javascript - How to cancel sending data while the username is taken -

when fills out registration form , username taken, refreshes page , says "the username taken, choose another".

but data person filled in removed.

how can stop refreshing if username taken, user can change it?

protected string msg = ""; protected void page_load(object sender, eventargs e) {     if (this.ispostback)     {         string firstname = request.form["firstname"];         string lastname = request.form["lastname"];         string phonenumber = request.form["phonenumber"];         string prefix = request.form["prefix"];         phonenumber = prefix + phonenumber;         int birthyear = int.parse(request.form["birthyear"]);         string email = request.form["email"];         string username = request.form["username"];         string password = request.form["password"];         string gender = request.form["gender"];         string dbpath = this.mappath("app_data/database.mdf");         dal dal = new dal(dbpath);         utilityuser uu = new utilityuser(dal);         if (uu.exist(username))         {             msg = "שם המשתמש שבחרת תפוס"; **//username taken**         }         else         {             uu.register(firstname, lastname, phonenumber, birthyear, email, username, password, gender);             msg = "נרשמת בהצלחה!"; **// register succeed**         }     } } 

this form check username:

var username = aspnetform.username.value;     if (username.length < 6) {         document.getelementbyid("labelusername").innerhtml = '<img src=\'images/x.png\'>'; //**x icon**         fields += "שם משתמש - שדה זה חייב להכיל בין 6 ל־12 תווים\n";         correct = false;     }     else {         document.getelementbyid("labelusername").innerhtml = '<img src=\'images/v.png\'>'; //**v icon**     } 

you need use ajax functionality. checking username availability ajax call. don't need send data server. not seem right

if using asp.net webforms use page methods. following code

your web method

 using system.web.services;   namespance namespacename{  class classname  {   [webmethod]  public static string checkav(string uname)   {     //your sql plus other logic       if(useravailable){return "true";}      else{return "false";}    } 

you html

 <input type="text" id="txtun" placeholder="username" onchange="return check();"/> 

your javascript

 function check()  {     pagemethods.checkav($(#txtun"").val(),success,fail);  }   function success(response)  {    if(response=="false")    {      $("#r").html("user available");    }    else    {$("#r").html("user not available");}  }   function fail(error){alert("error occured");} 

and need use in script manager

  <asp:scriptmanager id="sm" runat="server" enablepagemethods="true"></asp:scriptmanager>