JavaScript How to Convert String to Uppercase

Use the toUpperCase() method on a string to convert it to uppercase, like str.toUpperCase().

Examples

Inputhello

OutputHELLO

InputJavaScript

OutputJAVASCRIPT

Input123abc!

Output123ABC!

How to Think About It

To convert a string to uppercase, think of changing every letter to its capital form. The toUpperCase() method does this by scanning the string and replacing each lowercase letter with its uppercase version, leaving other characters unchanged.

Algorithm

1

Get the input string.

2

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

3

Return the new uppercase string.

Code

javascript

const str = 'Hello World!';
const upperStr = str.toUpperCase();
console.log(upperStr);

Output

HELLO WORLD!

Dry Run

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

1

Original string

str = 'Hello World!'

2

Convert to uppercase

upperStr = str.toUpperCase() -> 'HELLO WORLD!'

3

Print result

console.log(upperStr) outputs 'HELLO WORLD!'

Step	String Value
1	Hello World!
2	HELLO WORLD!
3	HELLO WORLD!

Why This Works

Step 1: Using toUpperCase()

The toUpperCase() method creates a new string with all letters converted to uppercase.

Step 2: Original string unchanged

The original string remains the same because strings are immutable in JavaScript.

Step 3: Non-letter characters

Characters like numbers and symbols are not changed by toUpperCase().

Alternative Approaches

Using a loop and manual conversion

javascript

const str = 'Hello';
let result = '';
for (const char of str) {
  const upperChar = char >= 'a' && char <= 'z' ? String.fromCharCode(char.charCodeAt(0) - 32) : char;
  result += upperChar;
}
console.log(result);

This manually converts letters but is longer and less readable than <code>toUpperCase()</code>.

Using locale-aware toLocaleUpperCase()

javascript

const str = 'i';
console.log(str.toLocaleUpperCase('tr'));

Handles locale-specific uppercase rules, useful for languages like Turkish.

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 uppercase result, so space also grows linearly.

Which Approach is Fastest?

toUpperCase() is optimized and fastest for general use; manual loops are slower and more error-prone.

Approach	Time	Space	Best For
toUpperCase()	O(n)	O(n)	Simple and fast uppercase conversion
Manual loop conversion	O(n)	O(n)	Learning or custom logic but slower
toLocaleUpperCase()	O(n)	O(n)	Locale-specific uppercase rules