🔰iterating elements

JS ⟩ object ⟩ built-in ⟩ Array ⟩ iterating arrays

// ⭐️ for-of: treats "holes" as undefined
for (const value of array) {}
for (const [index, value] of array.entries()) {}    // with index

// ⭐️ forEach: ignores "holes"
arr.forEach(value => ...)        // .forEach() method

⭐️ 重點

Unlike the for-of loop, forEach() is aware of sparse arrays and does not invoke the function for nonexistent elements.

🔴 主題

• iterating arrays

  • for-of loop

  • arr.forEach() method

  • for-of vs. forEach

👥 相關

• 👉 for-of vs. forEach.

💈範例

let a = [];        // sparse array
a[1] = 'hi';
a[5] = 'world';

let forofCount = 0;
let foreachCount = 0;

// ⭐️ for-of: treats "holes" as `undefined`.
for (const value of a) {
    forofCount += 1;
    console.log(forofCount, value);
}
// 1 undefined, 2 'hi', 3 undefined, ... 6 'world'

// ⭐️ forEach: ignores "holes".
a.forEach(value => {
    foreachCount += 1;
    console.log(foreachCount, value);
})
// 1 'hi', 2 'world'

📗 參考

• JS.info ⟩ iterables

• JavaScript: The Definitive Guide ⟩ 7.6 Iterating Arrays

📘 手冊