Session on Html

You have to make a call back to the server to set the Session you can do this with an AJAX call that sends the value to a server side process that then sets the session something like this (using jQuery)

$.post('setSession.asp', {t: T})

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Your script above indicates a slight misunderstanding on how javascript works.

When you run your function it all runs within the context of the browser - no server code is available.

Your code indicates you are trying to set a session variable on the server and then immediately after that output the value in JavaScript to show it has changed.

To do what you want to do you need to get a handle on some of the aspects of client server interaction in a web context. When you need to talk to the server you do so (for the most part) asynchronously - which means you send a request to the server and you carry on with other things. When the server returns you have code that is waiting to pick up the response and deal with it - but in the context of your application this could be a significant amount of time after your initial request was sent. To get around this we write event handlers (callbacks) that respond to the server return. This requires a bit of a shift in thinking because we need to suspend the code that needs the return and place that instead inside the callback.

To extend your example a little further. In this example we send a value to the server and the server simply sends that value back.

<script src="http://code.jquery.com/jquery.js"></script>
<script>
var T = 'Tail';
$(function() {
  $.post('setSession.asp',{t: T}, function(resp) {
     // This function is called when the server has sent its response
     // The resp parameter is whatever we decide to send back to
     // the calling application. In this case we are just mirroring
     // what was sent
     T = resp;
  });
});</script>

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The asp for this would look something like

<%
Session("AllowT") = Request("t")
Response.Write Session("AllowT")
%>

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Now this does not really tell us anything - because how do we know the value was actually set. Let's change it a bit so that we can tell what value was set on the server. This is tricky because we first have to set a value before we can test it but because we are running asynchronously so we can't just put the statements one after the other. We have two options
1. We put the test inside the callback from the set
2. We create a separate event handler that we control (like a button click) that tests the value for us

I am going to use 2

<button onclick="test()">Test</button>
<script src="http://code.jquery.com/jquery.js"></script>
<script>
var T = 'Tail';
$(function() {
  $.post('setSession.asp',{t: T}, function(resp) {
     // This function is called when the server has sent its response
     // The resp parameter is whatever we decide to send back to
     // the calling application. In this case we are just mirroring
     // what was sent
     T = resp;
  });
});
function test()
{
  $.get('testGet.asp',function(resp) {
    alert('Session: ' + resp);
  });
}
</script>

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This will demonstrate that the value set in the $.post is in fact set on the server.