TypedArray.prototype.findLastIndex()

The findLastIndex() method returns the index of the first element in a typed array 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.

Try it

Syntax

// Arrow function
findLastIndex((element) => { /* ... */ } )
findLastIndex((element, index) => { /* ... */ } )
findLastIndex((element, index, array) => { /* ... */ } )

// Callback function
findLastIndex(callbackFn)
findLastIndex(callbackFn, thisArg)

// Inline callback function
findLastIndex(function(element) { /* ... */ })
findLastIndex(function(element, index) { /* ... */ })
findLastIndex(function(element, index, array){ /* ... */ })
findLastIndex(function(element, index, array) { /* ... */ }, thisArg)

Parameters

callbackFn

A function used to test elements in the typed array.

The function is called with the following arguments:

element

The current element in the typed array.

index

The index (position) of the current element in the typed array.

array

The typed array that findLastIndex() was called on.

The callback must return a truthy value to indicate an appropriate element has been found. The index of this element is returned by findLastIndex().

thisArg Optional

Object to use as this when executing callbackFn.

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.

Description

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 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 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).

Examples

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

Specifications

Specification
Proposal-array-find-from-last
# sec-%typedarray%.prototype.findlastindex

Browser compatibility

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See also