Fizzbuzz game with for loop

I'm trying to do a function that'll print Fizz, Buzz or Fizzbuzz depending on the number can be divided by 3, 5 or both. (this is what I mean: http://ift.tt/YjQuKK)

This is how far I got before I got stuck.

 var fizzbuzz = function(start,stop) {
    for (var x=1;x <= stop; x++)
        var string =',';
    if (x%3 == 0) {
            string += 'Fizz';
    }
    if (x%5 ==  0){
        string += 'Buzz';
    }
    if (x%5 && x%3){
        string += 'Fizzbuzz';
    }
    return string;
};

I'm getting the answer "," and I'm not sure why.