extends
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2016年3月.
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
class DateFormatter extends Date {
getFormattedDate() {
const months = [
"Jan",
"Feb",
"Mar",
"Apr",
"May",
"Jun",
"Jul",
"Aug",
"Sep",
"Oct",
"Nov",
"Dec",
];
return `${this.getDate()}-${months[this.getMonth()]}-${this.getFullYear()}`;
}
}
console.log(new DateFormatter("August 19, 1975 23:15:30").getFormattedDate());
// Expected output: "19-Aug-1975"
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。
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® 2026 Language Specification # sec-class-definitions |
瀏覽器相容性
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