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 關鍵字被使用於類別（class）宣告或類別（class）表達式中來建立擴展的子類別 。

嘗試一下

語法

class ChildClass extends ParentClass { ... }

解釋

extends 關鍵字可用於建立一個自訂類別或內建類別的子類別。

其繼承之原型 .prototype 必須是 Object 或 null。

範例

使用 extends

第一個範例是根據 Polygon類別建立一個名為 Square 的子類別。此範例提取自線上示例。

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 於內建類別

這個範例擴展了內建的 Date 類別。此範例提取自線上範例。

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()
    );
  }
}

擴展 null

像擴展普通類別一樣擴展 null，但新對象的原型不會繼承 Object.prototype。

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

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

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

標準

Specification
ECMAScript Language Specification
# sec-class-definitions

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