@babel/plugin-transform-new-target
info
This plugin is included in @babel/preset-env
Example
JavaScript
function Foo() {
console.log(new.target);
}
Foo(); // => undefined
new Foo(); // => Foo
JavaScript
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.
JavaScript
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).
JavaScript
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
// inheritance is commonly implemented.)
Reflect.construct(Foo, [], Baz); // => undefined (incorrect)
Installation
- npm
- Yarn
- pnpm
npm install --save-dev @babel/plugin-transform-new-target
yarn add --dev @babel/plugin-transform-new-target
pnpm add --save-dev @babel/plugin-transform-new-target
Usage
With a configuration file (Recommended)
babel.config.json
{
"plugins": ["@babel/plugin-transform-new-target"]
}
Via CLI
Shell
babel --plugins @babel/plugin-transform-new-target script.js
Via Node API
JavaScript
require("@babel/core").transformSync("code", {
plugins: ["@babel/plugin-transform-new-target"],
});