Find the index of the last matching item of an array

Scan from the end and return the first index whose item matches.

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

function lastIndex(values, predicate) { return values.findLastIndex(predicate); }

Usage

lastIndex([1, 3, 5, 7, 9, 2, 4, 6, 8], n => n % 2 === 1); // → 4
lastIndex([1, 3, 5, 7, 9, 8, 6, 4, 2], n => n > 6); // → 5
lastIndex([], () => true); // → -1

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• Array.prototype.findLastIndex is ES2023: Node.js 18+ and current browsers. The original reduced the whole array; findLastIndex stops at the first match from the end.

The function above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Clone an array — Create a new array without copying the objects inside it.
• Remove duplicate values — Keep the first occurrence of each value, in its original order.
• Flatten a nested array — Turn a nested array into one sequence of values.