how to pass javascript return variable to code behind

You cannot interact between client side and server side without sending and HTTP request to the server.
You can do it using Ajax: http://en.wikipedia.org/wiki/Ajax_%28programming%29
One convenient way of using Ajax is using jQuery .Ajax(): http://api.jquery.com/jquery.ajax/
Or you can do so using Hidden Field.
Create a hidden field variable in the client side as

In the javascript you can assign some value to the hidden variable as

and you can read the hidden field variable in the code behind as
string variable = hdnfldVariable.Value;
I hope this will help you.