How to Use indexOf for Search in JavaScript

Use the indexOf method on a string to find the position of a substring. It returns the index of the first match or -1 if not found.

Syntax

The indexOf method is called on a string and takes the substring you want to find as the first argument. Optionally, you can provide a second argument to specify the starting position for the search.

• string.indexOf(searchValue) - searches from the start.
• string.indexOf(searchValue, fromIndex) - starts searching from fromIndex.

javascript

string.indexOf(searchValue, fromIndex)

Example

This example shows how to find the position of a word inside a sentence. It also shows what happens if the word is not found.

javascript

const sentence = "Hello, welcome to JavaScript programming.";

const position1 = sentence.indexOf("welcome");
console.log(position1); // 7

const position2 = sentence.indexOf("Python");
console.log(position2); // -1

Output

7
-1

Common Pitfalls

One common mistake is to check if indexOf returns true or false. Since it returns a number, you must check if the result is -1 to know if the substring was not found.

Also, remember that indexOf is case-sensitive, so searching for "Welcome" will not find "welcome".

javascript

const text = "JavaScript is fun.";

// Wrong way:
if (text.indexOf("script")) {
  console.log("Found it!");
} else {
  console.log("Not found.");
}

// Right way:
if (text.indexOf("script") !== -1) {
  console.log("Found it!");
} else {
  console.log("Not found.");
}

Output

Not found.
Found it!

Quick Reference

Usage	Description
string.indexOf(substring)	Returns index of first occurrence or -1 if not found
string.indexOf(substring, startIndex)	Starts search at given index
Check result !== -1	To confirm substring exists
Case-sensitive	Search matches exact letter case

Key Takeaways

Use indexOf to find the position of a substring in a string.

indexOf returns -1 if the substring is not found.

Always check if the result is not -1 to confirm a match.

indexOf is case-sensitive; letter case must match exactly.

You can specify a starting index to begin the search.