optional.js

/*
   Optionally load javascripts when some condition is met.  This code
   ensures the scripts are loaded sequencially to avoid dependency
   problems.

   In this case: when an element with id "yelp" exists load jquery and
   execute some javascript (depending on jquery) to replace the
   innerHtml of that element with "YELP".
*/

if (document.getElementById("yelp")) {
  (function(scripts) {
    var f = function(scripts) {
      var e = document.createElement("script");
      e.type = "text/javascript";
      if (scripts[0].src) e.src = scripts[0].src;
      if (scripts[0].text) e.text = scripts[0].text;
      e.onload = e.onreadystatechange = function() {
        if (this.done || (this.readyState && this.readyState != "loaded" && this.readyState != "complete")) return;
        this.done = true;
        if (scripts.length > 1) f(scripts.slice(1));
      };

      var s = document.getElementsByTagName("script")[0];
      s.parentNode.insertBefore(e, s);
    };
    f(scripts);
  })([{src: "jquery.js"},
      {text: "$('#yelp').html('YELP')"}]);
}