How to check if item is scroll to on the screen

scroll-track.js

function scroll_item() {
  var tracked_item = $(".list_item").last(); // grabs the last "item" with the class list_item

  // $(window).outerHeight()  is the height of the browser window
  // tracked_item.offset().top is the vertical amount of pixels from(below) the top of the web page document

  var scrollPosition = tracked_item.offset().top - $(window).outerHeight(); // figures out how many pixels below the
  // initial browser window is the tracked item. So if the window is 700px vertically and the item that is being tracked
  // is 1200px from the very top of the document.. then the user needs to scroll 500 more pixels to reach the
  // tracked item.

  $(window).scroll(function(event) {
    // applies an event listener to the document window
    // $(window).scrollTop() is how many pixels that are hidden(above) the element ( window in this case )
    if (scrollPosition > $(window).scrollTop()) {
      // checks if there are more pixels left to scroll(scrollPosition)
      // then what has been scrolled already($(window).scrollTop())
      return; // since the amount left is greater then what has been scrolled... do nothing, tracked_item isn't visible yet.
    }
    // finally the amount of pixels scrolled ($(window).scrollTop()) is larger then the amount needed to scroll
    // to the tracked_item.
    $(this).off(event); // temporarily remove this scroll event from the window.
    add_to_items(); // lazy load and append new item. If desired, at the end of the add_to_items make
    // sure to re-call scroll_item() to reattached the event listener to the window.
  });
}