It doesn't even seem that optimized. The fact that the replace function sticks undefined back into the string means it has to coerce undefined into the string '

It doesn't even seem that optimized. The fact that the replace function sticks undefined back into the string means it has to coerce undefined into the string 'undefined' then manipulate the string needlessly (he even admits you end up with a string of 'undefined&undefined&undefined' that you throw away). The version eventually created in the comments seemed much better: function compress(data){ var match = [], q = {}, ret = "", re = /([^=&]+)=([^&]*)/g; while (match = re.exec(data)){ q[match[1]] = (q[match[1]] ? q[match[1]] + "," : "") + match[2]; } for ( var key in q ) ret = (ret ? ret + "&" : "") + key + "=" + q[key]; return ret; } The underlying string manipulation of the browser can be horribly bad. I think it was IE6 that had a quadratic time concatenation issue. Granted, query strings aren't really going to be long enough to matter for this, but if you are for some reason parsing a lot of these at once, speed could matter.