Reversing vowels in a string JavaScript

We are required to write a JavaScript function that takes a string as input and reverse only the vowels of a string.

Problem Example

If the input string is:

const str = 'Hello';

Then the output should be:

'Holle'

Notice how 'e' and 'o' swap positions while consonants 'H', 'l', 'l' remain in their original places.

Solution Using Two Pointers

The most efficient approach uses two pointers from opposite ends of the string:

const str = 'Hello';

const reverseVowels = (str = '') => {
    const vowels = new Set(['a','e','i','o','u','A','E','I','O','U']);
    let left = 0, right = str.length - 1;
    let foundLeft = false, foundRight = false;
    str = str.split("");
    
    while(left 

Holle


How It Works

The algorithm uses a two-pointer technique:


    
Convert string to array for easy character swapping
    
Use a Set to store all vowels (both lowercase and uppercase) for O(1) lookup
    
Move left pointer forward until it finds a vowel
    
Move right pointer backward until it finds a vowel
    
Swap the vowels when both pointers find them
    
Continue until pointers meet


Testing with More Examples

const reverseVowels = (str = '') => {
    const vowels = new Set(['a','e','i','o','u','A','E','I','O','U']);
    let left = 0, right = str.length - 1;
    let foundLeft = false, foundRight = false;
    str = str.split("");
    
    while(left 

JAvAScritp
uoiea
bcdfg
UoIeA


Conclusion

This two-pointer approach efficiently reverses vowels in O(n) time complexity. The algorithm preserves consonant positions while only swapping vowels from opposite ends of the string.