The findIndex() method returns an index in the typed array, if an element in the typed 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 typed array instead of its index.


// Arrow function
findIndex((element) => { ... } )
findIndex((element, index) => { ... } )
findIndex((element, index, array) => { ... } )

// Callback function
findIndex(callbackFn, thisArg)

// Inline callback function
findIndex(function callbackFn(element) { ... })
findIndex(function callbackFn(element, index) { ... })
findIndex(function callbackFn(element, index, array){ ... })
findIndex(function callbackFn(element, index, array) { ... }, thisArg)



Function to execute on each value in the typed array, taking three arguments:


The current element being processed in the typed array.


The index of the current element being processed in the typed array.


The typed array findIndex() was called upon.

thisArg Optional

Object to use as this when executing callbackFn.

Return value

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


The findIndex() method executes the callbackFn function once for each element present in the typed array until it finds one where callbackFn returns a true value. If such an element is found, findIndex() immediately returns the index of that element. Otherwise, findIndex() returns -1. callbackFn is invoked only for indexes of the typed array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.

callbackFn is invoked with three arguments: the value of the element, the index of the element, and the typed 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 typed array on which it is called.

The range of elements processed by findIndex() is set before the first invocation of callbackFn. Elements that are appended to the typed array after the call to findIndex() begins will not be visited by callbackFn. If an existing, unvisited element of the typed array is changed by callbackFn, its value passed to the visiting callbackFn 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 a typed array

The following example finds the index of an element in the typed 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;

var uint8 = new Uint8Array([4, 6, 8, 12]);
var uint16 = new Uint16Array([4, 6, 7, 12]);

console.log(uint8.findIndex(isPrime)); // -1, not found
console.log(uint16.findIndex(isPrime)); // 2


TypedArray.prototype.findIndex = Array.prototype.findIndex = Array.prototype.findIndex || function(evaluator, thisArg) {
        'use strict';
        if (!this) {
          throw new TypeError('Array.prototype.some called on null or undefined');

        if (typeof(evaluator) !== 'function') {
            if (typeof(evaluator) === 'string') {
                // Attempt to convert it to a function
                if ( ! (evaluator = eval(evaluator)) ){
                    throw new TypeError();
            } else {
                throw new TypeError();

        var i;
        if (thisArg === undefined) {  // Optimize for thisArg
            for (i in this) {
                if (evaluator(this[i], i, this)) {
                    return i;
            return -1;
        for (i in this) {
            if (evaluator.call(thisArg, this[i], i, this)) {
                return i;
        return -1;


ECMAScript Language Specification (ECMAScript)
# sec-%typedarray%.prototype.findindex

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See also