In this JavaScript tutorial we’re going to learn about the findIndex JavaScript Array Method and how it can be used to find and return the index of the element from a given array which meets the criteria. If it cannot find an element it will return -1

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.

JavaScript Demo: Array.findIndex()

const array1 = [5, 12, 8, 130, 44];

const isLargeNumber = (element) => element > 13;

console.log(array1.findIndex(isLargeNumber));
// expected output: 3

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

Syntax

arr.findIndex(callback( element[, index[, array]] )[, thisArg])

Parameters

callback

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

It takes three arguments:

element

The current element being processed in the array.

index | Optional

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

array | Optional

The array findIndex() was called upon.

thisArg | Optional

Optional object to use as this when executing callback.

Return value

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

Description

The findIndex() method executes the callback function once for every index in the array until it finds the one where callback returns a truthy value (that is, a value that coerces to true).

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

Edge case alert: Unlike other array methods such as Array.some(), callback is run even for indexes with unassigned values.

callback 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 callback. If it is not provided, then undefined is used.

The range of elements processed by findIndex() is set before the first invocation of callback. callback 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 callback, its value passed to the callback will be the value at the time findIndex() visits the element’s index.

Elements that are deleted are still visited.

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(element) {
  let start = 2;
  while (start <= Math.sqrt(element)) {
    if (element % start < 1) {
      return false;
    } else {
      start++;
    }
  }
  return element > 1;
}

console.log([4, 6, 8, 12].findIndex(isPrime)); // -1, not found
console.log([4, 6, 7, 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

Polyfill

// https://tc39.github.io/ecma262/#sec-array.prototype.findindex
if (!Array.prototype.findIndex) {
  Object.defineProperty(Array.prototype, 'findIndex', {
    value: function(predicate) {
     // 1. Let O be ? ToObject(this value).
      if (this == null) {
        throw new TypeError('"this" is null or not defined');
      }

      var o = Object(this);

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0;

      // 3. If IsCallable(predicate) is false, throw a TypeError exception.
      if (typeof predicate !== 'function') {
        throw new TypeError('predicate must be a function');
      }

      // 4. If thisArg was supplied, let T be thisArg; else let T be undefined.
      var thisArg = arguments[1];

      // 5. Let k be 0.
      var k = 0;

      // 6. Repeat, while k < len
      while (k < len) {
        // a. Let Pk be ! ToString(k).
        // b. Let kValue be ? Get(O, Pk).
        // c. Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
        // d. If testResult is true, return k.
        var kValue = o[k];
        if (predicate.call(thisArg, kValue, k, o)) {
          return k;
        }
        // e. Increase k by 1.
        k++;
      }

      // 7. Return -1.
      return -1;
    },
    configurable: true,
    writable: true
  });
}

If you need to support truly obsolete JavaScript engines that do not support Object.defineProperty, it is best not to polyfill Array.prototype methods at all, as you cannot make them non-enumerable.

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