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@babel/plugin-transform-new-target

信息

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

Usage​

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"],
});