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How to validate server side code in Javascript

I am getting some value from server , if the value is false i need to show alert message on UI without refreshing or postbacking the page.
What i am doing currently is i get the value from server i declare one hidden control and store that value in hidden control and in aspx page i write one JS method to check if that value is false show the alert message, but the problem is i have to do this on click on a button which is present in code behind
so as soon as i click on that button JS code gets executed before i get value from server.
function fnvalidation() {
if (document.getElementById("<%=hdnCtrl.ClientID%>").value == false) {
alert('Please submit it again');
return false;
}
}
On page load i am registering like this :
btnSave.Attributes.Add("onclick", "return fnvalidation()");
C# Code :
bValidate = CommonUtility.ValidateOutput();
hdnCtrl.Value = bValidate.ToString();//Storing value in hidden variable
if(bvalidate == false)
{
//Call javascript method fnvalidation
}
How to call this JS method correctly when value is false coming from server ?
Thanks.

If your page needs to fetch data from the server without refreshing the page, then you need to use AJAX. Which comes down to two things:
Create an AJAX endpoint in server-side code.
Consume that AJAX endpoint from client-side code.
The server-side code likely has several options. HTTP Handlers, empty ASPX pages which clear and response and set a new content type, maybe even Web Methods? It's been a long time since I've used WebForms, so I'm not 100% sure what the recommended approach is right now. But I think Web Methods are what you're looking for. So your code-behind would have something like this:
public partial class MyPage : Page
{
[WebMethod]
public static string GetValue()
{
return "some value";
}
}
Then, assuming you're using jQuery for example, you would call that endpoint from your client-side code:
$.ajax({
type: "POST",
url: "MyPage.aspx/GetValue"
}).done(function (data) {
// respond to the value in "data"
}).error(function () {
// there was an error
});
Now your client-side button click handler can use that, or something like it, to fetch the value from the server. If that's all that button needs to do then don't make it an asp:Button but instead just an input type="button" so that it doesn't post back the whole page. Keeping it as a server-side button would mean canceling the event propagation in JavaScript, which gets ugly with asynchronous operations because there's no immediate way to resume event propagation, you'd have to manually invoke the post back.

I think you can use
Page.ClientScript.RegisterStartupScript
inside your condition if(bvalidate == false) condition like then no need to rely on the value to be set ( i.e in java script function you can remove checking the hidden field value simply alert the user.
Also you can remove this code btnSave.Attributes.Add("onclick", "return fnvalidation()")
Page.ClientScript.RegisterStartupScript(this.GetType(), "validateFnscript", "fnvalidation()",
true);

pass parameter to one function in the code behind in C# from javascript

I am trying to pass parameter to one function in the code behind in C# from javascript
<script type="text/javascript">
$(document).ready(function () {
$("#some_id").click(function () {
var id = document.getElementById('HiddenField2');
var a = <%=btn_Click(id)%>;
});
});
</script>
<asp:HiddenField ID="HiddenField2" runat="server" Value='<%# DataBinder.Eval(Container.DataItem, "item_id")%>' />
code behind
public string btn_Click(String item_id)
{
/*do some thing*/
return null;
}
But this piece of code always giving me error of context. That id is not defined in this context.
Can some please let me know what wrong i am doing?

First, var id is javascript and the code within <%= %> is C#. You can't pass a variable between the languages like that.
Second, the value of id in this case is going to be a DOM element which C# can't use anyways. If you want to get the value of HiddenField2 within the code behind you can use HiddenField2.Value.
Third, since you're using ASP.net, instead of using jQuery's .click handler you should use the onServerClick attribute to wire up the button click behavior to btn_Click.

The button click event in C# will be triggered by Postback when your browser post data back to asp.net
I do not understand why you use HiddenField here,so my suggestion do not consider about it
Solution 1:
first you can extract your code in btn_Click to a HttpHandler(*.ashx in asp.net),then use Ajax by using js framework like jQuery to send data to your HttpHandler,data returned by HttpHandler can be processed by js
sample code is here
Solution 2:
if your code in btn_Click is relevent to your page, just use ajax Get method, the data will send to your page,the data returned by your data will be processed by js too
In the end, if you are new to web, I recommend you to learn or implement asp.net MVC, in my opinion, it is more flexible than asp.net webform

Executing Server-Side Methods by Clicking on a DIV

I'm working on an ASP.Net project, with C#.
Usually, when I need to put Buttons that will execute some methods, I will use the ASP Controller (Button) inside a runat="server" form.
But I feel that this really limits the capabilities of my website, because when I used to work with JSP, I used jquery to reach a servlet to execute some codes and return a responseText.
I did not check yet how this is done in ASP.Net, but my question concerns controllers and the famous runat="server".
When I add a runat="server" to any HTML Element, I'm supposed to be able to manipulate this HTML element in C# (Server-Side), and this actually works, I can change the ID, set the InnerText or InnerHtml, but the thing that I can't get, is why can't I execute a method by clicking on this element?
The "onclick" attribute is for JavaScript I think, and OnServerClick doesn't seem to work as well. Is it something wrong with my codes? or this doesn't work at all?

You will have to handle the click in the div using the Jquery and call
server-side methods through JQuery

There are several way to execute server side methods by clicking on a div or anything on your page. The first is mentioned __dopostback, second is handling the click in javascript or with jQuery and calling a function in a handler or a page method in a webservice or a page method in your page behind code.
Here is the handler version:
$("#btn1").click(function() {
$.ajax({
url: '/Handler1.ashx?param1=someparam',
success: function(msg, status, xhr) {
//doSomething, manipulate your html
},
error: function() {
//doSomething
}
});
});
I think the second version is better, because you can make a partial postback without any updatepanel, asyncronously. The drawback is, the server side code is separated from your page behind code.
Handler:
public class Handler1: IHttpHandler
{
public void ProcessRequest(HttpContext context)
{
context.Response.ContentType = "application/json";
var param1= context.Request.QueryString["param1"];
//param1 value will be "someparam"
// do something cool like filling a datatable serialize it with newtonsoft jsonconvert
var dt= new DataTable();
// fill it
context.Response.Write(JsonConvert.SerializeObject(dt));
}
}
If everything is cool, you get the response in the ajax call in the success section, and the parameter called "msg" will be your serialized JSON datatable.

You can execute a method from jquery click in server, using __doPostBack javascript function, see this threat for more details How to use __doPostBack()

Add this code in your jquery on div onclick and pass DIv id whcih call click
__doPostBack('__Page', DivID);
On page load add this code
if (IsPostBack)
{
//you will get id of div which called function
string eventargs = Request["__EVENTARGUMENT"];
if (!string.IsNullOrEmpty(eventargs))
{
//call your function
}
}

Make the div runat="server" and id="divName"
in page_Load event in cs:
if (IsPostBack)
{
if (Request["__EVENTARGUMENT"] != null && Request["__EVENTARGUMENT"] == "divClick")
{
//code to run in click event of divName
}
}
divName.Attributes.Add("ondivClick", ClientScript.GetPostBackEventReference(divName, "divClick"));
Hope it helps :)

if you are referring to divs with runat="server" attributes, they don't have onserverclick events, that's why it doesn't work

Call javascript function with parameters and read the return values in page load

I have javascript function in default.aspx like this:
function ReadValue(Name) {
Return value;
}
Now from pageLoad method, I want to pass parameter and get the return value to server side variable.

To send data from the client to the server, you have to ... send data from the client to the server. :-) Typically, you do that with Ajax, although there are other ways as well (doing a standard form POST, or even doing a GET of some kind although if your server is going to act on the information by changing server state, you shouldn't use GET).

I would say the best idea is to make a Javascript function and make a server side Hiddenfield. Access that hidden field and store your values (comma delimited) in your hiddenfield. and access it on page load.
I do it this way.
<asp:HiddenField ID="hdn" runat="server" />
document.getElementById("<%= hdn.ClientID%>").value = "your value";

another way is by putting the values in sever side controls
for exmaple in jquery you can do something like this:
function pageLoad(sender, args) {
$("#txtMyTextBoxID").val('my value')
}
aspx:
<asp:TextBox ID="txtMyTextBoxID" runat="server" ClientIDMode="Static"></asp:TextBox>

ASP.NET MVC - how to get the value from a textbox in my View?

If I have a textbox in my view:
<div><%= Html.TextBox("Comments", Model.Comments)%></div>
I want to post the contents of this textbox to the controller with an Ajax call. I only need this one value though, so I don't want to post the whole form back.
<%= Ajax.ActionLink("update", "UpdateComments",
new { comments = /* ????? */ },
new AjaxOptions { HttpMethod="POST" })%>
How do I get the textbox value?

Rather than writing server-side ajax code, you should use client-side Ajax (e.g. jQuery) to get the runtime value of the textbox and post that value.

using jQuery, you could retrieve the value in the following manner.
$("#Comments").val();

Ajax.ActionLink is a helper function, that generates a JS-enabled link, that sends data via AJAX. Because it is generated serverside, and the value is generated on the client side, you can not pass a single value like that. You either have to manually write HTML and JS, or submit a whole form containing this element (and watch out not to nest it inside another form).

You can use Ajax in jquery. Like this
function FunctionName() {
$.Post(URl, function(data) {
});
}