Array.prototype.findIndex()

The findIndex() method returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1, indicating that no element passed the test.

See also the find() method, which returns the value of an array element, instead of its index.

Syntax

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

// Callback function
findIndex(callbackFn)
findIndex(callbackFn, thisArg)

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

Parameters

callbackFn

A function to execute on each value in the array until the function returns true, indicating that the satisfying element was found.

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

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.

Note: if the index of the first element in the array that passes the test is 0, the return value of findIndex will be interpreted as Falsy in conditional statements.

Description

The findIndex() method executes the callbackFn function once for every index in the array 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).

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(num) {
  for (let i = 2; num > i; i++) {
    if (num % i == 0) {
      return false;
    }
  }
  return num > 1;
}

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

Specifications

Specification
ECMAScript Language Specification
# sec-array.prototype.findindex

Browser compatibility

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See also