Number of occurrence

Return the number of occurrence of a certain letter in the given string.

Sample Code:

function countLetters(str,letter) {
// Enter your logic here...
}

Examples:

Input: 'Javascript','a'
Output: 2
Explanation: 2 a's to be found.

Input: 'I put the pro in procrastinate',' '
Output: 5
Explanation: 5 spaces to be found.