Check Element is in Viewport or not

isOnScreen.js

$.fn.isOnScreen = function(){
  var win = $(window);
  var viewport = {
      top : win.scrollTop(),
      left : win.scrollLeft()
  };
  viewport.right = viewport.left + win.width();
  viewport.bottom = viewport.top + win.height();
  var bounds = this.offset();
  bounds.right = bounds.left + this.outerWidth();
  bounds.bottom = bounds.top + this.outerHeight();
  return (!(viewport.right < bounds.left || viewport.left > bounds.right || viewport.bottom < bounds.top || viewport.top > bounds.bottom));
};

$(document).ready(function(){
  $(window).scroll(function(){
      if ($('#inViewport').isOnScreen()) {
          // The element is visible, do something
          alert("in viewport!");
      } else {
          // The element is NOT visible, do something else
      }
  });
});