The findLastIndex() method iterates a typed array in reverse order and returns the index of the first element that satisfies the provided testing function. If no values satisfy the testing function, -1 is returned.

See also the findLast() method, which returns the value of the found element rather than its index.

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findLastIndex(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 findLastIndex() was called upon.

thisArg Optional

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

Return value

The index of the last (highest-index) element in the array that passes the test. Otherwise -1 if no matching element is found.


The findLastIndex() method executes the callbackFn function once for each index of the typed array in descending-index order until the callbackFn returns a truthy value. findLastIndex() then returns the index of that element and stops iterating through the typed array. If callbackFn never returns a truthy value, findLastIndex() returns -1.

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 findLastIndex(), it will be used as the this for each invocation of the callbackFn. If it is not provided, then undefined is used.

The findLastIndex() method does not mutate the typed array on which it is called, but the function provided to callbackFn can.

The range of elements processed by findLastIndex() is set before the first invocation of callbackFn. Elements that are appended to the typed array after the call to findLastIndex() 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 findLastIndex() visits that element's index.

Warning: Concurrent modification of the kind described in the previous paragraph frequently leads to hard-to-understand code and is generally to be avoided (except in special cases).


Find the index of a prime number in a typed array

The following example finds the index of the last element in the typed array that is a prime number (or returns -1 if there is no prime number).

function isPrime(element) {
  if (element % 2 === 0 || element < 2) {
    return false;
  for (let factor = 3; factor <= Math.sqrt(element); factor += 2) {
    if (element % factor === 0) {
      return false;
  return true;

let uint8 = new Uint8Array([4, 6, 8, 12]);
// -1 (no primes in array)
uint8 = new Uint8Array([4, 5, 7, 8, 9, 11, 12]);
// 5


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

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See also