plugin-proposal-class-properties.md

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---
id: babel-plugin-proposal-class-properties
title: @babel/plugin-proposal-class-properties
sidebar_label: class-properties
---

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

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

With a configuration file (Recommended)

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").transformSync("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.

⚠️ Consider migrating to the top level setPublicClassFields assumption

// babel.config.json
{
  "assumptions": {
    "setPublicClassFields": true
  }
}

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 { "setPublicClassFields": 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 { "setPublicClassFields": 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