Please help me understand this JavaScript code from "Ajax in Action"

I saw this code in Ajax in Action book and there are two things I'm not able to understand (Keep in mind I just started web programming and I'm still trying to understand how JavaScript works).

1. On line 37 or in function loadXMLDoc, why did the author declared a local variable "var loader=this;" and then used it in the call "net.ContentLoader.onReadyState.call(loader);" instead of just using "net.ContentLoader.onReadyState.call(this);"

2. Why did the author used "net.ContentLoader.onReadyState.call(loader);", instead of "this.onReadyState();"

    /*
    url-loading object and a request queue built on top of it
    */

    /* namespacing object */
    var net=new Object();

    net.READY_STATE_UNINITIALIZED=0;
 开发者_如何学编程   net.READY_STATE_LOADING=1;
    net.READY_STATE_LOADED=2;
    net.READY_STATE_INTERACTIVE=3;
    net.READY_STATE_COMPLETE=4;


    /*--- content loader object for cross-browser requests ---*/
    net.ContentLoader=function(url,onload,onerror,method,params,contentType){
      this.req=null;
      this.onload=onload;
      this.onerror=(onerror) ? onerror : this.defaultError;
      this.loadXMLDoc(url,method,params,contentType);
    }

    net.ContentLoader.prototype.loadXMLDoc=function(url,method,params,contentType){
      if (!method){
        method="GET";
      }
      if (!contentType && method=="POST"){
        contentType='application/x-www-form-urlencoded';
      }
      if (window.XMLHttpRequest){
        this.req=new XMLHttpRequest();
      } else if (window.ActiveXObject){
        this.req=new ActiveXObject("Microsoft.XMLHTTP");
      }
      if (this.req){
        try{
          var loader=this;
          this.req.onreadystatechange=function(){
            net.ContentLoader.onReadyState.call(loader);
          }
          this.req.open(method,url,true);
          if (contentType){
            this.req.setRequestHeader('Content-Type', contentType);
          }
          this.req.send(params);
        }catch (err){
          this.onerror.call(this);
        }
      }
    }


    net.ContentLoader.onReadyState=function(){
      var req=this.req;
      var ready=req.readyState;
      var httpStatus=req.status;
      if (ready==net.READY_STATE_COMPLETE){
        if (httpStatus==200 || httpStatus==0){
          this.onload.call(this);
        }else{
          this.onerror.call(this);
        }
      }
    }

    net.ContentLoader.prototype.defaultError=function(){
      alert("error fetching data!"
        +"\n\nreadyState:"+this.req.readyState
        +"\nstatus: "+this.req.status
        +"\nheaders: "+this.req.getAllResponseHeaders());
    }

A try/catch statement in ECMA-/Javascript creates a new Context. Technically, this is similar to an eval statement and therefore an eval Context.

By calling net.ContentLoader.onReadyState.call(loader); the author explicitly calls the method onReadyState with the context of the loaded object (and that is what this within the callee is referencing then). A callee is a function (-context...) with was called by a caller (-context).

Long story short, ECMAscripts .call() and .apply() methods allow you to set a specific Context for a function when invoked. This is necessary here, because try/catch creates a new Context and the value of this within the called method would be wrong.

While the above statement is true, it's not responsible here. It's not the Context from try / catch which is the problem , it's furthermore the Context by the created anonymous function

this.req.onreadystatechange=function(){
    net.ContentLoader.onReadyState.call(loader);
}

Using this within that anonymous method would "again" reference a different Context. That is why the author cached the value from this in loader and invokes the method with that cached Context.