JavaScript Puzzle - 1

Courtesy: A Javascript conference video. I forgot the name of the conference. Will update.

var END = 9007199254740992; //Math.pow(2,53)
  var START = END - 100;

var count = 0;
for(var i = START; i<= END; i++){
   count++;
}

alert(count);

What this alerts? And, give me the reason for that.