`new.target`属性允许你检测函数或构造方法是否是通过new运算符被调用的。在通过new运算符被初始化的函数或构造方法中，`new.target`返回一个指向构造方法或函数的引用。在普通的函数调用中，`new.target` 的值是`undefined`。

## 语法

    new.target

## 描述

`new.target`语法由一个关键字"`new`"，一个点，和一个属性名"target"组成。通常"`new.`"`的`作用是提供属性访问的上下文，但这里"`new.`"其实不是一个真正的对象。不过在构造方法调用中，`new.target`指向被`new`调用的构造函数，所以"`new.`"成为了一个虚拟上下文。

`new.target`属性适用于所有函数访问的元属性。在 arrow functions 中，`new.target` 指向最近的外层函数的`new.target`（An arrow function expression does not have its own this, arguments, super , or new.target) 。

## 示例

### 函数调用中的 new.target

在普通的函数调用中（和作为构造函数来调用相对），`new.target`的值是`undefined`。这使得你可以检测一个函数是否是作为构造函数通过new被调用的。

    function Foo() {
      if (!new.target) throw "Foo() must be called with new";
      console.log("Foo instantiated with new");
    }

    Foo(); // throws "Foo() must be called with new"
    new Foo(); // logs "Foo instantiated with new"

### 构造方法中的 new.target

在类的构造方法中，`new.target`指向直接被`new`执行的构造函数。并且当一个父类构造方法在子类构造方法中被调用时，情况与之相同。

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

    class B extends A { constructor() { super(); } }

    var a = new A(); // logs "A"
    var b = new B(); // logs "B"

    class C { constructor() { console.log(new.target); } }
    class D extends C { constructor() { super(); } }

    var c = new C(); // logs class C{constructor(){console.log(new.target);}}
    var d = new D(); // logs class D extends C{constructor(){super();}}

从上面类 C 和 D 的例子可以看出来，new.target 指向的是初始化类的类定义。比如当 D 通过 new 初始化的时候，打印出了 D 的类定义，C 的例子与之类似，打印出的是 C 的类定义。

## 规范

Specification | Status | Comment
--- | --- | ---
ECMAScript 2015 (6th Edition, ECMA-262) Built-in Function Objects | Standard | Initial definition.
ECMAScript Latest Draft (ECMA-262) Built-in Function Objects | Draft |

## 浏览器兼容

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Update compatibility data on GitHub

| Desktop | Mobile | Server
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | Android webview | Chrome for Android | Edge Mobile | Firefox for Android | Opera for Android | Safari on iOS | Samsung Internet | Node.js
`new.target` | Chrome Full support 46 | Edge Full support Yes | Firefox Full support 41 | IE No support No | Opera Full support Yes | Safari Full support Yes | WebView Android Full support 46 | Chrome Android Full support 46 | Edge Mobile Full support Yes | Firefox Android Full support 41 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support 5.0 | nodejs Full support 5.0.0

### Legend

Full support
  Full support
No support
  No support

## 相关链接

- Functions
- Classes
- `new`
- `this`