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


Browser compatibility

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findIndex 45 12 37 32 9.1 45 37 32 9.3 5.0 45 1.0 4.0.0

See also

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