JavaScriptStringsBeginner

String.includes()

Checks if a string contains another string (true/false).

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string.includes(searchString, position?)
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What it does

Overview

Checks if a string contains another string (true/false).

String.prototype.includes() is a case-sensitive method that determines whether a string contains a specific sequence of characters, returning a boolean value. It follows the SameValueZero algorithm for comparison, which for string primitives is functionally equivalent to strict equality of characters. While more readable than the traditional 'indexOf() !== -1' pattern, it does not support regular expressions; attempting to pass a RegExp object as the search string will throw a TypeError. Performance is generally O(n) relative to the length of the string, though modern engines use highly optimized search algorithms like Boyer-Moore or Horspool. Note that searching for an empty string ('') always returns true, even if the starting position exceeds the string length. This method was introduced in ES6 (ECMAScript 2015) and is not supported in Internet Explorer.

Quick reference

Syntax

string.includes(searchString, position?)

Inputs

Parameters

searchStringstring · Substring to look for.
position (optional)number · Index to start searching from.

See it in practice

Examples

1

Basic Substring Detection

const sentence = 'The quick brown fox jumps over the lazy dog.';
const word = 'fox';

console.log(sentence.includes(word));
Output:
true

Determines if the substring 'fox' exists anywhere within the target string.

2

Searching with a Start Position

const text = 'To be, or not to be, that is the question.';

// Check if 'To' exists starting from index 1
console.log(text.includes('To', 1));
Output:
false

The second argument specifies the index to begin the search. Since 'To' starts at index 0, searching from index 1 returns false.

3

Case-Insensitive Validation

const email = 'User@Example.com';
const search = 'user';

const isValid = email.toLowerCase().includes(search.toLowerCase());
console.log(isValid);
Output:
true

Because includes() is case-sensitive, both strings must be normalized (e.g., to lowercase) to perform a case-insensitive match.

Debug faster

Common Errors

1

TypeError

Cause: Passing a Regular Expression as the first argument instead of a string.

Fix: Use a plain string for .includes() or use RegExp.prototype.test() if pattern matching is required.

'hello world'.includes(/hello/);
2

LogicError

Cause: Assuming the method is case-insensitive, leading to missed matches.

Fix: Always normalize both the source and the search term using .toLowerCase() or .toUpperCase().

const status = 'Active';
if (status.includes('active')) { /* Will not execute */ }

Runtime support

Compatibility

Node.js, BrowserAll modern browsers, IE not supportedES6 (2015)

Source: MDN Web Docs

Common questions

Frequently Asked Questions

Checks if a string contains another string (true/false).

searchString: Substring to look for. position: Index to start searching from.

TypeError: Use a plain string for .includes() or use RegExp.prototype.test() if pattern matching is required. LogicError: Always normalize both the source and the search term using .toLowerCase() or .toUpperCase().