The indexOf() method in JavaScript is used to find the index of the first occurrence of a specified value within a string. The method returns a 0-based index, making it a fundamental JavaScript string manipulation way.

The JavaScript indexOf() method helps locate a substring within a string and returns its position. This method is case-sensitive, meaning that "Hello".indexOf("hello") will return -1 due to the mismatch in the case.

Syntax

str.indexOf(searchValue , index);

Parameters

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

Return value

• If the searchValue is found, the method returns the index of its first occurrence.
• If the searchValue is not found, the method returns -1.

Example 1: Finding the First Occurrence of a Substring

Here’s a simple example that finds the position of a substring within a string:

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

// Finding index of occurrence of 'Train'
let index = str.indexOf('Train');
console.log(index);

9

Example 2: Using indexOf() to Locate Substrings

The indexOf() method can be used to search for longer substrings:

// JavaScript to illustrate indexOf() method

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

// Finding index of occurrence of 'Train'
let index = str.indexOf('ed Tr');

console.log(index);

6

Example 3: Handling Case Sensitivity in indexOf() Method

The indexOf() method is case-sensitive. If the cases don’t match, the method will return -1:

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

// Finding index of occurrence of 'Train'
let index = str.indexOf('train');

console.log(index);

-1

Example 4: Using an Index to Start the Search

You can specify a starting index from where the search begins. This is useful when you want to skip certain parts of the string:

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

// Finding index of occurrence of 'Train'
let index = str.indexOf('Train');

console.log(index);

9

Supported browsers

• Chrome 1
• Edge 12
• Firefox 1
• Opera 3
• Safari 1

JavaScript String indexOf() Method

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