❗only-iterate-once iterable

JS⟩ iteration ⟩ iterable ⟩ only-iterate-once iterable

some iterables only iterate once❗️

⭐️ 重點

only-iterate-once iterables：

• iterable iterators - iterators iterators only iterate once❗️.

• s - generators are iterable iterators.

• array.keys(), map.entries(), ... - built-in iterable iterators.

🗺️ 圖解

💈範例

💾 replit：only iterate once

// generator function returns "generators"
// - generators are "iterable iterators".
// - iterators only iterate once❗
function* genAB() {
    yield 'A';
    yield 'B';
}

// main
const ab = ['a', 'b'];     // array (iterable)
const it = ab.values();    // iterator (iterable)
const AB = genAB();        // generator (iterable iterator)

// ⭐ arrays can iterate multiple times.
[...ab, ...ab],            // [ 'a', 'b', 'a', 'b' ]

// ⭐ iterators only iterate once❗
[...it, ...it],            // [ 'a', 'b' ]❗
[...AB, ...AB],            // [ 'A', 'B' ]❗

// ⭐ have to call generator function multiple times
//    to return multiple new iterators.
[...genAB(), ...genAB()],  // [ 'A', 'B', 'A', 'B' ] ⭐

📗 參考

• 2ality ⟩ A downside of iterable iterators