Array.prototype.findIndex()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.
The findIndex()
method of Array
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 the find()
method, which returns the first element that satisfies the testing function (rather than its index).
Try it
Syntax
findIndex(callbackFn)
findIndex(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 index of the first element in the array that passes the test. Otherwise, -1
.
Description
The findIndex()
is an iterative method. It calls a provided callbackFn
function once for each element in an array in ascending-index order, until callbackFn
returns a truthy value. findIndex()
then returns the index of that element and stops iterating through the array. If callbackFn
never returns a truthy value, findIndex()
returns -1
. 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 findIndex()
method is generic. It only expects the this
value to have a length
property and integer-keyed properties.
Examples
Find the index of a 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(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, 9, 12].findIndex(isPrime)); // -1, not found
console.log([4, 6, 7, 9, 12].findIndex(isPrime)); // 2 (array[2] is 7)
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 findIndex()
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 for undefined
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
The findIndex()
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,
"-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 Language Specification # sec-array.prototype.findindex |
Browser compatibility
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