I would recommend taking advantage of the fact that nodeJS will always be ES5. Remember this isn't the browser folks you can depend on the language's implementation on being stable. That said I would recommend against ever using a for-in loop in nodeJS, unless you really want to do deep recursion up the prototype chain. For simple, traditional looping I would recommend making good use of Object.keys method, in ES5. If you view the following JSPerf test, especially if you use Chrome (since it has the same engine as nodeJS), you will get a rough idea of how much more performant using this method is than using a for-in loop (roughly 10 times faster). Here's a sample of the code:
var keys = Object.keys( obj );
for( var i = 0,length = keys.length; i < length; i++ ) {
obj[ keys[ i ] ];
}