JS Practice — Built-in Objects

Array.prototype.indexOf() — find first duplicate value

📖 Usage Example

const arr = [3, 1, 4, 1, 5]
const first = arr.indexOf(1)   // → 1
const second = arr.indexOf(1, first + 1)  // → 3

Chain indexOf() calls with a fromIndex to locate successive duplicate values.

Array.prototype.indexOf() — detecting duplicates

By comparing indexOf(val) with lastIndexOf(val), you can tell whether a value appears more than once. The first value where they differ is the first duplicate.

Challenge: Implement firstDuplicate(arr) that returns the first value that appears more than once, or -1 if all values are unique.

firstDuplicate([1, 2, 3, 2, 1]) // → 2 (first value that repeats)
firstDuplicate([1, 2, 3])        // → -1

💡 Hints (2)

• A value is a duplicate when `arr.indexOf(val) !== arr.lastIndexOf(val)`.
• Iterate and return on the first such value found.