Add post variable to form with jQuery

Add post variable to form with jQuery

I have the below code which adds the value of the parameter to my form by
default. This works well but if you set the form to post then it no longer
works. What is the best approach to setting this value?
var request = {};
var pairs = location.search.substring(1).split('&');
for (var i = 0; i < pairs.length; i++) {
var pair = pairs[i].split('=');
request[pair[0]] = pair[1];
}
var getfirstname = document.getElementById('firstname');
var firstname = request['firstname'];
getfirstname.setAttribute('value', firstname);
<form method="get" action="search">
<input type="text" name="firstname" id="firstname" />
<input type="submit" alt="Go" value="Go" border="0" />
</form>
www.example.com?firstname=john
<form method="get" action="search">
<input type="text" name="firstname" id="firstname" value="John" />
<input type="submit" alt="Go" value="Go" border="0" />
</form>