Javascript: Factor out identical callbacks in Express app

all!

Using Node/Express/Socket.IO, I wanted to refactor out identical callbacks.

I have:

io.sockets.on('connection', function(socket) {
    socket.on('find all notes' , function() { noteProvider.findAllNotes(function(err, result) {
        if(err) {
            socket.emit('err', err);
        } else {
            socket.emit('result', result);
        }
    }) });
    socket.on('find note by id', function(id) { noteProvider.findNoteById(id, function(err, result) {
        if(err) {
            socket.emit('err', err);
        } else {
            socket.emit('result', result);
        }
    }) });
}

but want rather something like:

io.sockets.on('connection', function(socket) {
    socket.on('find all notes' , function() { noteProvider.findAllNotes(callback) });
    socket.on('find note by id', function(id) { noteProvider.findNoteById(id, callback) });
}

How can I refactor out the callback? These 2 examples does not work:

Does not work 1:

var callback = function(err, result) {
    if(err) {
        socket.emit('err', err);
    } else {
        socket.emit('result', result);
    }
}

Does not work 2:

io.sockets.on('connection', function(socket) {
    socket.on('find all notes' , function() { noteProvider.findAllNotes(callback(socket, err, result)) });
    socket.on('find note by id', function(id) { noteProvider.findNoteById(id, callback(socket, err, result)) });
}

with

var callback = function(socket, err, result) {
    if(err) {
        socket.emit('err', err);
    } else {
        socket.emit('result', result);
    }
}

How can I keep my code DRY?

Frode

If you declare "does not work 1" where it has access to the socket variable it should work. That could be done like this:

io.sockets.on('connection', function(socket) {
  var callback = function(err, result) {
    if(err) {
      socket.emit('err', err);
    } else {
      socket.emit('result', result);
    }
  }

  socket.on('find all notes' , function() { noteProvider.findAllNotes(callback) });
  socket.on('find note by id', function(id) { noteProvider.findNoteById(id, callback) });
});

It seems you tried to solve that in "does not work 2", which would work if you return a function in the callback:

var callback = function(socket) {
  // Returning the callback which will be called when noteProvider is done.
  // Since it's in the closure of this function it will have a reference to socket.
  return function (err, result) {
    if(err) {
        socket.emit('err', err);
    } else {
        socket.emit('result', result);
    }
  }
}

io.sockets.on('connection', function(socket) {
  socket.on('find all notes' , function() { noteProvider.findAllNotes(callback(socket)) });
  socket.on('find note by id', function(id) { noteProvider.findNoteById(id, callback(socket)) });
}