A small problem of JavaScript interaction with the server

You must have used a very common scenario of JS interaction with the server. (If you have time, summarize the interaction between the server and the client script)
As follows:
The server obtains the value of a hidden control:

< Input Type = "Hidden" ID = "Allscore" Value = "" Runat = "Server" />

you need to assign a value to it using JS and obtain it on the server.
the scenario is very simple.
then, let's see how to assign a value to it using JS: var allscore = document. getelementbyid ( " allscore " ). value;
var temp = 123 ;
allscore = temp; alert (allscore);

Obviously, allscore = 123 can be obtained, but on the server side,
Allscore =... is always blank and no value is obtained. Crazy ~
I thought about it for a long time ~
Only consult experts.
Last changedVaRAllscore=Document. getelementbyid ("Allscore");
Allscore. Value=Temp;

Done ~
It's really depressing. I don't know how to explain it. That expert doesn't know how to explain it. It's just experience ~
Did anyone tell me?