JavaScript How to Convert String to Lowercase

Use the toLowerCase() method on a string to convert all its characters to lowercase, like str.toLowerCase().

Examples

Input"HELLO"

Output"hello"

Input"JavaScript Is Fun!"

Output"javascript is fun!"

Input"1234!@#"

Output"1234!@#"

How to Think About It

To convert a string to lowercase, think about changing every uppercase letter to its lowercase version while leaving other characters unchanged. The toLowerCase() method does this by scanning the string and replacing uppercase letters with their lowercase counterparts.

Algorithm

1

Get the input string.

2

Call the <code>toLowerCase()</code> method on the string.

3

Return the new string with all letters in lowercase.

Code

javascript

const original = "Hello World!";
const lower = original.toLowerCase();
console.log(lower);

Output

hello world!

Dry Run

Let's trace "Hello World!" through the code

1

Input string

"Hello World!"

2

Apply toLowerCase()

"hello world!"

3

Output result

"hello world!"

Original String	After toLowerCase()
Hello World!	hello world!

Why This Works

Step 1: What toLowerCase() does

The toLowerCase() method creates a new string where all uppercase letters are converted to lowercase.

Step 2: No change to non-letters

Characters that are not letters, like numbers or symbols, stay the same.

Step 3: Returns new string

It does not change the original string but returns a new one with lowercase letters.

Alternative Approaches

Using toLocaleLowerCase()

javascript

const original = "İstanbul";
const lower = original.toLocaleLowerCase();
console.log(lower);

This method respects locale-specific rules, useful for languages with special lowercase rules.

Complexity: O(n) time, O(n) space

Time Complexity

The method processes each character once, so time grows linearly with string length.

Space Complexity

A new string is created to hold the lowercase result, so space also grows linearly.

Which Approach is Fastest?

toLowerCase() is the standard and fastest for most cases; toLocaleLowerCase() is slower but needed for locale-specific conversions.

Approach	Time	Space	Best For
toLowerCase()	O(n)	O(n)	General lowercase conversion
toLocaleLowerCase()	O(n)	O(n)	Locale-aware lowercase conversion