babel/packages/babel-plugin-proposal-class-properties

@babel/plugin-proposal-class-properties

This plugin transforms class properties

Example

Below is a class with four class properties which will be transformed.

  class Bork {
    //Property initializer syntax
    instanceProperty = "bork";
    boundFunction = () => {
      return this.instanceProperty;
    };

    //Static class properties
    static staticProperty = "babelIsCool";
    static staticFunction = function() {
      return Bork.staticProperty;
    };
  }

  let myBork = new Bork;

  //Property initializers are not on the prototype.
  console.log(myBork.__proto__.boundFunction); // > undefined

  //Bound functions are bound to the class instance.
  console.log(myBork.boundFunction.call(undefined)); // > "bork"

  //Static function exists on the class.
  console.log(Bork.staticFunction()); // > "babelIsCool"

Installation

npm install --save-dev @babel/plugin-proposal-class-properties

Usage

Via .babelrc (Recommended)

.babelrc

Without options:

{
  "plugins": ["@babel/plugin-proposal-class-properties"]
}

With options:

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

Via CLI

babel --plugins @babel/plugin-proposal-class-properties script.js

Via Node API

require("@babel/core").transform("code", {
  plugins: ["@babel/plugin-proposal-class-properties"]
});

Options

loose

boolean, defaults to false.

When true, class properties are compiled to use an assignment expression instead of Object.defineProperty.

Example

  class Bork {
    static a = 'foo';
    static b;

    x = 'bar';
    y;
  }

Without { "loose": true }, the above code will compile to the following, using Object.defineProperty:

var Bork = function Bork() {
  babelHelpers.classCallCheck(this, Bork);
  Object.defineProperty(this, "x", {
    configurable: true,
    enumerable: true,
    writable: true,
    value: 'bar'
  });
  Object.defineProperty(this, "y", {
    configurable: true,
    enumerable: true,
    writable: true,
    value: void 0
  });
};

Object.defineProperty(Bork, "a", {
  configurable: true,
  enumerable: true,
  writable: true,
  value: 'foo'
});
Object.defineProperty(Bork, "b", {
  configurable: true,
  enumerable: true,
  writable: true,
  value: void 0
});

However, with { "loose": true }, it will compile using assignment expressions:

var Bork = function Bork() {
  babelHelpers.classCallCheck(this, Bork);
  this.x = 'bar';
  this.y = void 0;
};

Bork.a = 'foo';
Bork.b = void 0;

References

• Proposal: ES Class Fields & Static Properties