How to Use indexOf in JavaScript: Syntax and Examples

In JavaScript, indexOf is a method used to find the position of a substring in a string or an element in an array. It returns the first index where the item is found or -1 if it is not present.

Syntax

The indexOf method can be used on strings and arrays with the following syntax:

• For strings: string.indexOf(searchValue, fromIndex)
• For arrays: array.indexOf(searchElement, fromIndex)

searchValue/searchElement: The item to find.

fromIndex (optional): The position to start searching from (default is 0).

javascript

const str = 'hello world';
const index = str.indexOf('world', 0);

const arr = [10, 20, 30, 20];
const idx = arr.indexOf(20, 1);

Example

This example shows how to find the position of a substring in a string and an element in an array using indexOf. It also shows what happens when the item is not found.

javascript

const text = 'I love JavaScript';
const pos1 = text.indexOf('love');
const pos2 = text.indexOf('Python');

const numbers = [5, 10, 15, 20];
const pos3 = numbers.indexOf(15);
const pos4 = numbers.indexOf(25);

console.log(pos1);  // 2
console.log(pos2);  // -1
console.log(pos3);  // 2
console.log(pos4);  // -1

Output

2
-1
2
-1

Common Pitfalls

One common mistake is not checking if indexOf returns -1, which means the item was not found. Using the result without this check can cause errors.

Also, indexOf is case-sensitive when used on strings, so searching for 'Love' instead of 'love' will fail.

javascript

const phrase = 'Hello World';

// Wrong: assuming the substring exists
const pos = phrase.indexOf('world');
console.log(pos); // -1 (not found, but might be unexpected)

// Right: check before using
if (phrase.indexOf('world') !== -1) {
  console.log('Found!');
} else {
  console.log('Not found');
}

// Case sensitivity example
console.log(phrase.indexOf('World')); // 6
console.log(phrase.indexOf('world')); // -1

Output

-1
Not found
6
-1

Quick Reference

Method	Description	Returns
string.indexOf(searchValue, fromIndex)	Finds substring position in string	Index or -1 if not found
array.indexOf(searchElement, fromIndex)	Finds element position in array	Index or -1 if not found
fromIndex (optional)	Start searching from this index	Defaults to 0

Key Takeaways

Use indexOf to find the first position of a substring or element in strings and arrays.

If the item is not found, indexOf returns -1.

Always check for -1 before using the result to avoid errors.

Remember that indexOf is case-sensitive when used on strings.

You can specify a starting index to begin the search with the optional second argument.