I had also got the same warning
[DEP0013] DeprecationWarning: Calling an asynchronous function without callback is deprecated.
I saw this statement-suspect:
Fs . writeFile (path, aString, cb, encoding);
So the problem was the cb (= 'callback') and the encoding arguments are in the wrong order. So I got rid of the warning simply by changing the above to below:
Fs . writeFile (path, aString, encoding, cb);
But the problem is really with the ERRONEOUS warning-message So if the warning had informed as below
"WWARNING: calling fs.writeFile() with a string-argument
where a function is expected"
So the point here was I was NOT calling the writeFile() without the callback-argument, which is shown as deprecated. I was trying to call the writeFile() WITH A WRONG TYPE OF ARGUMENT. So that must be an ERROR and not the warning.
I hope it solved your issue.