socket.emit() vs. socket.send()

What's the difference between these two?

I noticed that if I changed from socket.emit to socket.send in a working program, the server failed to receive the message, although I don't understand why.

I also noticed that in my program if I changed from socket.emit to socket.send, the server receives a message, but it seems to receive it multiple times. When I use console.log() to see what the server received, it shows something different from when I use socket.emit.

Why this behavior? How do you know when to use socket.emit or socket.send?

With socket.emit you can register custom event like that:

server:

var io = require('socket.io').listen(80);

io.sockets.on('connection', function (socket) {
  socket.emit('news', { hello: 'world' });
  socket.on('my other event', function (data) {
    console.log(data);
  });
});

client:

var socket = io.connect('http://localhost');
socket.on('news', function (data) {
  console.log(data);
  socket.emit('my other event', { my: 'data' });
});

Socket.send does the same, but you don't register to 'news' but to message:

server:

var io = require('socket.io').listen(80);

io.sockets.on('connection', function (socket) {
  socket.send('hi');
});

client:

var socket = io.connect('http://localhost');
socket.on('message', function (message) {
  console.log(message);
});

Simple and precise (Source: Socket.IO google group):

socket.emit allows you to emit custom events on the server and client

socket.send sends messages which are received with the 'message' event

TL;DR:

socket.send(data, callback) is essentially equivalent to calling socket.emit('message', JSON.stringify(data), callback)

Without looking at the source code, I would assume that the send function is more efficient edit: for sending string messages, at least?

So yeah basically emit allows you to send objects, which is very handy.

Take this example with socket.emit:

sendMessage: function(type, message) {
	socket.emit('message', {
		type: type,
		message: message
	});
}

and for those keeping score at home, here is what it looks like using socket.send:

sendMessage: function(type, message) {
	socket.send(JSON.stringify({
		type: type,
		message: message
	}));
}

socket.send is implemented for compatibility with vanilla WebSocket interface. socket.emit is feature of Socket.IO only. They both do the same, but socket.emit is a bit more convenient in handling messages.

In basic two way communication systems, socket.emit has proved to be more convincing and easy to use (personal experience) and is a part of Socket.IO which is primarily built for such purposes

https://socket.io/docs/client-api/#socket-send-args-ack

socket.send // Sends a message event

socket.emit(eventName[, ...args][, ack]) // you can custom eventName