Comment by twii

You assume a lot.

We can refactor another round:

  function logFsResult( type, err ){
  	var msg= '';
  	switch ( type ) {
  		case 'append': msg= ( err ) ? 'Failed to update file' : 'Successfully updated file';
  			break;
  		case 'write': msg= ( err ) ? 'Failed to write file' : 'Successfully created file';
  			break;
  		default: msg= 'logFsResult error, missing or invalid first argument: '+ (type || '')
  	}
  	( err ) ? console.err( msg ) : console.log( msg );
  }

  function handleFile( filename, fileExists ) {
  	const timestamp = new Date().toISOString();
  	( fileExists )
  		?	fs.appendFile( filename, `Updated file on ${timestamp}\n`, logFsResult.bind(null, 'append') )
  		:	fs.writeFile( filename, `Created file on ${timestamp}\n`, logFsResult.bind(null, 'write') );
  }