JS面向对象语法问题

[改一下昵称]

var sign = new Object;

sign.signDiv = document.getElementById('signUp');
sign.signUpButton = document.getElementById('signUpButton');
    // sign.signUpButton.onclick = sign.signUp;

sign.userName = document.getElementById('userName').value ;
// error:Cannot read property 'value' of null
sign.removeSignDiv = function(){ 
         // remove the sign up div.  
		 this.signDiv.parentNode.removeChild( this.signDiv );
	 }
sign.signUp = function()
{     // sign up to user.php
     ajax.open( 'GET', 'user.php?name='+ userName , true);
	 ajax.onreadystatechange = function(){
	     if( ajax.status == 200 && ajax.readyState == 4 ){
		     this.removeSignDiv();
			 // setInterval("getMessage();" , 5000);
		    }
	    }
	 ajax.send();
}

我确定HTML元素id都没有错，chrome却报错. 而且尝试这么调用 sign.signUp(); 也会报错：没有这个方法
我的写法上有什么问题吗？

[liangws]

这里的this绑定的不是sign..
sign.signUp = function()
{ // sign up to user.php
ajax.open( 'GET', 'user.php?name='+ userName , true);
ajax.onreadystatechange = function(){
if( ajax.status == 200 && ajax.readyState == 4 ){
this.removeSignDiv();
// setInterval("getMessage();" , 5000);
}
}
ajax.send();
}

改成下面，看看

sign.signUp = function()
{ // sign up to user.php
var that = this;
ajax.open( 'GET', 'user.php?name='+ userName , true);
ajax.onreadystatechange = function(){
if( ajax.status == 200 && ajax.readyState == 4 ){
that.removeSignDiv();
// setInterval("getMessage();" , 5000);
}
}
ajax.send();
}

[改一下昵称]

找到原因了，脚本放在body前面所以取不到对象。谢谢