JS Practice — Built-in Objects

Array.prototype.some() — detect duplicates

📖 Usage Example

const arr = [1, 2, 3, 4, 5]
arr.some(n => { console.log(n); return n === 3 })
// logs 1, 2, 3 — stops early

some() short-circuits as soon as a truthy result is found, saving unnecessary iterations.

Array.prototype.some() — early exit with indexOf

some() is ideal for detecting duplicates: check whether any element appears at a different index than its first occurrence using indexOf.

Challenge: Implement containsDuplicate(arr) that returns true if any element appears more than once.

containsDuplicate([1, 2, 3, 2]) // → true
containsDuplicate([1, 2, 3])    // → false

💡 Hints (2)

• `indexOf` always returns the **first** occurrence index. If `idx` differs from `indexOf(val)`, the element is a duplicate.
• This is O(n²) — for large arrays, a `Set` approach is more performant.