JavaScript Program to Count Words in a String

Use str.trim().split(/\s+/).length in JavaScript to count words in a string by trimming spaces and splitting by whitespace.

Examples

InputHello world

Output2

Input This is a test string.

Output5

Input

Output0

How to Think About It

To count words, first remove extra spaces at the start and end using trim(). Then split the string by spaces or any whitespace using a regular expression like /\s+/. The number of resulting parts is the word count. If the string is empty or only spaces, the count is zero.

Algorithm

1

Get the input string.

2

Remove leading and trailing spaces using trim.

3

Split the trimmed string by one or more whitespace characters.

4

Count the number of parts from the split.

5

If the string is empty after trimming, return zero.

6

Return the count as the number of words.

Code

javascript

function countWords(str) {
  if (!str.trim()) return 0;
  return str.trim().split(/\s+/).length;
}

console.log(countWords('Hello world')); // 2
console.log(countWords('  This is a test string.  ')); // 5
console.log(countWords('')); // 0

Output

2
5
0

Dry Run

Let's trace the input ' This is a test string. ' through the code.

1

Input string

' This is a test string. '

2

Trim spaces

'This is a test string.'

3

Split by whitespace

['This', 'is', 'a', 'test', 'string.']

4

Count words

5

Step	Value
Trimmed string	This is a test string.
Split array	['This', 'is', 'a', 'test', 'string.']
Word count	5

Why This Works

Step 1: Trim the string

Using trim() removes spaces at the start and end so they don't count as words.

Step 2: Split by whitespace

Splitting by /\s+/ divides the string into words separated by any spaces or tabs.

Step 3: Count the parts

The length of the split array is the total number of words in the string.

Alternative Approaches

Using match with regex

javascript

function countWords(str) {
  const words = str.match(/\b\w+\b/g);
  return words ? words.length : 0;
}

console.log(countWords('Hello world')); // 2

This method uses regex to find word boundaries and counts matches; it handles punctuation better but may be slower.

Manual loop counting

javascript

function countWords(str) {
  let count = 0;
  let inWord = false;
  for (const char of str) {
    if (char.trim() !== '') {
      if (!inWord) {
        count++;
        inWord = true;
      }
    } else {
      inWord = false;
    }
  }
  return count;
}

console.log(countWords('Hello world')); // 2

This approach counts words by checking characters one by one; it's more manual but avoids splitting.

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

Time Complexity

The program scans the string once to trim and split, so time grows linearly with string length.

Space Complexity

Splitting creates an array of words, so space grows with the number of words.

Which Approach is Fastest?

Splitting by whitespace is simple and fast for most cases; regex match is more flexible but slightly slower; manual counting uses less memory but more code.

Approach	Time	Space	Best For
Split by whitespace	O(n)	O(n)	Simple and fast word count
Regex match	O(n)	O(n)	Handles punctuation better
Manual loop	O(n)	O(1)	Memory efficient, more control