The findIndex() method returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1, indicating that no element passed the test.

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


arr.findIndex(callback( element[, index[, array]] )[, thisArg])



A function to execute on each value in the array until the function returns true, indicating that the satisfying element was found.

It takes three arguments:

The current element being processed in the array.
index Optional
The index of the current element being processed in the array.
array Optional
The array findIndex() was called upon.
thisArg Optional
Optional object to use as this when executing callback.

Return value

The index of the first element in the array that passes the test. Otherwise, -1.


The findIndex() method executes the callback function once for every index in the array until it finds the one where callback returns a truthy value.

If such an element is found, findIndex() immediately returns the element's index. If callback never returns a truthy value (or the array's length is 0), findIndex() returns -1.

Edge case alert: Unlike other array methods such as Array.some(), callback is run even for indexes with unassigned values.

callback is invoked with three arguments:

  1. The value of the element
  2. The index of the element
  3. The Array object being traversed

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

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

Elements that are deleted are still visited.


// https://tc39.github.io/ecma262/#sec-array.prototype.findindex
if (!Array.prototype.findIndex) {
  Object.defineProperty(Array.prototype, 'findIndex', {
    value: function(predicate) {
     // 1. Let O be ? ToObject(this value).
      if (this == null) {
        throw new TypeError('"this" is null or not defined');

      var o = Object(this);

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0;

      // 3. If IsCallable(predicate) is false, throw a TypeError exception.
      if (typeof predicate !== 'function') {
        throw new TypeError('predicate must be a function');

      // 4. If thisArg was supplied, let T be thisArg; else let T be undefined.
      var thisArg = arguments[1];

      // 5. Let k be 0.
      var k = 0;

      // 6. Repeat, while k < len
      while (k < len) {
        // a. Let Pk be ! ToString(k).
        // b. Let kValue be ? Get(O, Pk).
        // c. Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
        // d. If testResult is true, return k.
        var kValue = o[k];
        if (predicate.call(thisArg, kValue, k, o)) {
          return k;
        // e. Increase k by 1.

      // 7. Return -1.
      return -1;
    configurable: true,
    writable: true

If you need to support truly obsolete JavaScript engines that do not support Object.defineProperty, it is best not to polyfill Array.prototype methods at all, as you cannot make them non-enumerable.


Find the index of a prime number in an array

The following example returns the index of the first element in the array that is a prime number, or -1 if there is no prime number.

function isPrime(num) {
  for (let i = 2; num > i; i++) {
    if (num % i == 0) {
      return false;
  return num > 1;

console.log([4, 6, 8, 9, 12].findIndex(isPrime)); // -1, not found
console.log([4, 6, 7, 9, 12].findIndex(isPrime)); // 2 (array[2] is 7)

Find index using arrow function

The following example finds the index of a fruit using an arrow function:

const fruits = ["apple", "banana", "cantaloupe", "blueberries", "grapefruit"];

const index = fruits.findIndex(fruit => fruit === "blueberries");

console.log(index); // 3
console.log(fruits[index]); // blueberries


Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
findIndex 45 12 25 No 32 8
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
findIndex 45 45 4 32 8 5.0
findIndex 4.0.0
Disabled From version 0.12: this feature is behind the --harmony runtime flag.

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.