
+    manipulateOptions(opts, parserOpts) {
+      parserOpts.plugins.push("classPrivateMethods");
+    },
+
+    pre() {
+      enableFeature(this.file, FEATURES.privateMethods, loose);
+    },
+  };
+});

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/assignment/exec.js

@@ -0,0 +1,11 @@
+class Foo {
+    constructor() {
+      this.publicField = this.#privateMethod();
+    }
+
+    #privateMethod() {
+      return 42;
+    }
+  }
+
+  expect((new Foo).publicField).toEqual(42);

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/assignment/input.js

@@ -0,0 +1,9 @@
+class Foo {
+  constructor() {
+    this.publicField = this.#privateMethod();
+  }
+
+  #privateMethod() {
+    return 42;
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/assignment/output.js

@@ -0,0 +1,15 @@
+var Foo = function Foo() {
+  "use strict";
+
+  babelHelpers.classCallCheck(this, Foo);
+  Object.defineProperty(this, _privateMethod, {
+    value: _privateMethod2
+  });
+  this.publicField = babelHelpers.classPrivateFieldLooseBase(this, _privateMethod)[_privateMethod]();
+};
+
+var _privateMethod = babelHelpers.classPrivateFieldLooseKey("privateMethod");
+
+var _privateMethod2 = function _privateMethod2() {
+  return 42;
+};

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/context/exec.js

@@ -0,0 +1,41 @@
+class Foo {
+    constructor(status) {
+      this.status = status;
+      expect(() => this.#getStatus = null).toThrow(TypeError);
+    }
+
+    #getStatus() {
+      return this.status;
+    }
+
+    getCurrentStatus() {
+      return this.#getStatus();
+    }
+
+    setCurrentStatus(newStatus) {
+      this.status = newStatus;
+    }
+
+    getFakeStatus(fakeStatus) {
+      const getStatus = this.#getStatus;
+      return function () {
+        return getStatus.call({ status: fakeStatus });
+      };
+    }
+
+    getFakeStatusFunc() {
+      return {
+        status: 'fake-status',
+        getFakeStatus: this.#getStatus,
+      };
+    }
+  }
+
+  const f = new Foo('inactive');
+  expect(f.getCurrentStatus()).toBe('inactive');
+
+  f.setCurrentStatus('new-status');
+  expect(f.getCurrentStatus()).toBe('new-status');
+
+  expect(f.getFakeStatus('fake')()).toBe('fake');
+  expect(f.getFakeStatusFunc().getFakeStatus()).toBe('fake-status');

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/context/input.js

@@ -0,0 +1,31 @@
+class Foo {
+  constructor(status) {
+    this.status = status;
+  }
+
+  #getStatus() {
+    return this.status;
+  }
+
+  getCurrentStatus() {
+    return this.#getStatus();
+  }
+
+  setCurrentStatus(newStatus) {
+    this.status = newStatus;
+  }
+
+  getFakeStatus(fakeStatus) {
+    const fakeGetStatus = this.#getStatus;
+    return function() {
+      return fakeGetStatus.call({ status: fakeStatus });
+    };
+  }
+
+  getFakeStatusFunc() {
+    return {
+      status: 'fake-status',
+      getFakeStatus: this.#getStatus,
+    };
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/context/output.js

@@ -0,0 +1,51 @@
+var Foo =
+/*#__PURE__*/
+function () {
+  "use strict";
+
+  function Foo(status) {
+    babelHelpers.classCallCheck(this, Foo);
+    Object.defineProperty(this, _getStatus, {
+      value: _getStatus2
+    });
+    this.status = status;
+  }
+
+  babelHelpers.createClass(Foo, [{
+    key: "getCurrentStatus",
+    value: function getCurrentStatus() {
+      return babelHelpers.classPrivateFieldLooseBase(this, _getStatus)[_getStatus]();
+    }
+  }, {
+    key: "setCurrentStatus",
+    value: function setCurrentStatus(newStatus) {
+      this.status = newStatus;
+    }
+  }, {
+    key: "getFakeStatus",
+    value: function getFakeStatus(fakeStatus) {
+      var fakeGetStatus = babelHelpers.classPrivateFieldLooseBase(this, _getStatus)[_getStatus];
+
+      return function () {
+        return fakeGetStatus.call({
+          status: fakeStatus
+        });
+      };
+    }
+  }, {
+    key: "getFakeStatusFunc",
+    value: function getFakeStatusFunc() {
+      return {
+        status: 'fake-status',
+        getFakeStatus: babelHelpers.classPrivateFieldLooseBase(this, _getStatus)[_getStatus]
+      };
+    }
+  }]);
+  return Foo;
+}();
+
+var _getStatus = babelHelpers.classPrivateFieldLooseKey("getStatus");
+
+var _getStatus2 = function _getStatus2() {
+  return this.status;
+};

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/exfiltrated/exec.js

@@ -0,0 +1,15 @@
+let exfiltrated;
+class Foo {
+    #privateMethod() {}
+
+    constructor() {
+        if (exfiltrated === undefined) {
+            exfiltrated = this.#privateMethod;
+        }
+        expect(exfiltrated).toStrictEqual(this.#privateMethod);
+    }
+}
+
+new Foo();
+// check for private method function object equality
+new Foo();

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/exfiltrated/input.js

@@ -0,0 +1,10 @@
+let exfiltrated;
+class Foo {
+    #privateMethod() {}
+
+    constructor() {
+        if (exfiltrated === undefined) {
+            exfiltrated = this.#privateMethod;
+        }
+    }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/exfiltrated/output.js

@@ -0,0 +1,18 @@
+var exfiltrated;
+
+var Foo = function Foo() {
+  "use strict";
+
+  babelHelpers.classCallCheck(this, Foo);
+  Object.defineProperty(this, _privateMethod, {
+    value: _privateMethod2
+  });
+
+  if (exfiltrated === undefined) {
+    exfiltrated = babelHelpers.classPrivateFieldLooseBase(this, _privateMethod)[_privateMethod];
+  }
+};
+
+var _privateMethod = babelHelpers.classPrivateFieldLooseKey("privateMethod");
+
+var _privateMethod2 = function _privateMethod2() {};

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method-loose/options.json

@@ -0,0 +1,15 @@
+{
+  "plugins": [
+    [
+      "external-helpers",
+      {
+        "helperVersion": "7.1.6"
+      }
+    ],
+    ["proposal-private-methods", { "loose": true }],
+    ["proposal-class-properties", { "loose": true }],
+    "transform-classes",
+    "transform-block-scoping",
+    "syntax-class-properties"
+  ]
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/assignment/exec.js

@@ -0,0 +1,11 @@
+class Foo {
+    constructor() {
+      this.publicField = this.#privateMethod();
+    }
+  
+    #privateMethod() {
+      return 42;
+    }
+  }
+  
+  expect((new Foo).publicField).toEqual(42);

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/assignment/input.js

@@ -0,0 +1,9 @@
+class Foo {
+  constructor() {
+    this.publicField = this.#privateMethod();
+  }
+
+  #privateMethod() {
+    return 42;
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/assignment/output.js

@@ -0,0 +1,15 @@
+var Foo = function Foo() {
+  "use strict";
+
+  babelHelpers.classCallCheck(this, Foo);
+
+  _privateMethod.add(this);
+
+  this.publicField = babelHelpers.classPrivateMethodGet(this, _privateMethod, _privateMethod2).call(this);
+};
+
+var _privateMethod = new WeakSet();
+
+var _privateMethod2 = function _privateMethod2() {
+  return 42;
+};

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/context/exec.js

@@ -0,0 +1,41 @@
+class Foo {
+    constructor(status) {
+      this.status = status;
+      expect(() => this.#getStatus = null).toThrow(TypeError);
+    }
+
+    #getStatus() {
+      return this.status;
+    }
+
+    getCurrentStatus() {
+      return this.#getStatus();
+    }
+
+    setCurrentStatus(newStatus) {
+      this.status = newStatus;
+    }
+
+    getFakeStatus(fakeStatus) {
+      const getStatus = this.#getStatus;
+      return function () {
+        return getStatus.call({ status: fakeStatus });
+      };
+    }
+
+    getFakeStatusFunc() {
+      return {
+        status: 'fake-status',
+        getFakeStatus: this.#getStatus,
+      };
+    }
+  }
+
+  const f = new Foo('inactive');
+  expect(f.getCurrentStatus()).toBe('inactive');
+
+  f.setCurrentStatus('new-status');
+  expect(f.getCurrentStatus()).toBe('new-status');
+
+  expect(f.getFakeStatus('fake')()).toBe('fake');
+  expect(f.getFakeStatusFunc().getFakeStatus()).toBe('fake-status');

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/context/input.js

@@ -0,0 +1,31 @@
+class Foo {
+  constructor(status) {
+    this.status = status;
+  }
+
+  #getStatus() {
+    return this.status;
+  }
+
+  getCurrentStatus() {
+    return this.#getStatus();
+  }
+
+  setCurrentStatus(newStatus) {
+    this.status = newStatus;
+  }
+
+  getFakeStatus(fakeStatus) {
+    const fakeGetStatus = this.#getStatus;
+    return function() {
+      return fakeGetStatus.call({ status: fakeStatus });
+    };
+  }
+
+  getFakeStatusFunc() {
+    return {
+      status: 'fake-status',
+      getFakeStatus: this.#getStatus,
+    };
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/context/output.js

@@ -0,0 +1,50 @@
+var Foo =
+/*#__PURE__*/
+function () {
+  "use strict";
+
+  function Foo(status) {
+    babelHelpers.classCallCheck(this, Foo);
+
+    _getStatus.add(this);
+
+    this.status = status;
+  }
+
+  babelHelpers.createClass(Foo, [{
+    key: "getCurrentStatus",
+    value: function getCurrentStatus() {
+      return babelHelpers.classPrivateMethodGet(this, _getStatus, _getStatus2).call(this);
+    }
+  }, {
+    key: "setCurrentStatus",
+    value: function setCurrentStatus(newStatus) {
+      this.status = newStatus;
+    }
+  }, {
+    key: "getFakeStatus",
+    value: function getFakeStatus(fakeStatus) {
+      var fakeGetStatus = babelHelpers.classPrivateMethodGet(this, _getStatus, _getStatus2);
+      return function () {
+        return fakeGetStatus.call({
+          status: fakeStatus
+        });
+      };
+    }
+  }, {
+    key: "getFakeStatusFunc",
+    value: function getFakeStatusFunc() {
+      return {
+        status: 'fake-status',
+        getFakeStatus: babelHelpers.classPrivateMethodGet(this, _getStatus, _getStatus2)
+      };
+    }
+  }]);
+  return Foo;
+}();
+
+var _getStatus = new WeakSet();
+
+var _getStatus2 = function _getStatus2() {
+  return this.status;
+};

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/exfiltrated/exec.js

@@ -0,0 +1,15 @@
+let exfiltrated;
+class Foo {
+    #privateMethod() {}
+
+    constructor() {
+        if (exfiltrated === undefined) {
+            exfiltrated = this.#privateMethod;
+        }
+        expect(exfiltrated).toStrictEqual(this.#privateMethod);
+    }
+}
+
+new Foo();
+// check for private method function object equality
+new Foo();

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/exfiltrated/input.js

@@ -0,0 +1,10 @@
+let exfiltrated;
+class Foo {
+    #privateMethod() {}
+
+    constructor() {
+        if (exfiltrated === undefined) {
+            exfiltrated = this.#privateMethod;
+        }
+    }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/exfiltrated/output.js

@@ -0,0 +1,17 @@
+var exfiltrated;
+
+var Foo = function Foo() {
+  "use strict";
+
+  babelHelpers.classCallCheck(this, Foo);
+
+  _privateMethod.add(this);
+
+  if (exfiltrated === undefined) {
+    exfiltrated = babelHelpers.classPrivateMethodGet(this, _privateMethod, _privateMethod2);
+  }
+};
+
+var _privateMethod = new WeakSet();
+
+var _privateMethod2 = function _privateMethod2() {};

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/options.json

@@ -0,0 +1,15 @@
+{
+  "plugins": [
+    [
+      "external-helpers",
+      {
+        "helperVersion": "7.1.6"
+      }
+    ],
+    "proposal-private-methods",
+    "proposal-class-properties",
+    "transform-classes",
+    "transform-block-scoping",
+    "syntax-class-properties"
+  ]
+}

packages/babel-plugin-proposal-private-methods/test/index.js

@@ -0,0 +1,3 @@
+import runner from "@babel/helper-plugin-test-runner";
+
+runner(__dirname);

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

@@ -7,7 +7,11 @@ export default declare(api => {
     name: "syntax-class-properties",
 
     manipulateOptions(opts, parserOpts) {
-      parserOpts.plugins.push("classProperties", "classPrivateProperties");
+      parserOpts.plugins.push(
+        "classProperties",
+        "classPrivateProperties",
+        "classPrivateMethods",
+      );
     },
   };
 });

packages/babel-types/src/asserts/generated/index.js

@@ -687,6 +687,12 @@ export function assertClassPrivateProperty(
 ): void {
   assert("ClassPrivateProperty", node, opts);
 }
+export function assertClassPrivateMethod(
+  node: Object,
+  opts?: Object = {},
+): void {
+  assert("ClassPrivateMethod", node, opts);
+}
 export function assertImport(node: Object, opts?: Object = {}): void {
   assert("Import", node, opts);
 }

packages/babel-types/src/builders/generated/index.js

@@ -620,6 +620,10 @@ export function ClassPrivateProperty(...args: Array<any>): Object {
   return builder("ClassPrivateProperty", ...args);
 }
 export { ClassPrivateProperty as classPrivateProperty };
+export function ClassPrivateMethod(...args: Array<any>): Object {
+  return builder("ClassPrivateMethod", ...args);
+}
+export { ClassPrivateMethod as classPrivateMethod };
 export function Import(...args: Array<any>): Object {
   return builder("Import", ...args);
 }

packages/babel-types/src/definitions/es2015.js

@@ -87,6 +87,7 @@ defineType("ClassBody", {
         assertEach(
           assertNodeType(
             "ClassMethod",
+            "ClassPrivateMethod",
             "ClassProperty",
             "ClassPrivateProperty",
             "TSDeclareMethod",

packages/babel-types/src/definitions/experimental.js

@@ -5,7 +5,10 @@ import defineType, {
   assertValueType,
   chain,
 } from "./utils";
-import { classMethodOrPropertyCommon } from "./es2015";
+import {
+  classMethodOrPropertyCommon,
+  classMethodOrDeclareMethodCommon,
+} from "./es2015";
 
 defineType("AwaitExpression", {
   builder: ["argument"],
@@ -131,6 +134,28 @@ defineType("ClassPrivateProperty", {
   },
 });
 
+defineType("ClassPrivateMethod", {
+  builder: ["kind", "key", "params", "body", "static"],
+  visitor: [
+    "key",
+    "params",
+    "body",
+    "decorators",
+    "returnType",
+    "typeParameters",
+  ],
+  aliases: ["Method", "Private", "Function"],
+  fields: {
+    ...classMethodOrDeclareMethodCommon,
+    key: {
+      validate: assertNodeType("PrivateName"),
+    },
+    body: {
+      validate: assertNodeType("BlockStatement"),
+    },
+  },
+});
+
 defineType("Import", {
   aliases: ["Expression"],
 });

packages/babel-types/src/validators/generated/index.js

@@ -2159,6 +2159,20 @@ export function isClassPrivateProperty(node: Object, opts?: Object): boolean {
 
   return false;
 }
+export function isClassPrivateMethod(node: Object, opts?: Object): boolean {
+  if (!node) return false;
+
+  const nodeType = node.type;
+  if (nodeType === "ClassPrivateMethod") {
+    if (typeof opts === "undefined") {
+      return true;
+    } else {
+      return shallowEqual(node, opts);
+    }
+  }
+
+  return false;
+}
 export function isImport(node: Object, opts?: Object): boolean {
   if (!node) return false;
 
@@ -3492,7 +3506,8 @@ export function isFunction(node: Object, opts?: Object): boolean {
     "FunctionExpression" === nodeType ||
     "ObjectMethod" === nodeType ||
     "ArrowFunctionExpression" === nodeType ||
-    "ClassMethod" === nodeType
+    "ClassMethod" === nodeType ||
+    "ClassPrivateMethod" === nodeType
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3737,7 +3752,8 @@ export function isMethod(node: Object, opts?: Object): boolean {
   if (
     nodeType === "Method" ||
     "ObjectMethod" === nodeType ||
-    "ClassMethod" === nodeType
+    "ClassMethod" === nodeType ||
+    "ClassPrivateMethod" === nodeType
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -4119,6 +4135,7 @@ export function isPrivate(node: Object, opts?: Object): boolean {
   if (
     nodeType === "Private" ||
     "ClassPrivateProperty" === nodeType ||
+    "ClassPrivateMethod" === nodeType ||
     "PrivateName" === nodeType
   ) {
     if (typeof opts === "undefined") {

packages/babel-types/src/validators/isReferenced.js

@@ -46,6 +46,7 @@ export default function isReferenced(node: Object, parent: Object): boolean {
     // no: class { NODE() {} }
     // yes: class { [NODE]() {} }
     case "ClassMethod":
+    case "ClassPrivateMethod":
     case "ObjectMethod":
       if (parent.key === node) {
         return !!parent.computed;