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

See also the findLastIndex() method, which returns the index of the found element instead of its value.

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findLast(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 findLast() was called upon.

thisArg Optional

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

Return value

The element in the typed array with the highest index value that satisfies the provided testing function; undefined if no matching value is found.


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

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

The findLast() 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 findLast() is set before the first invocation of callbackFn. Elements that are appended to the typed array after the call to findLast() 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 findLast() 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 a prime number in a typed array

The following example finds the value of the last element in the typed array that is a prime number (or returns undefined 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]);
console.log(uint8.findLast(isPrime)); // undefined (no primes in array)
uint8 = new Uint8Array([4, 5, 7, 8, 9, 11, 12]);
console.log(uint8.findLast(isPrime)); // 11

All elements are visited and may be modified by the callback

The following examples show that all elements are visited, and that the value passed to the callback is their value when visited:

// Declare array with no elements at indexes 2, 3, and 4
// The missing elements will be initialized to zero.
const uint8 = new Uint8Array([0, 1, , , , 5, 6]);

// Iterate through the elements in reverse order.
// Note that all elements are visited.
uint8.findLast((value, index) => {
  console.log(`Visited index ${index} with value ${value}`);

// Shows all indexes, including deleted
uint8.findLast((value, index) => {
  // Modify element 3 on first iteration
  if (index === 6) {
    console.log("Set uint8[3] to 44");
    uint8[3] = 44;
  // Element 3 is still visited but will have a new value.
  console.log(`Visited index ${index} with value ${value}`);
// Visited index 6 with value 6
// Visited index 5 with value 5
// Visited index 4 with value 0
// Visited index 3 with value 0
// Visited index 2 with value 0
// Visited index 1 with value 1
// Visited index 0 with value 0
// Set uint8[3] to 44
// Visited index 6 with value 6
// Visited index 5 with value 5
// Visited index 4 with value 0
// Visited index 3 with value 44
// Visited index 2 with value 0
// Visited index 1 with value 1
// Visited index 0 with value 0


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

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See also