Using async, if you want something to allow listeners to be attached later:

  var outcome = Boxon();
  fs.readFile( "test.txt", outcome );
  // Attach listener later:
  outcome( function( err, content ){ ... } );

See https://github.com/JeanHuguesRobert/l8/wiki/AboutBoxons

Boxon objects are light compared to promises, but they interop well. Best of both worlds!