JavaScript Program to Capitalize First Letter of Each Word

Use str.split(' ').map(word => word.charAt(0).toUpperCase() + word.slice(1)).join(' ') to capitalize the first letter of each word in a string.

Examples

Inputhello world

OutputHello World

InputjavaScript is fun

OutputJavaScript Is Fun

Input multiple spaces here

Output Multiple Spaces Here

How to Think About It

To capitalize the first letter of each word, split the string into words by spaces, then for each word, change the first letter to uppercase and keep the rest as is, finally join the words back with spaces.

Algorithm

1

Get the input string.

2

Split the string into an array of words using spaces.

3

For each word, change the first character to uppercase and keep the rest unchanged.

4

Join all the words back into a single string with spaces.

5

Return the resulting string.

Code

javascript

function capitalizeFirstLetter(str) {
  return str.split(' ').map(word => {
    if (word.length === 0) return word;
    return word.charAt(0).toUpperCase() + word.slice(1);
  }).join(' ');
}

const input = "hello world";
console.log(capitalizeFirstLetter(input));

Output

Hello World

Dry Run

Let's trace the input 'hello world' through the code

1

Split string

Split 'hello world' into ['hello', 'world']

2

Capitalize each word

For 'hello', first letter 'h' to 'H', result 'Hello'; for 'world', 'w' to 'W', result 'World'

3

Join words

Join ['Hello', 'World'] into 'Hello World'

Word	Capitalized Word
hello	Hello
world	World

Why This Works

Step 1: Splitting the string

Splitting the string by spaces creates an array of words to process individually.

Step 2: Capitalizing first letter

For each word, charAt(0).toUpperCase() changes the first letter to uppercase, and slice(1) keeps the rest unchanged.

Step 3: Joining words back

Joining the array with spaces reconstructs the string with capitalized words.

Alternative Approaches

Using regular expression

javascript

function capitalizeWithRegex(str) {
  return str.replace(/\b\w/g, char => char.toUpperCase());
}

console.log(capitalizeWithRegex('hello world'));

This method uses regex to find word boundaries and capitalize the first letter directly, which is concise but may be less clear for beginners.

Using for loop

javascript

function capitalizeWithLoop(str) {
  const words = str.split(' ');
  for (let i = 0; i < words.length; i++) {
    if (words[i].length > 0) {
      words[i] = words[i][0].toUpperCase() + words[i].slice(1);
    }
  }
  return words.join(' ');
}

console.log(capitalizeWithLoop('hello world'));

This approach uses a classic loop for clarity and control, good for learners who prefer explicit iteration.

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

Time Complexity

The program processes each character once during splitting and mapping, so time grows linearly with input size.

Space Complexity

Extra space is used for the array of words and the new string, proportional to input size.

Which Approach is Fastest?

The regex method is usually fastest due to internal optimizations, but the split-map-join method is more readable.

Approach	Time	Space	Best For
Split-Map-Join	O(n)	O(n)	Readability and clarity
Regular Expression	O(n)	O(n)	Concise and fast for simple cases
For Loop	O(n)	O(n)	Explicit control and beginner-friendly