The JavaScript string.indexOf() method

Q: What does indexOf() return if the substring is not found?

-1. So if (str.indexOf("x") === -1) means the substring is absent.

Q: What is the difference between indexOf() and includes()?

indexOf() returns the position (or -1); includes() returns a boolean. Use includes() for presence checks.

Q: How do I find the last occurrence of a substring?

Use lastIndexOf(), which searches from the end of the string backward.

Method JavaScript All modern browsers Updated June 23, 2026

Quick answer

The indexOf() method returns the index of the first occurrence of a substring, or -1 if it isn't found. "hello".indexOf("l") is 2. When you only need to know whether the text is present, includes() reads more clearly.

Overview

indexOf() finds where a substring first appears, counting from 0, and returns -1 if it's not there at all. That -1 is the value you test against. It's case-sensitive, like its string cousins.

A second argument lets you start the search from a given position, which is the trick for finding the next match after one you've already located — call it in a loop, each time starting just past the last hit. To search from the end backward, there's lastIndexOf().

Since includes() arrived, a lot of indexOf() usage has shifted to it for simple presence checks, because includes() returns a boolean directly. Keep indexOf() for when you actually need the position — for example to slice() the string at that point.

Syntax

const i = str.indexOf(searchString)
const i = str.indexOf(searchString, fromIndex)

Parameters

The string.indexOf() method accepts the following parameters.

Parameter	Description
`searchString`	The substring to find. Case-sensitive.
`fromIndex`	Optional. The index to start searching from. Defaults to `0`.

Example

Live example

<pre id="out" style="font:15px ui-monospace,monospace"></pre>
<script>
  const email = 'user@example.com';

  const at     = email.indexOf('@');
  const domain = email.slice(at + 1);

  document.getElementById('out').textContent =
    '@ at index: ' + at + '\n' +
    'domain:     ' + domain; // @ at index: 4 / domain: example.com
</script>

Best practices

Test against -1 for "not found" — that's the signal value.
Use indexOf() when you need the position; use includes() for a yes/no check.
Pass a fromIndex to find later occurrences in a loop.
Pair it with slice() to cut a string at the match position.

Frequently asked questions

What does indexOf() return if the substring is not found?

-1. So if (str.indexOf("x") === -1) means the substring is absent.

What is the difference between indexOf() and includes()?

indexOf() returns the position (or -1); includes() returns a boolean. Use includes() for presence checks.

Is string indexOf() case-sensitive?

Yes. Lowercase both strings first if you need a case-insensitive search.

How do I find the last occurrence of a substring?

Use lastIndexOf(), which searches from the end of the string backward.

string.includes() string.localeCompare()

References