TypedArray.prototype.findLast()

The findLast() method of {{jsxref("TypedArray")}} instances iterates the typed array in reverse order and returns the value of the first element that satisfies the provided testing function. If no elements satisfy the testing function, {{jsxref("undefined")}} is returned. This method has the same algorithm as {{jsxref("Array.prototype.findLast()")}}.

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

```js interactive-example function isNegative(element /, index, array /) { return element < 0; }

const int8 = new Int8Array([10, 0, -10, 20, -30, 40, 50]);

console.log(int8.find(isNegative)); // Expected output: -30

## Syntax

```js-nolint
findLast(callbackFn)
findLast(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 findLast() was called upon.
• thisArg {{optional_inline}}
• : A value to use as this when executing callbackFn. See iterative methods.

Return value#

The last (highest-index) element in the typed array that satisfies the provided testing function; {{jsxref("undefined")}} if no matching element is found.

Description#

See {{jsxref("Array.prototype.findLast()")}} for more details. This method is not generic and can only be called on typed array instances.

Examples#

Find the last prime number in a typed array#

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

[!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#

• Polyfill of TypedArray.prototype.findLast in core-js
• JavaScript typed arrays guide
• {{jsxref("TypedArray")}}
• {{jsxref("TypedArray.prototype.find()")}}
• {{jsxref("TypedArray.prototype.findIndex()")}}
• {{jsxref("TypedArray.prototype.findLastIndex()")}}
• {{jsxref("TypedArray.prototype.includes()")}}
• {{jsxref("TypedArray.prototype.filter()")}}
• {{jsxref("TypedArray.prototype.every()")}}
• {{jsxref("TypedArray.prototype.some()")}}
• {{jsxref("Array.prototype.findLast()")}}