Array.prototype.findLastIndex()

The findLastIndex() method of Array instances iterates the 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.

See also the findLast() method, which returns the value of last element that satisfies the testing function (rather than its index).

Syntax

findLastIndex(callbackFn)
findLastIndex(callbackFn, thisArg)

Parameters

callbackFn
- : A function to execute for each element in the 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 array.
  - index
    - : The index of the current element being processed in the array.
  - array
    - : The array findLastIndex() was called upon.
thisArg
- : A value to use as this when executing callbackFn. See iterative methods.

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 is an iterative method. It calls a provided callbackFn function once for each element in an array in descending-index order, until callbackFn returns a truthy value. findLastIndex() then returns the index of that element and stops iterating through the array. If callbackFn never returns a truthy value, findLastIndex() returns -1.

callbackFn is invoked for every index of the array, not just those with assigned values. Empty slots in sparse arrays behave the same as undefined.

findLastIndex() does not mutate the array on which it is called, but the function provided as callbackFn can. Note, however, that the length of the array is saved before the first invocation of callbackFn. Therefore:

callbackFn will not visit any elements added beyond the array's initial length when the call to findLastIndex() began.
Changes to already-visited indexes do not cause callbackFn to be invoked on them again.
If an existing, yet-unvisited element of the array is changed by callbackFn, its value passed to the callbackFn will be the value at the time that element gets visited. Deleted elements are visited as if they were undefined.

Warning: Concurrent modifications of the kind described above frequently lead to hard-to-understand code and are generally to be avoided (except in special cases).

The findLastIndex() method is generic. It only expects the this value to have a length property and integer-keyed properties.

Examples

Find the index of the last prime number in an array

The following example returns the index of the last element in the array that is a prime number, or -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;
}

console.log([4, 6, 8, 12].findLastIndex(isPrime)); // -1, not found
console.log([4, 5, 7, 8, 9, 11, 12].findLastIndex(isPrime)); // 5

Using findLastIndex() on sparse arrays

You can search for undefined in a sparse array and get the index of an empty slot.

console.log([1, , 3].findLastIndex((x) => x === undefined)); // 1

Calling findLastIndex() on non-array objects

The findLastIndex() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length.

const arrayLike = {
  length: 3,
  0: 2,
  1: 7.3,
  2: 4,
  3: 3, // ignored by findLastIndex() since length is 3
};
console.log(
  Array.prototype.findLastIndex.call(arrayLike, (x) => Number.isInteger(x)),
); // 2