JS Practice — Built-in Objects

Array.prototype.forEach() — mutate array in-place

📖 Usage Example

const log = []
[1, 2, 3].forEach(n => log.push(n * 2))
log  // → [2, 4, 6]

Use forEach() when you need side effects like pushing to another array or calling an API.

Array.prototype.forEach() — index-based mutation

The forEach callback receives three arguments: (value, index, array). You can use the index to mutate elements in-place by writing back to array[index].

Challenge: Implement multiplyInPlace(arr, factor) that multiplies each element in the array by factor using forEach.

const arr = [1, 2, 3]
multiplyInPlace(arr, 2)
// arr is now [2, 4, 6]

💡 Hints (2)

• The third argument to the callback is the original array — use `array[index] = ...` to mutate it.
• `forEach` returns `undefined` — it is only useful for its side effects.