array.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 no element passed the test.

See also the find() method, which returns the value of an array element, instead of that element's 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 the desired element was found. It takes 3 arguments:

element

The current element being processed in the array.

indexOptional

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

arrayOptional

The array findIndex was called upon.

thisArgOptional

Optional. Object to use as this when executing callback.

Return value

An index in the array if an element passes the test; otherwise, -1.

Description

The findIndex method executes the callback function once for every array index 0..length-1 (inclusive) in the array until it finds one where callback returns a truthy value (a value that coerces to true).

If such an element is found, findIndex immediately returns that found element's index. If the callback never returns a truthy value or the array's length is 0, findIndex returns -1. Unlike some other array methods such as Array.some, in sparse arrays the callback is called even for indexes of entries not present in the array.

callback is invoked with 3 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. Elements appended to the array after the call to findIndex begins will not be processed by callback. 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 that findIndex visits that 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 an element in the array that is a prime number, or -1 if there is no prime number.

function isPrime(element, index, array) {
  var 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 don't support Object.defineProperty, it's best not to polyfill Array.prototype methods at all, as you can't make them non-enumerable.

Specifications

Browser compatibilityUpdate compatibility data on GitHub

See also

• Array.prototype.find()
• Array.prototype.indexOf()