Array.prototype.findIndex()
BaselineWidely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.
ThefindIndex()
method ofArray
instances returns the index of the first element in an array that satisfies the provided testing function.If no elements satisfy the testing function, -1 is returned.
See also thefind()
method, which returns the first element that satisfies the testing function (rather than its index).
Try it
const array1 = [5, 12, 8, 130, 44];const isLargeNumber = (element) => element > 13;console.log(array1.findIndex(isLargeNumber));// Expected output: 3
Syntax
findIndex(callbackFn)findIndex(callbackFn, thisArg)
Parameters
callbackFn
A function to execute for each element in the array. It should return atruthy value to indicate a matching element has been found, and afalsy value otherwise. The function is called with the following arguments:
thisArg
OptionalA value to use as
this
when executingcallbackFn
. Seeiterative methods.
Return value
The index of the first element in the array that passes the test. Otherwise,-1
.
Description
ThefindIndex()
is aniterative method. It calls a providedcallbackFn
function once for each element in an array in ascending-index order, untilcallbackFn
returns atruthy value.findIndex()
then returns the index of that element and stops iterating through the array. IfcallbackFn
never returns a truthy value,findIndex()
returns-1
. Read theiterative methods section for more information about how these methods work in general.
callbackFn
is invoked forevery index of the array, not just those with assigned values. Empty slots insparse arrays behave the same asundefined
.
ThefindIndex()
method isgeneric. It only expects thethis
value to have alength
property and integer-keyed properties.
Examples
Find the index of the first prime number in an array
The following example returns the index of the first element in the array that is a prime number, or-1
if there is no prime number.
function isPrime(n) { if (n < 2) { return false; } if (n % 2 === 0) { return n === 2; } for (let factor = 3; factor * factor <= n; factor += 2) { if (n % factor === 0) { return false; } } return true;}console.log([4, 6, 8, 9, 12].findIndex(isPrime)); // -1, not foundconsole.log([4, 6, 7, 9, 12].findIndex(isPrime)); // 2 (array[2] is 7)
Note:TheisPrime()
implementation is for demonstration only. For a real-world application, you would want to use a heavily memoized algorithm such as theSieve of Eratosthenes to avoid repeated calculations.
Using the third argument of callbackFn
Thearray
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 usesfilter()
to extract the positive values and then usesfindIndex()
to find the first element that is less than its neighbors.
const numbers = [3, -1, 1, 4, 1, 5, 9, 2, 6];const firstTrough = numbers .filter((num) => num > 0) .findIndex((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(firstTrough); // 1
Using findIndex() on sparse arrays
You can search forundefined
in a sparse array and get the index of an empty slot.
console.log([1, , 3].findIndex((x) => x === undefined)); // 1
Calling findIndex() on non-array objects
ThefindIndex()
method reads thelength
property ofthis
and then accesses each property whose key is a nonnegative integer less thanlength
.
const arrayLike = { length: 3, "-1": 0.1, // ignored by findIndex() since -1 < 0 0: 2, 1: 7.3, 2: 4,};console.log( Array.prototype.findIndex.call(arrayLike, (x) => !Number.isInteger(x)),); // 1
Specifications
Specification |
---|
ECMAScript® 2026 Language Specification # sec-array.prototype.findindex |