extends

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2017.

extends 关键字用于类声明或者类表达式中,以创建一个类,该类是另一个类的子类。

语法

class ChildClass extends ParentClass { ... }

描述

extends 关键字用来创建一个普通类或者内建对象的子类。

继承的 .prototype 必须是一个 Object 或者 null

示例

使用 extends

第一个例子是根据名为 Polygon 类创建一个名为 Square 的类。这个例子是从这个在线演示中提取出来的。

js
class Square extends Polygon {
  constructor(length) {
    // Here, it calls the parent class' constructor with lengths
    // provided for the Polygon's width and height
    super(length, length);
    // Note: In derived classes, super() must be called before you
    // can use 'this'. Leaving this out will cause a reference error.
    this.name = "Square";
  }

  get area() {
    return this.height * this.width;
  }
}

使用 extends与内置对象

这个示例继承了内置的 Date 对象。这个例子是从这个在线演示中提取出来的。

js
class myDate extends Date {
  constructor() {
    super();
  }

  getFormattedDate() {
    var months = [
      "Jan",
      "Feb",
      "Mar",
      "Apr",
      "May",
      "Jun",
      "Jul",
      "Aug",
      "Sep",
      "Oct",
      "Nov",
      "Dec",
    ];
    return (
      this.getDate() + "-" + months[this.getMonth()] + "-" + this.getFullYear()
    );
  }
}

扩展 null

可以像扩展普通类一样扩展 null,但是新对象的原型将不会继承 Object.prototype (en-US)

js
class nullExtends extends null {
  constructor() {}
}

Object.getPrototypeOf(nullExtends); // Function.prototype
Object.getPrototypeOf(nullExtends.prototype); // null

new nullExtends(); //ReferenceError: this is not defined

规范

Specification
ECMAScript Language Specification
# sec-class-definitions

浏览器兼容性

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扩展阅读