Count the number of vowels in a given string

Count the number of vowels appeared in a given string.

Sample Code:

function countVowel(str) {
// Enter your logic here...
}

Examples:

Input: 'Codetisan'
Output: 4
Explanation: 'Codetisan' contains 4 vowels (o, e, i, a)

Input: 'shhhh'
Output: 0
Explanation: 'shhhh' has no vowel.