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@babel/plugin-transform-private-property-in-object

info

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 install --save-dev @babel/plugin-transform-private-property-in-object

Usage​

babel.config.json
{
  "plugins": ["@babel/plugin-transform-private-property-in-object"]
}

Via CLI​

Shell
babel --plugins @babel/plugin-transform-private-property-in-object

Via Node API​

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

Options​

loose​

boolean, defaults to false.

note

The loose mode configuration setting must be the same as @babel/transform-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.

caution

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");
tip

You can read more about configuring plugin options here

References​