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.


typedarray.findIndex(callback[, 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.
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 each element present in the typed array until it finds one where callback returns a true value. If such an element is found, findIndex immediately returns the index of that element. Otherwise, findIndex returns -1. callback 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.

callback 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 callback. Elements that are appended to the typed array after the call to findIndex begins will not be visited by callback. If an existing, unvisited element of the typed 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.


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;


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


ECMAScript (ECMA-262)
The definition of '%TypedArray%.prototype.findIndex' in that specification.

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
findIndexChrome Full support 45Edge Full support 14Firefox Full support 37IE No support NoOpera Full support 32Safari Full support 9.1WebView Android Full support 45Chrome Android Full support 45Firefox Android Full support 37Opera Android Full support 32Safari iOS Full support 9.3Samsung Internet Android Full support 5.0nodejs Full support 4.0.0


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See also