on demand loading js

js_loader.js

JsLoader = function(srcMap){
 this.srcMap = {};
 for(key in srcMap){
   this.srcMap[key] = {state: "unload",src: srcMap[key],cb: []}
 }
};
JsLoader.prototype = {
  load: function(m,cb){
    var that = this;
    if(!this.srcMap[m]){
      throw m + " was not found";
    }
    if(this.srcMap[m].state == "unload"){
      var fjs = document.getElementsByTagName("script")[0]
      var script = document.createElement("script");
      this.srcMap[m].state = "loading"
      that.srcMap[m].cb.push(cb);
      script.onload = function(){
        that.srcMap[m].state = "loaded"
        while(that.srcMap[m].cb.length > 0){
          that.srcMap[m].cb[0]();
          that.srcMap[m].cb.shift()
        }
      }
      script.onreadystatechange = function(){
        if (script.readyState == "loaded" || script.readyState=="complete"){
          that.srcMap[m].state = "loaded"
          while(that.srcMap[m].cb.length > 0){
            that.srcMap[m].cb[0]();
            that.srcMap[m].cb.shift()
          }
        }
      };
      script.src = this.srcMap[m].src;
      fjs.parentNode.insertBefore(script,fjs);
    }else if(this.srcMap[m].state == "loading"){
      this.srcMap[key].cb.push(cb);
    }else{
      cb()
    }
  }
}