JavaScript Array findIndex() Method

This article will help us to understand all the facts as well as required information in detail along with the implementation of the Array.findIndex() Method.

Let us start with an example of the method Array findIndex() which is used to find the first index of positive number from an array.

• Example:

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.
    document.write(found);
</script>

• Output:

2

The 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 user while calling). Otherwise, if no data is found then 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 the current element belongs to.
• thisValue: This parameter is optional, if a value 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. Below examples illustrate the Array findIndex() function in JavaScript:

• 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.

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

print([4, 6, 8, 12].findIndex(isOdd)); 

• Output:

-1

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

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

print([4, 6, 7, 12].findIndex(isOdd)); 

• Output:

2

Codes for the above function are defined as follows: 

Program 1:

html

<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
    document.write(array.findIndex(finding_index));
</script>

Output:

2

Program 2:

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.
    document.write([4, 6, 8, 12].findIndex(isPrime));
    document.write("<br />")
    // Printing 2 the index of prime number (7) found.
    document.write([4, 6, 7, 12].findIndex(isPrime));
</script>

Output:

-1
2

Example-3: In this example we will use arrow functions to find out the index of an element from an array.

Javascript

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]);

Output:

2
mango

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