plugin-proposal-class-properties.md

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---
id: version-7.0.0-babel-plugin-proposal-class-properties
title: @babel/plugin-proposal-class-properties
sidebar_label: proposal-class-properties
original_id: babel-plugin-proposal-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.

For an explanation of the consequences of using either, see Definition vs. Assignment (TL;DR in Part 5)

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;

You can read more about configuring plugin options here

References

• Proposal: Public and private instance fields
• Proposal: Static class features