JS Built-ins
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ArrayArray.prototype.findIndexbeginner

Array.prototype.findIndex() — index of first match

📖 Usage Example
const nums = [5, 12, 8, 130]
nums.findIndex(n => n > 10)  // → 1  (index of 12)

Use findIndex() to get the index of the first element that satisfies a condition.

Array.prototype.findIndex()

Array.prototype.findIndex(callback) returns the index of the first element for which the callback returns truthy, or -1 if no match is found. It is the index-returning counterpart of find().

Challenge: Implement firstEvenIndex(nums) that returns the index of the first even number.

firstEvenIndex([1, 3, 4, 6]) // → 2
firstEvenIndex([1, 3, 5])    // → -1
💡 Hints (2)
  • • `findIndex()` returns the index, while `find()` returns the element.
  • • Returns `-1` when no element satisfies the predicate.