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 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 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().
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.
- : The array
- : 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
- : A value to use as
this
when executingcallbackFn
. See iterative methods.
- : A value to use as
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.
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
.
findLast()
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 tofindLast()
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 thecallbackFn
will be the value at the time that element gets visited. Deleted elements are visited as if they wereundefined
.
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 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 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