Skip to main content

@babel/plugin-proposal-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.

JavaScript
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

Without options:

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

With options:

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

Via CLI

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

Via Node API

JavaScript
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

JavaScript
class Bork {
static a = "foo";
static b;

x = "bar";
y;
}

When setPublicClassFields is false, the above code will compile to the following, using Object.defineProperty:

JavaScript
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,
});

When setPublicClassFields is set to true, it will compile using assignment expressions:

JavaScript
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