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JavaScript String indexOf() Method

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  • Last Updated : 19 Oct, 2022
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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. The syntax of the function is as follows: 

Syntax:

str.indexOf(searchValue , index)

Arguments: The first argument to the function search value is the string that is to be searched in the base string. The second argument to the function 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.

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

Example:

JavaScript




// JavaScript to illustrate indexOf() function
<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: 

print('Departed Train'.indexOf('Train')); 

Output: In this example, the function indexOf() finds the index of the string Train. Since the first and only index where this string is present is 9, therefore this function returns 9 as the answer.

9

Example 2: 

print('Departed Train'.indexOf('ed Tr')); 

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

6

Example 3: 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.

print('Departed Train'.indexOf('train')); 

Output: 

-1

Example 4: 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.

print('Departed Train before another Train'.indexOf('Train')); 

Output:  

9

The code for the above function is provided below:

Program 1:

JavaScript




// JavaScript to illustrate indexOf() function
<script>
function func() {
 
    // Original string
    var str = 'Departed Train';
 
    // Finding index of occurrence of 'Train'
    var index = str.indexOf('Train');
    console.logdocument.write(index);
}
func();
</script>

Output: 

9

Program 2:

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

Program 3:

JavaScript




// JavaScript to illustrate indexOf() function
<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

Program 4:

JavaScript




// JavaScript to illustrate indexOf() function
<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 

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

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