JS Practice — Built-in Objects

Array.prototype.lastIndexOf() — search from a given index

📖 Usage Example

const arr = [1, 2, 3, 2, 1]
arr.lastIndexOf(2, 2)  // → 1  (search backwards from index 2)

Pass a fromIndex to lastIndexOf() to limit how far back the backward search goes.

Array.prototype.lastIndexOf() — fromIndex

The optional second argument fromIndex tells lastIndexOf where to start searching backwards. Only positions ≤ fromIndex are considered.

Challenge: Implement lastPosFrom(arr, val, from) that uses arr.lastIndexOf(val, from).

lastPosFrom([1, 2, 3, 2, 1], 2, 2) // → 1  (search up to index 2)
lastPosFrom([1, 2, 3, 2, 1], 2, 3) // → 3

💡 Hints (3)