Call function once when scrolled toa certain position

gistfile1.txt

window.onscroll = myScroll;
var counter = 0; // Global Variable
function myScroll(){
 var val = document.getElementById("value");
 val.innerHTML = 'pageYOffset = ' + window.pageYOffset;
 if(counter == 0){ // if counter is 1, it will not execute
   if(window.pageYOffset > 300){
      alert('You have scrolled to second div');
      counter++; // increment the counter by 1, new value = 1
   }
 }
}