How to serialize an Object into a list of URL query parameters?

Without knowing the keys of a JavaScript Object, how can I turn something like…

var obj = {
   param1: 'something',
   param2: 'somethingelse',
   param3: 'another'
}

obj[param4] = 'yetanother';

…into…

var str="param1=something&param2=somethingelse&param3=another&param4=yetanother";

…?