Call function on scroll only once

Let's try to solve your problem in javascript as all of the answers here are in jquery. You can use global variable as browser retains its value as long as the page is not refreshed. All those variables declared outside the function are called global variables and can be accessed by any function.

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
     }
   }
  }

#wrapper,#tester {
   width: 300px;
  height: 300px;
  border: 1px solid black;
  padding: 10px;
  }
#wrapper p {
  text-align: center;
  }
#tester {
  border: 1px solid crimson;
  }
#value {
   position: fixed;
  left: auto;
  right: 40px;
  top: 10px;
  }

<p id = "value"></p>
<div id="wrapper">
    <p>scroll down to div to see the alert</p>
</div>
<div id="tester"></div>

Try using .one():

$(window).one('scroll',function() {
   // Stuff
});

Or, unlink the event inside:

$(window).on('scroll',function() {
   // After Stuff
   $(window).off('scroll');
});

Guess you might need this code:

// this will check if element is in viewport

function checkVisible(elm, eval) {
  eval = eval || "object visible";
  var viewportHeight = $(window).height(), // Viewport Height
    scrolltop = $(window).scrollTop(), // Scroll Top
    y = $(elm).offset().top,
    elementHeight = $(elm).height();

  if (eval == "object visible") return y < viewportHeight + scrolltop && y > scrolltop - elementHeight;
  if (eval == "above") return y < viewportHeight + scrolltop;
}

$(window).on("scroll", function () {
  if (checkVisible($("#tester"))) {
    alert("Visible!!!");
    $(window).off("scroll");
  } else {
    // do nothing
  }
});

Fiddle: http://jsfiddle.net/c68nz3q6/