check how many times a substring appears in a string

1a = '128532012'
2print(a.count('0'))
3#Prints: 1
4
5print(a.count('12'))
6#Prints: 2
7

1function letterCount(string, letter, caseSensitive) {
2    var count = 0;
3    if ( !caseSensitive) {
4        string = string.toUpperCase();
5        letter = letter.toUpperCase();
6    }
7    for (var i=0, l=string.length; i<string.length; i += 1) {
8        if (string[i] === letter) {
9            count += 1;
10        }
11    }
12    return count;
13}
14
15var exampleString = 'I will practice survival skills';
16console.log(letterCount(exampleString, 'i', true)); // 4 (doesn't match uppercase I at beginning)
17console.log(letterCount(exampleString, 'i', false)); 5 (matches both upper and lowercase i)
18