TypedArray.prototype.findLastIndex()

The findLastIndex() method of TypedArray instances iterates the typed array in reverse order and returns the index of the first element 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.findLastIndex().

Try it

function isNegative(element /*, index, array */) {
  return element < 0;
}

const int8 = new Int8Array([10, -20, 30, -40, 50]);

console.log(int8.findLastIndex(isNegative));
// Expected output: 3

Syntax

findLastIndex(callbackFn)
findLastIndex(callbackFn, thisArg)

Parameters

callbackFn

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:

element: The current element being processed in the typed array.
index: The index of the current element being processed in the typed array.
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 typed array that passes the test. Otherwise -1 if no matching element is found.

Description

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

Examples

undefined

Find the index of the last prime number in a typed array

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

function isPrime(n) {
  if (n < 2) {
    return false;
  }
  if (n % 2 === 0) {
    return n === 2;
  }
  for (let factor = 3; factor * factor <= n; factor += 2) {
    if (n % factor === 0) {
      return false;
    }
  }
  return true;
}

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

Note: The isPrime() implementation is for demonstration only. For a real-world application, you would want to use a heavily memoized algorithm such as the Sieve of Eratosthenes to avoid repeated calculations.

Specifications

ECMAScript® 2027 Language Specification