Array.prototype.findIndex()

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).

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Syntax

js
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:

element

The current element being processed in the array.

index

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

array

The array findIndex() was called upon.

thisArg Optional

A value to use as this when executing callbackFn. 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.

js
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.

js
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.

js
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.

js
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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See also