Array.prototype.findLast()

The findLast() method returns the value of the last element in an array that satisfies the provided testing function. If no elements satisfy the testing function, undefined is returned.

Try it

If you need to find:

  • the first element that matches, use find().
  • the index of the last matching element in the array, use findLastIndex().
  • the index of a value, use Array.prototype.indexOf(). (It's similar to findIndex(), but checks each element for equality with the value instead of using a testing function.)
  • whether a value exists in an array, use Array.prototype.includes(). Again, it checks each element for equality with the value instead of using a testing function.
  • if any element satisfies the provided testing function, use Array.prototype.some().

Syntax

// Arrow function
findLast((element) => { /* … */ } )
findLast((element, index) => { /* … */ } )
findLast((element, index, array) => { /* … */ } )

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

// Inline callback function
findLast(function(element) { /* … */ })
findLast(function(element, index) { /* … */ })
findLast(function(element, index, array){ /* … */ })
findLast(function(element, index, array) { /* … */ }, thisArg)

Parameters

callbackFn

A function used to test elements in the array.

The function is called with the following arguments:

element

The current element in the array.

index

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

array

The array that findLast() was called on.

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

thisArg Optional

Object to use as this when executing callbackFn.

Return value

The value of the element in the array with the highest index value that satisfies the provided testing function; undefined if no matching element is found.

Description

The findLast() method executes the callbackFn function once for each element of the array in descending-index order until callbackFn returns a truthy value. findLast() then returns the value of that element and stops iterating through the array. If callbackFn never returns a truthy value, findLast() returns undefined.

callbackFn is invoked for every index of the array, not just those with assigned values. This means it may be less efficient for sparse arrays, compared to methods that only visit assigned values.

If a thisArg parameter is provided to findLast(), it will be used as the this value inside each invocation of the callbackFn. If it is not provided, then undefined is used.

The findLast() method does not mutate the array on which it is called, but the function provided to callbackFn can. The elements processed by findLast() are set before the first invocation of callbackFn. Therefore:

  • callbackFn will not visit any elements added to the array after the call to findLast() begins.
  • Elements that are assigned to indexes that have already been visited will not be revisited by callbackFn.
  • Elements that are assigned to indexes outside the range, will not be visited by callbackFn.
  • 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 findLast() visits that element's index.
  • Elements that are deleted are still visited.

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 last object in an array matching on element properties

This example shows how you might create a test based on the properties of array elements.

const inventory = [
  { name: 'apples', quantity: 2 },
  { name: 'bananas', quantity: 0 },
  { name: 'fish', quantity: 1 },
  { name: 'cherries', quantity: 5 }
];

// return true inventory stock is low
function isNotEnough(item) {
  return item.quantity < 2;
}

console.log(inventory.findLast(isNotEnough));
// { name: "fish", quantity: 1 }

Using arrow function and destructuring

The previous example might be written using an arrow function and object destructuring:

const inventory = [
  { name: 'apples', quantity: 2 },
  { name: 'bananas', quantity: 0 },
  { name: 'fish', quantity: 1 },
  { name: 'cherries', quantity: 5 },
];

const result = inventory.findLast(({ quantity }) => quantity < 2);

console.log(result);
// { name: "fish", quantity: 1 }

Find last prime number in an array

The following example finds the last element in the array that is a prime number (or returns undefined 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].findLast(isPrime)); // undefined, not found
console.log([4, 5, 7, 8, 9, 11, 12].findLast(isPrime)); // 11

Nonexistent and deleted elements are visited

The following examples show that nonexistent and deleted elements are visited, and that the value passed to the callback is their value when visited:

// Declare array with no elements at indexes 2, 3, and 4
const array = [0, 1, , , , 5, 6];

// Shows all indexes, not just those with assigned values
array.findLast((value, index) => {
  console.log(`Visited index ${index} with value ${value}`);
});

// Shows all indexes, including deleted
array.findLast((value, index) => {
  // Delete element 5 on first iteration
  if (index === 6) {
    console.log(`Deleting array[5] with value ${array[5]}`);
    delete array[5];
  }
  // Element 5 is still visited even though deleted
  console.log(`Visited index ${index} with value ${value}`);
});
// expected output:
// > "Visited index 6 with value 6"
// > "Visited index 5 with value 5"
// > "Visited index 4 with value undefined"
// > "Visited index 3 with value undefined"
// > "Visited index 2 with value undefined"
// > "Visited index 1 with value 1"
// > "Visited index 0 with value 0"
// > "Deleting array[5] with value 5"
// > "Visited index 6 with value 6"
// > "Visited index 5 with value undefined"
// > "Visited index 4 with value undefined"
// > "Visited index 3 with value undefined"
// > "Visited index 2 with value undefined"
// > "Visited index 1 with value 1"
// > "Visited index 0 with value 0"

Specifications

Specification
Proposal-array-find-from-last
# sec-array.prototype.findlast

Browser compatibility

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See also