While reading through the Ajax in Action book, I came across a code snippet that has left me with a couple of questions. As someone who is new to web programming and still getting to grips with JavaScript, I am hoping for some clarity on the following:
In line 37 or within the function loadXMLDoc, why does the author declare a local variable "var loader=this;" and then use it in the call "net.ContentLoader.onReadyState.call(loader);" instead of simply using "net.ContentLoader.onReadyState.call(this);"
What is the reason behind the author's choice to use "net.ContentLoader.onReadyState.call(loader);" rather than "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;
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());
}