JavaScript String indexOf() Method

The JavaScript str.indexOf() function finds the index of the first occurrence of the argument string in the given string. The value returned is 0-based.

Syntax:

str.indexOf(searchValue , index)

Parameters: 

• searchValue: The searchValue is the string that is to be searched in the base string. 
• index: The index defines the starting index from where the search value is to be searched in the base string.

Return value: This function returns the index of the string (0-based) where the search value is found for the first time. If the search value cannot be found in the string then the function returns -1.

Example: Below is an example of the String indexOf() Method. 

JavaScript

<script>
    function func() {

        // Original string
        var str = 'Departed Train';

        // Finding index of occurrence of 'Train'
        var index = str.indexOf('Train');
        console.log(index);
    }
    func();
</script>

Output:

9

Examples of the above function are provided below:

Example 1: In this example, the function indexOf() finds the index of the string ed Tr. Since the first and only index where this string is present is 6, therefore this function returns 6 as the answer.

JavaScript

<script>
    // JavaScript to illustrate indexOf() function
    function func() {

        // Original string
        var str = 'Departed Train';

        // Finding index of occurrence of 'Train'
        var index = str.indexOf('ed Tr');
        console.log(index);
    }
    func();
</script>

Output: 

6

Example 2: In this example, the function indexOf() finds the index of the string Train. Since the searchValue is not present in the string, therefore this function returns -1 as the answer.

JavaScript

<script>
    function func() {

        // Original string
        var str = 'Departed Train';

        // Finding index of occurrence of 'Train'
        var index = str.indexOf('train');
        console.log(index);
    }
    func();
</script>

Output: 

-1

Example 3: In this example, the function indexOf() finds the index of the string Train. Since the first index of the searchValue is 9, therefore this function returns 9 as the answer.

JavaScript

<script>
    function func() {

        // Original string
        var str = 'Departed Train before another Train';

        // Finding index of occurrence of 'Train'
        var index = str.indexOf('Train');
        console.log(index);
    }
    func();
</script>

Output: 

9 

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

Supported browser:

• Chrome 1 and above
• Edge 12 and above
• Firefox 1 and above
• Internet Explorer 3 and above
• Opera 3 and above
• Safari 1 and above