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.


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


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

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support No support 37.0 (37.0) No support No support No support
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support 37.0 (37.0) No support No support No support

See also

Document Tags and Contributors

 Contributors to this page: eduardoboucas, fscholz
 Last updated by: eduardoboucas,