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

Example?

In

JavaScript

class Foo {
  #bar = "bar";

  test(obj) {
    return #bar in obj;
  }
}

Out

JavaScript

class Foo {
  constructor() {
    _bar.set(this, {
      writable: true,
      value: "bar",
    });
  }

  test() {
    return _bar.has(this);
  }
}

var _bar = new WeakMap();

Installation?

• npm
• Yarn
• pnpm

pnpm add --save-dev @babel/plugin-proposal-private-property-in-object

Usage?

With a configuration file (Recommended)?

babel.config.json

{
  "plugins": ["@babel/plugin-proposal-private-property-in-object"]
}

Via CLI?

Shell

babel --plugins @babel/plugin-proposal-private-property-in-object

Via Node API?

JavaScript

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

Options?

loose?

boolean, defaults to false.

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

When true, private property in expressions will check own properties (as opposed to inherited ones) on the object, instead of checking for presence inside 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.

Example?

In

JavaScript

class Foo {
  #bar = "bar";

  test(obj) {
    return #bar in obj;
  }
}

Out

JavaScript

class Foo {
  constructor() {
    Object.defineProperty(this, _bar, {
      writable: true,
      value: "bar",
    });
  }

  test() {
    return Object.prototype.hasOwnProperty.call(this, _bar);
  }
}

var _bar = babelHelpers.classPrivateFieldLooseKey("bar");

You can read more about configuring plugin options here

References?

• Proposal: Ergonomic brand checks for Private Fields