代码中的警报框在哪里？

下面的代码给我返回了一个警告框，但是正如你在代码中所看到的，其中并没有使用警报——所以它是从哪里来的？

var mypostrequest=new ajaxRequest();
  mypostrequest.onreadystatechange=function(){
    if (mypostrequest.readyState==4){
      if (mypostrequest.status==200 || window.location.href.indexOf("http")==-1){
        if(mypostrequest.responseText == "true")
      {
         err.style.display = "none";   
         captcha.style.borderColor = "green";
         document.validForm.submit();
         return true;
     } else{ 
       captcha.style.borderColor = "red";
       err.style.display = "inline-block";
       err.innerHTML = "invalid captcha. Try again."
       captcha.focus();
         return false;
     }          
  }

它显示的警报是："true"。

这是ajaxRequest()：

function ajaxRequest(){
 var activexmodes=["Msxml2.XMLHTTP", "Microsoft.XMLHTTP"]; //activeX versions to check for in IE
 if (window.ActiveXObject){ //Test for support for ActiveXObject in IE first (as XMLHttpRequest in IE7 is broken)
  for (var i=0; i<activexmodes.length; i++){
   try{
    return new ActiveXObject(activexmodes[i]);
   }
   catch(e){
    //suppress error
   }
  }
 }
 else if (window.XMLHttpRequest) // if Mozilla, Safari etc
  return new XMLHttpRequest();
 else
  return false;

}