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

callbackFn

A function used to test elements in the array.

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.

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.

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.

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.

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)

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

• Polyfill of Array.prototype.findIndex in core-js
• Array.prototype.find()
• Array.prototype.indexOf()