jsRegExp.replaceWithFn.js - replace matched string with a callback function, with its parameters as "full match", "capturing 1st", "capturing 2nd" ... #note: using this technique with String.replace may be very useful.

jsRegExp.replaceWithFn.js

function compress(source) {
    var keys = {};
    source.replace(
        /([^=&]+)=([^&]*)/g,

    function (full, key, value) {

        keys[key] = (keys[key] ? keys[key] + "," : "") + value;
        console.log(keys[key]);
        return "";
    });

    var result = [];

    for (var key in keys) {
        result.push(key + "=" + keys[key]);
    }

    return result.join("&");
}
var res = compress("foo=1&foo=2&blah=a&blah=b&foo=3");
console.dir(res);