feat: Implement privateFieldsAsSymbols assumption for classes (#15435)

Co-authored-by: Nicolò Ribaudo <nicolo.ribaudo@gmail.com>

packages/babel-core/src/config/validation/options.ts

@@ -277,6 +277,7 @@ const knownAssumptions = [
   "noIncompleteNsImportDetection",
   "noNewArrows",
   "objectRestNoSymbols",
+  "privateFieldsAsSymbols",
   "privateFieldsAsProperties",
   "pureGetters",
   "setClassMethods",

packages/babel-helper-create-class-features-plugin/src/fields.ts

@@ -58,18 +58,22 @@ export function buildPrivateNamesMap(props: PropPath[]) {
 export function buildPrivateNamesNodes(
   privateNamesMap: PrivateNamesMap,
   privateFieldsAsProperties: boolean,
+  privateFieldsAsSymbols: boolean,
   state: File,
 ) {
   const initNodes: t.Statement[] = [];
 
   for (const [name, value] of privateNamesMap) {
-    // When the privateFieldsAsProperties assumption is enabled,
-    // 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 spec mode, only instance fields need a "private name" initializer
-    // because static fields are directly assigned to a variable in the
-    // buildPrivateStaticFieldInitSpec function.
+    // - When the privateFieldsAsProperties assumption is enabled,
+    //   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).
+    // - When the privateFieldsAsSymbols assumption is enabled,
+    //   both static and instance fields are transpiled using a
+    //   unique Symbol to define a non-enumerable property.
+    // - In spec mode, only instance fields need a "private name" initializer
+    //   because static fields are directly assigned to a variable in the
+    //   buildPrivateStaticFieldInitSpec function.
     const { static: isStatic, method: isMethod, getId, setId } = value;
     const isAccessor = getId || setId;
     const id = t.cloneNode(value.id);
@@ -80,6 +84,8 @@ export function buildPrivateNamesNodes(
       init = t.callExpression(state.addHelper("classPrivateFieldLooseKey"), [
         t.stringLiteral(name),
       ]);
+    } else if (privateFieldsAsSymbols) {
+      init = t.callExpression(t.identifier("Symbol"), [t.stringLiteral(name)]);
     } else if (!isStatic) {
       init = t.newExpression(
         t.identifier(!isMethod || isAccessor ? "WeakMap" : "WeakSet"),

packages/babel-helper-create-class-features-plugin/src/index.ts

@@ -48,19 +48,33 @@ export function createClassFeaturePlugin({
   inherits,
 }: Options): PluginObject {
   const setPublicClassFields = api.assumption("setPublicClassFields");
+  const privateFieldsAsSymbols = api.assumption("privateFieldsAsSymbols");
   const privateFieldsAsProperties = api.assumption("privateFieldsAsProperties");
   const constantSuper = api.assumption("constantSuper");
   const noDocumentAll = api.assumption("noDocumentAll");
 
+  if (privateFieldsAsProperties && privateFieldsAsSymbols) {
+    throw new Error(
+      `Cannot enable both the "privateFieldsAsProperties" and ` +
+        `"privateFieldsAsSymbols" assumptions as the same time.`,
+    );
+  }
+  const privateFieldsAsSymbolsOrProperties =
+    privateFieldsAsProperties || privateFieldsAsSymbols;
+
   if (loose === true) {
-    const explicit = [];
+    type AssumptionName = Parameters<PluginAPI["assumption"]>[0];
+    const explicit: `"${AssumptionName}"`[] = [];
 
     if (setPublicClassFields !== undefined) {
       explicit.push(`"setPublicClassFields"`);
     }
     if (privateFieldsAsProperties !== undefined) {
       explicit.push(`"privateFieldsAsProperties"`);
     }
+    if (privateFieldsAsSymbols !== undefined) {
+      explicit.push(`"privateFieldsAsSymbols"`);
+    }
     if (explicit.length !== 0) {
       console.warn(
         `[${name}]: You are using the "loose: true" option and you are` +
@@ -71,7 +85,7 @@ export function createClassFeaturePlugin({
           ` following top-level option:\n` +
           `\t"assumptions": {\n` +
           `\t\t"setPublicClassFields": true,\n` +
-          `\t\t"privateFieldsAsProperties": true\n` +
+          `\t\t"privateFieldsAsSymbols": true\n` +
           `\t}`,
       );
     }
@@ -191,6 +205,7 @@ export function createClassFeaturePlugin({
         const privateNamesNodes = buildPrivateNamesNodes(
           privateNamesMap,
           privateFieldsAsProperties ?? loose,
+          privateFieldsAsSymbols ?? false,
           file,
         );
 
@@ -199,7 +214,8 @@ export function createClassFeaturePlugin({
           path,
           privateNamesMap,
           {
-            privateFieldsAsProperties: privateFieldsAsProperties ?? loose,
+            privateFieldsAsProperties:
+              privateFieldsAsSymbolsOrProperties ?? loose,
             noDocumentAll,
             innerBinding,
           },
@@ -231,7 +247,7 @@ export function createClassFeaturePlugin({
                 privateNamesMap,
                 file,
                 setPublicClassFields ?? loose,
-                privateFieldsAsProperties ?? loose,
+                privateFieldsAsSymbolsOrProperties ?? loose,
                 constantSuper ?? loose,
                 innerBinding,
               ));
@@ -246,7 +262,7 @@ export function createClassFeaturePlugin({
               privateNamesMap,
               file,
               setPublicClassFields ?? loose,
-              privateFieldsAsProperties ?? loose,
+              privateFieldsAsSymbolsOrProperties ?? loose,
               constantSuper ?? loose,
               innerBinding,
             ));

packages/babel-helper-create-class-features-plugin/test/fixtures/plugin-proposal-class-properties/both-privateFieldsAsProperties-and-privateFieldsAsSymbols/input.js

@@ -0,0 +1 @@
+;

packages/babel-helper-create-class-features-plugin/test/fixtures/plugin-proposal-class-properties/both-privateFieldsAsProperties-and-privateFieldsAsSymbols/options.json

@@ -0,0 +1,8 @@
+{
+  "plugins": ["proposal-class-properties"],
+  "assumptions": {
+    "privateFieldsAsProperties": true,
+    "privateFieldsAsSymbols": true
+  },
+  "throws": "Cannot enable both the \"privateFieldsAsProperties\" and \"privateFieldsAsSymbols\" assumptions as the same time."
+}

packages/babel-helper-create-class-features-plugin/test/fixtures/plugin-proposal-class-properties/warn-loose-and-assumptions/stderr.txt

@@ -1,5 +1,5 @@
 [proposal-class-properties]: You are using the "loose: true" option and you are explicitly setting a value for the "setPublicClassFields" assumption. The "loose" option can cause incompatibilities with the other class features plugins, so it's recommended that you replace it with the following top-level option:
 	"assumptions": {
 		"setPublicClassFields": true,
-		"privateFieldsAsProperties": true
+		"privateFieldsAsSymbols": true
 	}

packages/babel-plugin-proposal-class-properties/test/fixtures/assumption-setPublicClassFields/static-super-loose/stderr.txt

@@ -1,5 +1,5 @@
 [proposal-class-properties]: You are using the "loose: true" option and you are explicitly setting a value for the "setPublicClassFields" assumption. The "loose" option can cause incompatibilities with the other class features plugins, so it's recommended that you replace it with the following top-level option:
 	"assumptions": {
 		"setPublicClassFields": true,
-		"privateFieldsAsProperties": true
+		"privateFieldsAsSymbols": true
 	}

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/basic/exec.js

@@ -0,0 +1,31 @@
+class Cl {
+  #privateField = "top secret string";
+
+  constructor() {
+    this.publicField = "not secret string";
+  }
+
+  get #privateFieldValue() {
+    return this.#privateField;
+  }
+
+  set #privateFieldValue(newValue) {
+    this.#privateField = newValue;
+  }
+
+  publicGetPrivateField() {
+    return this.#privateFieldValue;
+  }
+
+  publicSetPrivateField(newValue) {
+    this.#privateFieldValue = newValue;
+  }
+}
+
+const cl = new Cl();
+
+expect(cl.publicGetPrivateField()).toEqual("top secret string");
+
+cl.publicSetPrivateField("new secret string");
+expect(cl.publicGetPrivateField()).toEqual("new secret string");
+

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/basic/input.js

@@ -0,0 +1,23 @@
+class Cl {
+  #privateField = "top secret string";
+
+  constructor() {
+    this.publicField = "not secret string";
+  }
+
+  get #privateFieldValue() {
+    return this.#privateField;
+  }
+
+  set #privateFieldValue(newValue) {
+    this.#privateField = newValue;
+  }
+
+  publicGetPrivateField() {
+    return this.#privateFieldValue;
+  }
+
+  publicSetPrivateField(newValue) {
+    this.#privateFieldValue = newValue;
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/basic/output.js

@@ -0,0 +1,27 @@
+var _privateField = /*#__PURE__*/Symbol("privateField");
+var _privateFieldValue = /*#__PURE__*/Symbol("privateFieldValue");
+class Cl {
+  constructor() {
+    Object.defineProperty(this, _privateFieldValue, {
+      get: _get_privateFieldValue,
+      set: _set_privateFieldValue
+    });
+    Object.defineProperty(this, _privateField, {
+      writable: true,
+      value: "top secret string"
+    });
+    this.publicField = "not secret string";
+  }
+  publicGetPrivateField() {
+    return babelHelpers.classPrivateFieldLooseBase(this, _privateFieldValue)[_privateFieldValue];
+  }
+  publicSetPrivateField(newValue) {
+    babelHelpers.classPrivateFieldLooseBase(this, _privateFieldValue)[_privateFieldValue] = newValue;
+  }
+}
+function _get_privateFieldValue() {
+  return babelHelpers.classPrivateFieldLooseBase(this, _privateField)[_privateField];
+}
+function _set_privateFieldValue(newValue) {
+  babelHelpers.classPrivateFieldLooseBase(this, _privateField)[_privateField] = newValue;
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/get-only-setter/exec.js

@@ -0,0 +1,13 @@
+class Cl {
+  #privateField = 0;
+
+  set #privateFieldValue(newValue) {
+    this.#privateField = newValue;
+  }
+
+  constructor() {
+    expect(this.#privateFieldValue).toBeUndefined();
+  }
+}
+
+const cl = new Cl();

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/get-only-setter/input.js

@@ -0,0 +1,11 @@
+class Cl {
+  #privateField = 0;
+
+  set #privateFieldValue(newValue) {
+    this.#privateField = newValue;
+  }
+
+  constructor() {
+    this.publicField = this.#privateFieldValue;
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/get-only-setter/output.js

@@ -0,0 +1,18 @@
+var _privateField = /*#__PURE__*/Symbol("privateField");
+var _privateFieldValue = /*#__PURE__*/Symbol("privateFieldValue");
+class Cl {
+  constructor() {
+    Object.defineProperty(this, _privateFieldValue, {
+      get: void 0,
+      set: _set_privateFieldValue
+    });
+    Object.defineProperty(this, _privateField, {
+      writable: true,
+      value: 0
+    });
+    this.publicField = babelHelpers.classPrivateFieldLooseBase(this, _privateFieldValue)[_privateFieldValue];
+  }
+}
+function _set_privateFieldValue(newValue) {
+  babelHelpers.classPrivateFieldLooseBase(this, _privateField)[_privateField] = newValue;
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/helper/exec.js

@@ -0,0 +1,11 @@
+let foo;
+class Cl {
+  set #foo(v) { return 1 }
+  test() {
+    foo = this.#foo = 2;
+  }
+}
+
+new Cl().test();
+
+expect(foo).toBe(2);

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

@@ -0,0 +1,6 @@
+{
+  "plugins": ["proposal-private-methods", "proposal-class-properties"],
+  "assumptions": {
+    "privateFieldsAsSymbols": true
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/preserve-comments/input.js

@@ -0,0 +1,9 @@
+class C {
+  /* before get a */
+  get #a() { return 42 };
+  /* after get a */
+
+  /* before set a */
+  set #a(v) {}
+  /* after set a */
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/preserve-comments/output.js

@@ -0,0 +1,17 @@
+var _a = /*#__PURE__*/Symbol("a");
+class C {
+  constructor() {
+    /* before get a */
+    Object.defineProperty(this, _a, {
+      get: _get_a,
+      set: _set_a
+    });
+  }
+  /* after set a */
+}
+function _get_a() {
+  return 42;
+}
+/* after get a */
+/* before set a */
+function _set_a(v) {}

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/set-only-getter/exec.js

@@ -0,0 +1,14 @@
+class Cl {
+  #privateField = 0;
+
+  get #privateFieldValue() {
+    return this.#privateField;
+  }
+
+  constructor() {
+    expect(() => this.#privateFieldValue = 1).toThrow(TypeError);
+    expect(() => ([this.#privateFieldValue] = [1])).toThrow(TypeError);
+  }
+}
+
+const cl = new Cl();

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/set-only-getter/input.js

@@ -0,0 +1,12 @@
+class Cl {
+  #privateField = 0;
+
+  get #privateFieldValue() {
+    return this.#privateField;
+  }
+
+  constructor() {
+    this.#privateFieldValue = 1;
+    ([this.#privateFieldValue] = [1]);
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors-privateFieldsAsSymbols/set-only-getter/output.js

@@ -0,0 +1,19 @@
+var _privateField = /*#__PURE__*/Symbol("privateField");
+var _privateFieldValue = /*#__PURE__*/Symbol("privateFieldValue");
+class Cl {
+  constructor() {
+    Object.defineProperty(this, _privateFieldValue, {
+      get: _get_privateFieldValue,
+      set: void 0
+    });
+    Object.defineProperty(this, _privateField, {
+      writable: true,
+      value: 0
+    });
+    babelHelpers.classPrivateFieldLooseBase(this, _privateFieldValue)[_privateFieldValue] = 1;
+    [babelHelpers.classPrivateFieldLooseBase(this, _privateFieldValue)[_privateFieldValue]] = [1];
+  }
+}
+function _get_privateFieldValue() {
+  return babelHelpers.classPrivateFieldLooseBase(this, _privateField)[_privateField];
+}