一個比較通用的ajax，以作準確修改

 function AJAXRequest() {
 var xmlPool=[],AJAX=this,ac=arguments.length,av=arguments;
 var xmlVersion=["MSXML2.XMLHTTP","Microsoft.XMLHTTP"];
 var emptyFun=function(){};
 var av=ac>0?typeof(av[0])=="object"?av[0]:{}:{};
 var encode=av.charset?av.charset.toUpperCase()=="UTF-8"?encodeURIComponent:escape:encodeURIComponent;
 this.url=getp(av.url,"");
 this.oncomplete=getp(av.oncomplete,emptyFun);
 this.content=getp(av.content,"");
 this.method=getp(av.method,"POST");
 this.async=getp(av.async,true);
 this.onexception=getp(av.onexception,emptyFun);
 this.ontimeout=getp(av.ontimeout,emptyFun);
 this.timeout=getp(av.timeout,3600000);
 this.onrequeststart=getp(av.onstartrequest,emptyFun);
 this.onrequestend=getp(av.onendrequest,emptyFun);
 if(!getObj()) return false;
 function getp(p,d) { return p?p:d; }
 function getObj() {
  var i,j,tmpObj;
  for(i=0,j=xmlPool.length;i<j;i++) if(xmlPool[i].readyState==0||xmlPool[i].readyState==4) return xmlPool[i];
  try { tmpObj=new XMLHttpRequest; }
  catch(e) {
   for(i=0,j=xmlVersion.length;i<j;i++) {
    try { tmpObj=new ActiveXObject(xmlVersion[i]); } catch(e2) { continue; }
    break;
   }
  }
  if(!tmpObj) return false;
  else { xmlPool[xmlPool.length]=tmpObj; return xmlPool[xmlPool.length-1]; }
 }
 function $(id){return document.getElementById(id);}
 function $N(n,d){n=parseInt(n);return(isNaN(n)?d:n);}
 function $VO(v) {
  if(typeof(v)=="string") {
   if(v=$(v)) return v;
   else return false; }
  else return v;
 }
 function $ST(obj,text) {
  var nn=obj.nodeName.toUpperCase();
  if("INPUT|TEXTAREA".indexOf(nn)>-1) obj.value=text;
  else try{obj.innerHTML=text;} catch(e){};
 }
 function $CB(cb) {
  if(typeof(cb)=="function") return cb;
  else {
   cb=$VO(cb);
   if(cb) return(function(obj){$ST(cb,obj.responseText);});
   else return emptyFun; }
 }
 function send(purl,pc,pcbf,pm,pa) {
  var purl,pc,pcbf,pm,pa,ct,ctf=false,xmlObj=getObj(),ac=arguments.length,av=arguments;
  if(!xmlObj) return false;
  if(!pm||!purl||!pa) return false;
  var ev={url:purl, content:pc, method:pm};
  purl+=(purl.indexOf("?")>-1?"&":"?")+Math.random();
  xmlObj.open(pm,purl,pa);
  AJAX.onrequeststart(ev);
  if(pm=="POST") xmlObj.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
  ct=setTimeout(function(){ctf=true;xmlObj.abort();},AJAX.timeout);
  xmlObj.onreadystatechange=function() {
   if(ctf) { AJAX.ontimeout(ev); AJAX.onrequestend(ev); }
   else if(xmlObj.readyState==4) {
    ev.status=xmlObj.status;
    try{ clearTimeout(ct); } catch(e) {};
    try{ if(xmlObj.status==200) pcbf(xmlObj); else AJAX.onexception(ev); }
    catch(e) { AJAX.onexception(ev); }
    AJAX.onrequestend(ev);
   }
  }
  if(pm=="POST") xmlObj.send(pc); else xmlObj.send("");
 }
 this.setcharset=function(cs) {
  if(cs.toUpperCase()=="UTF-8") encode=encodeURIComponent; else encode=escape;
 }
 this.get=function() {
  var purl,pcbf,ac=arguments.length,av=arguments;
  purl=ac>0?av[0]:this.url;
  pcbf=ac>1?$CB(av[1]):this.oncomplete;
  if(!purl&&!pcbf) return false;
  send(purl,"",pcbf,"GET",true);
 }
 this.update=function() {
  var purl,puo,pinv,pcnt,ac=arguments.length,av=arguments;
  puo=ac>0?$CB(av[0]):emptyFun;
  purl=ac>1?av[1]:this.url;
  pinv=ac>2?$N(av[2],1000):null;
  pcnt=ac>3?$N(av[3],0):null;
  if(pinv) {
   send(purl,"",puo,"GET",true);
   if(pcnt&&--pcnt) {
    var cf=function(cc) {
     send(purl,"",puo,"GET",true);
     if(cc<1) return; else cc--;
     setTimeout(function(){cf(cc);},pinv);
    }
    setTimeout(function(){cf(--pcnt);},pinv);
   }
   else return(setInterval(function(){send(purl,"",puo,"GET",true);},pinv));
  }
  else send(purl,"",puo,"GET",true);
 }
 this.post=function() {
  var purl,pcbf,pc,ac=arguments.length,av=arguments;
  purl=ac>0?av[0]:this.url;
  pc=ac>1?av[1]:"";
  pcbf=ac>2?$CB(av[2]):this.oncomplete;
  if(!purl&&!pcbf) return false;
  send(purl,pc,pcbf,"POST",true);
 }
 this.postf=function() {
  var fo,vaf,pcbf,purl,pc,pm,ac=arguments.length,av=arguments;
  fo=ac>0?$VO(av[0]):false;
  if(!fo||(fo&&fo.nodeName!="FORM")) return false;
  vaf=fo.getAttribute("onsubmit");
  vaf=vaf?(typeof(vaf)=="string"?new Function(vaf):vaf):null;
  if(vaf&&!vaf()) return false;
  pcbf=ac>1?$CB(av[1]):this.oncomplete;
  purl=ac>2?(av[2]!=null?(av[2]!=""?av[2]:fo.action):fo.action):(fo.action?fo.action:this.url);
  pm=ac>3?(av[3]!=null?(av[3]!=""?av[3]:"POST"):"POST"):(fo.method?fo.method.toUpperCase():"POST");
  if(!pcbf&&!purl) return false;
  pc=this.formToStr(fo);
  if(!pc) return false;
  if(pm) {
   if(pm=="POST") send(purl,pc,pcbf,"POST",true);
   else if(purl.indexOf("?")>0) send(purl+"&"+pc,"",pcbf,"GET",true);
    else send(purl+"?"+pc,"",pcbf,"GET",true);
  }
  else send(purl,pc,pcbf,"POST",true);
 }
 /* formToStr
 // from SurfChen <[email protected]>
 // @url     http://www.surfchen.org/
 // @license http://www.gnu.org/licenses/gpl.html GPL
 // modified by xujiwei
 // @url     http://www.xujiwei.cn/
 */
 this.formToStr=function(fc) {
  var i,qs="",and="",ev="";
  for(i=0;i<fc.length;i++) {
   e=fc[i];
   if (e.id!=''||e.name!='') {
    if (e.type=='select-one'&&e.selectedIndex>-1) ev=e.options[e.selectedIndex].value;
    else if (e.type=='checkbox' || e.type=='radio') {
     if (e.checked==false) continue;
     ev=e.value;
    }
    else ev=e.value;
    ev=encode(ev);
    qs+=and+(e.id!=''?e.id:e.name)+'='+ev;
    and="&";
   }
  }
  return qs;
 }
}