Write a function countVowels that takes a string as input and returns the number of vowels in that string. Vowels include both lowercase and uppercase characters: 'a', 'e', 'i', 'o', 'u' and 'A', 'E', 'I', 'O', 'U'.
Example Inputs & Outputs
countVowels("hello") // → 2 (e, o)
countVowels("JavaScript") // → 3 (a, a, i)
countVowels("bcd") // → 0
countVowels("AEIOU") // → 5
countVowels("") // → 0
Constraints & Edge Cases
- Input will always be a string.
- String may contain spaces, punctuation, or numbers — these are not vowels.
- Function should be case-insensitive (handle both uppercase and lowercase).
- An empty string should return
0.
