The findIndex() method 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.

Try it

See also the find() method, which returns the first element that satisfies the testing function (rather than its index).


// Arrow function
findIndex((element) => { /* … */ } )
findIndex((element, index) => { /* … */ } )
findIndex((element, index, array) => { /* … */ } )

// Callback function
findIndex(callbackFn, thisArg)

// Inline callback function
findIndex(function(element) { /* … */ })
findIndex(function(element, index) { /* … */ })
findIndex(function(element, index, array){ /* … */ })
findIndex(function(element, index, array) { /* … */ }, thisArg)



A function used to test elements in the array.

The function is called with the following arguments:


The current element being processed in the array.


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


The array findIndex() was called upon.

The callback must return a truthy value to indicate an appropriate element has been found. The index of this element is then returned by findIndex().

thisArg Optional

Optional object to use as this when executing callbackFn.

Return value

The index of the first element in the array that passes the test. Otherwise, -1.


The findIndex() method executes the callbackFn function once for every index in the array, in ascending order, until it finds the one where callbackFn returns a truthy value.

If such an element is found, findIndex() immediately returns the element's index. If callbackFn never returns a truthy value (or the array's length is 0), findIndex() returns -1.

Note: Unlike other array methods such as Array.some(), callbackFn is run even for indexes with unassigned values.

callbackFn is invoked with three arguments:

  1. The value of the element
  2. The index of the element
  3. The Array object being traversed

If a thisArg parameter is passed to findIndex(), it will be used as the this inside each invocation of the callbackFn. If it is not provided, then undefined is used.

The range of elements processed by findIndex() is set before the first invocation of callbackFn. Elements which are assigned to indexes already visited, or to indexes outside the range, will not be visited by callbackFn. callbackFn will not process the elements appended to the array after the call to findIndex() begins. If an existing, unvisited element of the array is changed by callbackFn, its value passed to the callbackFn will be the value at the time findIndex() visits the element's index. Elements that are deleted are still visited.

Warning: Concurrent modification of the kind described in the previous paragraph frequently leads to hard-to-understand code and is generally to be avoided (except in special cases).


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)

Find index using arrow function

The following example finds the index of a fruit using an arrow function:

const fruits = ["apple", "banana", "cantaloupe", "blueberries", "grapefruit"];

const index = fruits.findIndex((fruit) => fruit === "blueberries");

console.log(index); // 3
console.log(fruits[index]); // blueberries


ECMAScript Language Specification
# sec-array.prototype.findindex

Browser compatibility

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See also