POST And HTML Forms

I am currently designing an HTML page with a form with a post method.
The page is calling itself so with the details that are passed will
vary how the page will look i.e. number of results from the sql query
that is being run. I'm trying to catch these using a javascript
function which is:

var qsParm = new Array();
function qs() {
var query = window.location.search.substring(1);
document.writeln(query);
var parms = query.split('&');
for (var i=0; i<parms.length; i++) {
var pos = parms[i].indexOf('=');
if (pos > 0) {
var key = parms[i].substring(0,pos);
var val = parms[i].substring(pos+1);
qsParm[key] = val;
}
}
}

and then just calling the array to get the value, in this case
qsParm['num_res']. The above function doesnt work for post but does for
get. I'm fairly new to javascript and am assuming this line is at
fault:

var query = window.location.search.substring(1);

but I'm not sure what it should be.