Count Vowels

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.