JavaScript Array indexOf() Method

The JavaScript Array indexOf() method returns the position of the first occurrence of a value in a string. If the value is not found, it returns -1. This method provides a simple way to determine the position of specific characters or substrings within strings.

Syntax:

array.indexOf(element, start)

Parameters:

• element: This parameter holds the element whose index will be returned.
• start: This parameter is optional and holds the starting point of the array, where the default value is 0 to begin the search.

Return value:

This method returns the index of the first occurrence of the element. If the element cannot be found in the array, then this method returns -1.

Example 1: Finding Index of Element in Array

This code demonstrates the use of the indexOf() method to find the index of the element “gfg” in the array name. The index of “gfg” is stored in the variable a and then logged to the console.

let name = ['gfg', 'cse', 'geeks', 'portal'];
a = name.indexOf('gfg')
// Printing result of method
console.log(a)

0

Example 2: Searching Element in Array

This code demonstrates the use of the indexOf() method to find the index of a specific element (2) in an array (A). It returns the index of the first occurrence of the element in the array (1 in this case). If the element is not found, it returns -1.

// Taking input as an array A
// having some elements.
let A = [1, 2, 3, 4, 5];

// indexOf() method is called to
// test whether the searching element
// is present in given array or not.
a = A.indexOf(2)

// Printing result of method.
console.log(a);

1

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

Supported Browsers:

• Google Chrome
• Edge 
• Firefox
• Opera
• Safari

JavaScript Array indexOf() Method- FAQs

How does the indexOf() method work?

The indexOf() method searches the array for the specified element, starting from the beginning. It uses strict equality (===) to compare elements and returns the index of the first occurrence of the element. If the element is not found, it returns -1.

How does indexOf() handle NaN values?

The indexOf() method uses strict equality (===) for comparison, which means it cannot find NaN values because NaN is not equal to NaN.

What happens if the fromIndex is out of bounds?

If fromIndex is greater than or equal to the array length, indexOf() returns -1. If fromIndex is negative, it is treated as array.length + fromIndex.

What is the performance of the indexOf() method?

The indexOf() method only finds the index of a specific element, while findIndex() can be used to find the index of an element that meets a certain condition defined by a function.

What are some common use cases for the indexOf() method?

Common use cases for the indexOf() method include:

• Finding the position of a specific element in an array.
• Checking if an element exists in an array by comparing the returned index to -1.
• Implementing functionality that requires knowing the first occurrence of an element.

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