@babel/plugin-proposal-private-methods

NOTE: This plugin is included in @babel/preset-env, in ES2022

Example?

JavaScript

class Counter extends HTMLElement {
  #xValue = 0;

  get #x() {
    return this.#xValue;
  }
  set #x(value) {
    this.#xValue = value;
    window.requestAnimationFrame(this.#render.bind(this));
  }

  #clicked() {
    this.#x++;
  }
}

Installation?

• npm
• Yarn
• pnpm

pnpm add @babel/plugin-proposal-private-methods --save-dev

Usage?

With a configuration file (Recommended)?

Without options:

babel.config.json

{
  "plugins": ["@babel/plugin-proposal-private-methods"]
}

With options:

babel.config.json

{
  "plugins": [["@babel/plugin-proposal-private-methods", { "loose": true }]]
}

Via CLI?

Shell

$ babel --plugins @babel/plugin-proposal-private-methods script.js

Via Node API?

JavaScript

require("@babel/core").transformSync("code", {
  plugins: ["@babel/plugin-proposal-private-methods"],
});

Options?

loose?

boolean, defaults to false.

Note: The loose mode configuration setting must be the same as @babel/plugin-proposal-class-properties.

When true, private methods will be assigned directly on its parent via Object.defineProperty rather than a WeakSet. This results in improved performance and debugging (normal property access vs .get()) at the expense of potentially leaking "privates" via things like Object.getOwnPropertyNames.

?? Consider migrating to the top level privateFieldsAsProperties assumption.

babel.config.json

{
  "assumptions": {
    "privateFieldsAsProperties": true,
    "setPublicClassFields": true
  }
}

Note that both privateFieldsAsProperties and setPublicClassFields must be set to true.

Let's use the following as an example:

JavaScript

class Foo {
  constructor() {
    this.publicField = this.#privateMethod();
  }

  #privateMethod() {
    return 42;
  }
}

By default, this becomes:

JavaScript

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;
};

With { privateFieldsAsProperties: true }, it becomes:

JavaScript

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;
};

You can read more about configuring plugin options here

References?

• Proposal: Private methods and getter/setters for JavaScript classes