Array.prototype.findIndex()

Summary

The findIndex() method returns an index in the array, if an element in the array satisfies the provided testing function. Otherwise -1 is returned.

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.

Description

The findIndex method executes the callback function once for each element present in the array until it finds one where callback returns a true value. If such an element is found, findIndex immediately returns the index of that element. Otherwise, findIndex returns -1. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.

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

Example: 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

This method has been added to the ECMAScript 6 specification and may not be available in all JavaScript implementations yet. However, you can polyfill Array.prototype.findIndex with the following snippet:

if (!Array.prototype.findIndex) {
  Array.prototype.findIndex = function(predicate) {
    if (this == null) {
      throw new TypeError('Array.prototype.findIndex called on null or undefined');
    }
    if (typeof predicate !== 'function') {
      throw new TypeError('predicate must be a function');
    }
    var list = Object(this);
    var length = list.length >>> 0;
    var thisArg = arguments[1];
    var value;

    for (var i = 0; i < length; i++) {
      value = list[i];
      if (predicate.call(thisArg, value, i, list)) {
        return i;
      }
    }
    return -1;
  };
}

Specifications

Browser compatibility

See also

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