Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.

The findIndex() method of TypedArray instances returns the index of the first element in a typed array that satisfies the provided testing function. If no elements satisfy the testing function, -1 is returned. This method has the same algorithm as Array.prototype.findIndex().

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findIndex(callbackFn, thisArg)



A function to execute for each element in the typed array. It should return a truthy value to indicate a matching element has been found, and a falsy value otherwise. The function is called with the following 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

A value to use as this when executing callbackFn. See iterative methods.

Return value

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


See Array.prototype.findIndex() for more details. This method is not generic and can only be called on typed array instances.


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) {
  let start = 2;
  while (start <= Math.sqrt(element)) {
    if (element % start++ < 1) {
      return false;
  return element > 1;

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

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


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

Browser compatibility

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See also