With a field as follows:

<INPUT TYPE="HIDDEN" NAME="variableName" VaLUE="">

Then in Javascript you can do:

document.form.variableName.value="value";
form.submit();

and the JSP can evaluate the param "variableName". Remember though, that a Parameter is always a String (or an array of strings if accessed through a different method) so you will need a way to transform this string to what you want, if you want something other than a String (e.g. an Integer).

Bt to tell you the truth I am not sure what you by a null value in javascript. Don't forget that anything that the JSP reads from the params, must then turn around and insert the value back into the hidden field to be read by the javascript on the client (or actually modify the javascript in the JSP before returning the html to the client).