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 array = [5, 12, 8, 130, 44];const isLargeNumber = (element) => element > 13;console.log(array.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:

element

The current element being processed in the array.

index

The index of the current element being processed in the array.

array

The arrayfindIndex() was called upon.

thisArgOptional

A value to use asthis 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)

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

Browser compatibility

See also

• Polyfill ofArray.prototype.findIndex incore-js
• es-shims polyfill ofArray.prototype.findIndex
• Indexed collections guide
• Array
• Array.prototype.find()
• Array.prototype.findLast()
• Array.prototype.findLastIndex()
• Array.prototype.indexOf()
• Array.prototype.lastIndexOf()
• TypedArray.prototype.findIndex()