JavascriptHow-ToBeginner · 3 min read

How to Check if String Contains Substring in JavaScript

In JavaScript, you can check if a string contains a substring using the includes() method, which returns true if the substring is found and false otherwise. For example, str.includes(substring) checks if substring exists inside str.

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Syntax

The includes() method is called on a string and takes one required argument: the substring you want to find. It returns a boolean value: true if the substring is present, and false if not.

Optional second argument specifies the position to start searching from.

javascript

string.includes(substring, startPosition)

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Example

This example shows how to check if the word "hello" is inside a string. It prints true if found, otherwise false.

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const str = "Hello, world!";
const substring = "hello";

// includes() is case-sensitive
const contains = str.includes(substring);
console.log(contains); // false

// To ignore case, convert both strings to lowercase
const containsIgnoreCase = str.toLowerCase().includes(substring.toLowerCase());
console.log(containsIgnoreCase); // true

Output

false true

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Common Pitfalls

Case sensitivity: includes() is case-sensitive, so "Hello" and "hello" are different.
Older browsers: includes() is not supported in very old browsers; indexOf() can be used instead.
Wrong usage: Using includes() on non-string types will cause errors.

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const str = "Hello, world!";

// Wrong: case-sensitive check
console.log(str.includes("Hello")); // true
console.log(str.includes("hello")); // false

// Right: case-insensitive check
console.log(str.toLowerCase().includes("hello")); // true

// Legacy way using indexOf
console.log(str.indexOf("hello") !== -1); // false
console.log(str.toLowerCase().indexOf("hello") !== -1); // true

Output

true false true false true

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Quick Reference

Method	Description	Returns
includes(substring, startPosition)	Checks if substring exists in string starting at optional position	true or false
indexOf(substring)	Returns index of substring or -1 if not found	number (>=0 or -1)
toLowerCase()	Converts string to lowercase for case-insensitive checks	new lowercase string

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Key Takeaways

Use includes() to check if a string contains a substring easily and clearly.

includes() is case-sensitive; convert strings to lowercase for case-insensitive checks.

For older browsers, use indexOf() and check if the result is not -1.

Always call includes() on a string to avoid errors.

The optional second argument in includes() sets the start position for searching.