Asp.net Ajax implements refreshing data acquisition

Register. ASPX page Code :
<SCRIPT type = "text/JavaScript">
Function checkuser ()
{
VaR option = {
Parameters: "userid =" + $ ("userid"). value,
Method: "Post ",
Onsuccess: function (Transport ){
VaR Rp = transport. responsetext;
If (Rp = "true ")
{
Alert ("this user nickname exists !! ");
}
Else
{
Alert ("this user nickname does not exist. Please enter it again !! ");
}
},
Onfailure: function (Transport ){
Alert ("data submission failed. Check the network or try again. ");
}
}
VaR request = new Ajax. Request ("Ajax. aspx? Userid = "+ escape ($ (" userid "). value), option );
}
</SCRIPT>

Nickname: <asp: textbox id = "userid" runat = "server" text = "" onblur = "checkuser ()"> </ASP: textbox>

Ajax. ASPX page code:
Private void page_load (Object sender, system. eventargs E)
{
String userid = request. querystring ["userid"];
String SQL = "select userid from blog_user where userid = '" + userid + "')";
Object o = dbconn. executescalar (SQL );
If (O = NULL)
Response. Write ("false ");
Else
Response. Write ("true ");
}