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@babel/plugin-proposal-private-methods

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

History
VersionChanges
v7.3.0Support private accessors (getters and setters)
v7.2.0Initial Release

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​

Shell
$ npm install @babel/plugin-proposal-private-methods --save-dev

Usage​

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​