> you compare err != null?? besides not using strict mode, what should err be? a...

> you compare err != null?? besides not using strict mode, what should err be? a String? So, what will happen if err is undefined or an empty String?

YOU are the one who made that comparison, not me. You originally wrote the following line:

   ( err ) ? console.err( msg ) : console.log( msg );

What do you think the "?" operator does with "err"?

> Then you call logFsResult with err while it is not used..

It seems you don't understand the code. I'm not calling "logFsResult", i am defining a function called logFsResult. You also did the same, you defined logFsResult to receive the "err" parameter.

  function logFsResult( type, err ){
  	var msg= '';
  	switch ( type ) {

> That's a trap, I should rather make my code as readable, scalable and bug free as possible regardless of ESxxx.

ES6 allows you to write more readable code than ES5. Take a look at the features.