Array.prototype.findLast()
Baseline 2022
Newly available
Since August 2022, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers.
The findLast()
method of 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, undefined
is returned.
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
indexOf()
. (It's similar tofindIndex()
, but checks each element for equality with the value instead of using a testing function.) -
whether a value exists in an array, use
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
some()
.
Try it
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:
thisArg
Optional-
A value to use as
this
when executingcallbackFn
. See iterative methods.
Return value
The last (highest-index) element in the array that satisfies the provided testing function; 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 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 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
Specification |
---|
ECMAScript Language Specification # sec-array.prototype.findlast |
Browser compatibility
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