Adding a parameter to the URL with JavaScript

Question

In a web application that makes use of AJAX calls, I need to submit a request but add a parameter to the end of the URL, for example:

Original URL:

Answer 1

Thank you all for your contribution. I used annakata code and modified to also include the case where there is no query string in the url at all. Hope this would help.

function insertParam(key, value) {
        key = escape(key); value = escape(value);

        var kvp = document.location.search.substr(1).split('&');
        if (kvp == '') {
            document.location.search = '?' + key + '=' + value;
        }
        else {

            var i = kvp.length; var x; while (i--) {
                x = kvp[i].split('=');

                if (x[0] == key) {
                    x[1] = value;
                    kvp[i] = x.join('=');
                    break;
                }
            }

            if (i < 0) { kvp[kvp.length] = [key, value].join('='); }

            //this will reload the page, it's likely better to store this until finished
            document.location.search = kvp.join('&');
        }
    }