if you already have that hidden input :
function product(a, b) {
return a * b;
}
function setInputValue(input_id, val) {
document.getElementById(input_id).setAttribute('value', val);
}
if not, you can create one, add it to the body and then set it's value :
function addInput(val) {
var input = document.createElement('input');
input.setAttribute('type', 'hidden');
input.setAttribute('value', val);
document.body.appendChild(input);
}
And then you can use(depending on the case) :
addInput(product(2, 3)); // if you want to create the input
// or
setInputValue('input_id', product(2, 3));