The findIndex() method returns an index in the array, if an element in the array satisfies the provided testing function. Otherwise -1 is returned.

See also the find() method, which returns the value of a found element in the array instead of its index.


<var>arr</var>.findIndex(<var>callback</var>[, <var>thisArg</var>])


Function to execute on each value in the array, taking three arguments:
The current element being processed in the array.
The index of the current element being processed in the array.
The array findIndex was called upon.
Optional. Object to use as this when executing callback.

Return value

An index in the array if an element passes the test; otherwise, -1.


The findIndex method executes the callback function once for every array index 0..length-1 (inclusive) in the array until it finds one where callback returns a truthy value (a value that coerces to true). If such an element is found, findIndex immediately returns the index for that iteration. If the callback never returns a truthy value or the array's length is 0, findIndex returns -1. Unlike some other array methods such as Array#some, in sparse arrays the callback is called even for indexes of entries not present in the array.

callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.

If a thisArg parameter is provided to findIndex, it will be used as the this for each invocation of the callback. If it is not provided, then undefined is used.

findIndex does not mutate the array on which it is called.

The range of elements processed by findIndex is set before the first invocation of callback. Elements that are appended to the array after the call to findIndex begins will not be visited by callback. If an existing, unvisited element of the array is changed by callback, its value passed to the visiting callback will be the value at the time that findIndex visits that element's index; elements that are deleted are not visited.


Find the index of a prime number in an array

The following example finds the index of an element in the array that is a prime number (or returns -1 if there is no prime number).

function isPrime(element, index, array) {
  var start = 2;
  while (start <= Math.sqrt(element)) {
    if (element % start++ < 1) {
      return false;
  return element > 1;

console.log([4, 6, 8, 12].findIndex(isPrime)); // -1, not found
console.log([4, 6, 7, 12].findIndex(isPrime)); // 2


This method has been added to the ECMAScript 6 specification and may not be available in all JavaScript implementations yet. However, you can polyfill Array.prototype.findIndex with the following snippet:

if (!Array.prototype.findIndex) {
  Array.prototype.findIndex = function(predicate) {
    'use strict';
    if (this == null) {
      throw new TypeError('Array.prototype.findIndex called on null or undefined');
    if (typeof predicate !== 'function') {
      throw new TypeError('predicate must be a function');
    var list = Object(this);
    var length = list.length >>> 0;
    var thisArg = arguments[1];
    var value;

    for (var i = 0; i < length; i++) {
      value = list[i];
      if (, value, i, list)) {
        return i;
    return -1;


Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Array.prototype.findIndex' in that specification.
Standard Initial definition.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Array.prototype.findIndex' in that specification.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Edge Opera Safari
Basic support 45.0 25.0 (25.0) No support Yes No support 7.1
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support 25.0 (25.0) No support No support 8.0

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

Array ECMAScript6 JavaScript Method polyfill Prototype