@babel/plugin-transform-new-target

@babel/plugin-transform-new-target

This plugins allows babel to transform new.target meta property into a (correct in most cases) this.constructor expression.

Example

function Foo() {
  console.log(new.target);
}

Foo(); // => undefined
new Foo(); // => Foo

class Foo {
  constructor() {
    console.log(new.target);
  }
}

class Bar extends Foo {
}

new Foo(); // => Foo
new Bar(); // => Bar

Caveats

This plugin relies on this.constructor, which means super must already have been called when using untransformed classes.

class Foo {}

class Bar extends Foo {
  constructor() {
    // This will be a problem if classes aren't transformed to ES5
    new.target;
    super();
  }
}

Additionally, this plugin cannot transform all Reflect.construct cases when using newTarget with ES5 function classes (transformed ES6 classes).

function Foo() {
  console.log(new.target);
}

// Bar extends Foo in ES5
function Bar() {
  Foo.call(this);
}
Bar.prototype = Object.create(Foo.prototype);
Bar.prototype.constructor = Bar;

// Baz does not extend Foo
function Baz() {}

Reflect.construct(Foo, []); // => Foo (correct)
Reflect.construct(Foo, [], Bar); // => Bar (correct)

Reflect.construct(Bar, []); // => Bar (incorrect, though this is how ES5
                            // inheritience is commonly implemented.)
Reflect.construct(Foo, [], Baz); // => undefined (incorrect)

Installation

npm install --save-dev @babel/plugin-transform-new-target

Usage

Via .babelrc (Recommended)

.babelrc

{
  "plugins": ["@babel/plugin-transform-new-target"]
}

Via CLI

babel --plugins @babel/plugin-transform-new-target script.js

Via Node API

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