A simple sticky-nav implementation

simple-sticky-nav.js

//http://stackoverflow.com/questions/123999/how-to-tell-if-a-dom-element-is-visible-in-the-current-viewport
function isElementInViewport (el) {
    //special bonus for those using jQuery
    if (typeof jQuery === "function" && el instanceof jQuery) {
        el = el[0];
    }
    var rect = el.getBoundingClientRect();
    return (
        rect.bottom > 0
    );
}

window.onscroll = function(){
  var stickynav = document.getElementById("js-sticky-nav-bar");
  var stickyPastElement = document.getElementsById("elementToBeStickyAfter");
  var visible = isElementInViewport(stickyPastElement);
  if (!visible) {
    stickynav.classList.add("stuck"); //.stuck class has position:fixed, top:0, etc.
  } else {
    stickynav.classList.remove("stuck");
  }
};