JS Practice — Built-in Objects

Array.prototype.indexOf() — find first index

📖 Usage Example

const arr = [10, 20, 30, 40]
arr.indexOf(30)  // → 2
arr.indexOf(20)  // → 1

Use indexOf() to find the position of the first occurrence of a value in an array.

Array.prototype.indexOf()

Array.prototype.indexOf(value, fromIndex?) returns the index of the first occurrence of value using strict equality (===), or -1 if not found.

Challenge: Implement firstIndexOf(arr, val) that returns arr.indexOf(val).

firstIndexOf([1, 2, 3, 2], 2) // → 1
firstIndexOf([1, 2, 3], 9)    // → -1

💡 Hints (2)

• `indexOf` uses strict equality (`===`) — it cannot find `NaN`.
• `indexOf` returns the first occurrence; use `lastIndexOf` for the last one.