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TypedArray.prototype.findLastIndex()

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The findLastIndex() method of {{jsxref("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 {{jsxref("Array.prototype.findLastIndex()")}}.

{{InteractiveExample("JavaScript Demo: TypedArray.prototype.findLastIndex()")}}

```js interactive-example 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

```js-nolint
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_inline}}
  • : 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 {{jsxref("Array.prototype.findLastIndex()")}} for more details. This method is not generic and can only be called on typed array instances.

Examples#

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#

{{Specifications}}

Browser compatibility#

{{Compat}}

See also#