Array.prototype.findLast()

{{JSRef}}

The findLast() method of {{jsxref("Array")}} instances iterates the 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.

If you need to find:

the first element that matches, use {{jsxref("Array/find", "find()")}} .
the index of the last matching element in the array, use {{jsxref("Array/findLastIndex", "findLastIndex()")}} .
the index of a value, use {{jsxref("Array/indexOf", "indexOf()")}} . (It’s similar to {{jsxref("Array/findIndex", "findIndex()")}} , but checks each element for equality with the value instead of using a testing function.)
whether a value exists in an array, use {{jsxref("Array/includes", "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 {{jsxref("Array/some", "some()")}} .

{{InteractiveExample("JavaScript Demo: Array.findLast()", "shorter")}}

const array1 = [5, 12, 50, 130, 44];

const found = array1.findLast((element) => element > 45);

console.log(found);
// Expected output: 130

Syntax

findLast(callbackFn)
findLast(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 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 array that satisfies the provided testing function; {{jsxref("undefined")}} if no matching element is found.

Description

The findLast() 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. findLast() then returns that element and stops iterating through the array. If callbackFn never returns a truthy value, findLast() returns {{jsxref("undefined")}} . Read the iterative methods section for more information about how these methods work in general.

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.

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

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 the last prime number in an array

The following example returns the last element in the array that is a prime number, or {{jsxref("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

Using the third argument of callbackFn

The array argument is useful if you want to access another element in the array, especially when you don’t have an existing variable that refers to the array. The following example first uses filter() to extract the positive values and then uses findLast() to find the last element that is less than its neighbors.

const numbers = [3, -1, 1, 4, 1, 5, 9, 2, 6];
const lastTrough = numbers
  .filter((num) => num > 0)
  .findLast((num, idx, arr) => {
    // Without the arr argument, there's no way to easily access the
    // intermediate array without saving it to a variable.
    if (idx > 0 && num >= arr[idx - 1]) return false;
    if (idx < arr.length - 1 && num >= arr[idx + 1]) return false;
    return true;
  });
console.log(lastTrough); // 2

Using findLast() on sparse arrays

Empty slots in sparse arrays are visited, and are treated the same as undefined.

// 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}`);
});
// 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

// 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}`);
});
// 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

Calling findLast() on non-array objects

The findLast() 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 findLast() since length is 3
};
console.log(
  Array.prototype.findLast.call(arrayLike, (x) => Number.isInteger(x)),
); // 4

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

docs.rodeo

MDN Web Docs mirror