var item = {
        field1: 100,
        field2: 200,
        field3: [ 1,2,3 ],
        field4: {
            field5: 300,
            field6: 'Quad',
            field7: [ 4,5,6]
        }
    };

var serialize2 = function(obj, prefix) {
        var str = [], p;
        for (p in obj) {
            if (obj.hasOwnProperty(p)) {
                var k = prefix ? prefix + "[" + p + "]" : p, v = obj[p];
                str.push((v !== null && typeof v === "object") ?
                    serialize2(v, k) :
                encodeURIComponent(k) + "=" + encodeURIComponent(v));
            }
        }
        return str.join("&");
    };

var serialize2 = function(obj, prefix) {
    var str = [], p;
    for (p in obj) {
        if (obj.hasOwnProperty(p)) {
            var k = prefix ? prefix + "." + p : p, v = obj[p]; // changed this line
            str.push((v !== null && typeof v === "object") ?
                serialize2(v, k) :
            encodeURIComponent(k) + "=" + encodeURIComponent(v));
        }
    }
    return str.join("&");
};