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The findIndex() method returns the index of the first element in the array that satisfies the provided testing function. Otherwise -1 is returned.

function isBigEnough(element) {
  return element >= 15;
}

[12, 5, 8, 130, 44].findIndex(isBigEnough); 
// index of 4th element in the Array is returned,
// so this will result in '3'

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

Syntax

arr.findIndex(callback[, thisArg])

Parameters

callback
Function to execute on each value in the array, taking three 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. 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 the index for that iteration. 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 three arguments: the value of the element, the index of the element, and the Array object being traversed.

If a thisArg parameter is provided to findIndex, it will be used as the this for each invocation of the callback. If it is not provided, then undefined is used.

findIndex does not mutate the array on which it is called.

The range of elements processed by findIndex is set before the first invocation of callback. Elements that are appended to the array after the call to findIndex begins will not be visited by callback. If an existing, unvisited element of the array is changed by callback, its value passed to the visiting callback will be the value at the time that findIndex visits that element's index; elements that are deleted are not visited.

Examples

Find the index of a prime number in an array

The following example finds the index of an element in the array that is a prime number (or returns -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;
    }
  }
  return element > 1;
}

console.log([4, 6, 8, 12].findIndex(isPrime)); // -1, not found
console.log([4, 6, 7, 12].findIndex(isPrime)); // 2

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;
    }
  });
}

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

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Array.prototype.findIndex' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Array.prototype.findIndex' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic Support45 (Yes)25 No327.1
FeatureAndroidChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic Support (Yes) (Yes) (Yes)1 No (Yes)8

See also