JavaScript Array findIndex() Method

The Javascript Array.findIndex() method is used to return the first index of the element in a given array that satisfies the provided testing function (passed in by the user while calling). Otherwise, if no data is found then the value of -1 is returned.

• It does not execute the method once it finds an element satisfying the testing method.
• It does not change the original array.

Syntax:

array.findIndex(function(currentValue, index, arr), thisValue)

Parameters: This method accepts five parameters as mentioned above and described below:

• function: It is the function of the array that works on each element.
• currentValue: This parameter holds the current element.
• index: It is an optional parameter that holds the index of the current element.
• arr: It is an optional parameter that holds the array object to which the current element belongs.
• thisValue: This parameter is optional if a value is to be passed to the function to be used as its “this” value else the value “undefined” will be passed as its “this” value.

Return value: It returns the array element index if any of the elements in the array pass the test, otherwise, it returns -1.

Example 1: In this example, the method findIndex() finds all the indices that contain odd numbers. Since no odd numbers are present therefore it returns -1.

Javascript

<script>
    function isOdd(element, index, array) {
      return (element%2 == 1);
    }

    console.log(([4, 6, 8, 12].findIndex(isOdd)));
</script>

Output:

-1

Example 2: In this example, the method findIndex() finds all the indices that contain odd numbers. Since 7 is an odd number therefore it returns its index.

Javascript

<script>
    function isOdd(element, index, array) {
      return (element%2 == 1);
    }

    console.log(([4, 6, 7, 12].findIndex(isOdd)));
</script>

Output:

2

Example 3: In this example, the method findIndex() finds all the indices that contain positive numbers. Since 0.30 is a positive number therefore it returns its index.

JavaScript

<script>
    // input array contain some elements.
    var array = [-10, -0.20, 0.30, -40, -50];

    // function (return element > 0).
    var found = array.findIndex(function (element) {
        return element > 0;
    });

    // Printing desired values.
    console.log(found);
</script>

Output:

2

Example 4: In this example, the method findIndex() finds all the indices that contain numbers greater than 25. Since 30 is greater than 25 therefore it returns its index.

Javascript

<script>
    // Input array contain elements
    var array = [10, 20, 30, 110, 60];

    // Testing method (element > 25).
    function finding_index(element) {
        return element > 25;
    }

    // Printing the index of element which is satisfies
    console.log(array.findIndex(finding_index));
</script>

Output:

2

Example: In this example, the method findIndex() finds all the indices that contain prime numbers.

JavaScript

<script>
    // Number is prime or not prime.
    function isPrime(n) {
        if (n === 1) {
            return false;
        } else if (n === 2) {
            return true;
        } else {
            for (var x = 2; x < n; x++) {
                if (n % x === 0) {
                    return false;
                }
            }
            return true;
        }
    }

    // Printing -1 because prime number is not found.
    console.log([4, 6, 8, 12].findIndex(isPrime));

    // Printing 2 the index of prime number (7) found.
    console.log([4, 6, 7, 12].findIndex(isPrime));
</script>

Output:

-1
 2

Example: In this example, we will use arrow functions to find out the index of an element from an array.

Javascript

<script>
    let fruits_array  = ["apple", "banana", "mango",
                         "banana", "grapes"];
    let searching_index = fruits_array.findIndex
    (fruit => fruit === "mango");
        console.log(searching_index);
        console.log(fruits_array[searching_index]);
</script>

Output:

2
mango

We have a complete list of Javascript Array methods, to check those please go through this Javascript Array Complete reference article.

Supported Browsers: The browsers supported by JavaScript Array findIndex() method are listed below:

• Google Chrome 45
• Microsoft Edge 12.0
• Mozilla Firefox 25.0
• Safari 7.1
• Opera 32.0

We have a Cheat Sheet on Javascript where we covered all the important topics of Javascript to check those please go through Javascript Cheat Sheet-A Basic guide to JavaScript.