extending built-in classes

☢️ Alert❗

Normally, when one class extends another, both static and non-static methods are inherited. ( 👉 JS.info ⟩ Static properties and methods )

But built-in classes are an exception. They don’t inherit statics from each other.❗️❗️❗️

比較 class B extends A 與 Date extends Object 的不同：

一般的 class A, B，它們之間有 prototype 的連結，所以 A 所有的 static properties B 也都有。
但 Date 與 Object 之間並沒有連結，因此雖然有 Object.keys() 方法，但卻沒有 Date.keys() 方法❗️

replit

// ⭐ extending Array
class PowerArray extends Array {
    // arr.isEmpty
    get isEmpty() { return this.length === 0 }
    // arr.last
    get last() { return this[this.length - 1] }
}

let arr = new PowerArray(1, 2, 5, 10, 50);
let filtered = arr.filter(x => x >= 10);    // ⭐ return `PowerArray`❗

arr.isEmpty,             // false
arr.last,                // 50
Array.isArray(arr),      // true
filtered,                // PowerArray(2) [ 10, 50 ] ⭐
filtered.isEmpty,        // false

PowerArray.__proto__,         // Array
PowerArray.isArray(filtered), // true (⭐ static methods inherited❗)

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Last updated 3 years ago

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