callback（false）和callback（true）有什么作用？

Question

I'm studying an example project of a nodejs chat, and I can't really understand what happens when callback(false) and callback(true) are called here...

io.sockets.on('connection', function(socket){
    socket.on('new user', function(data, callback){
        if(usernames.indexOf(data) != -1){
            callback(false);
        } else {
            callback(true);
            socket.username = data;
            usernames.push(socket.username);
            updateUsernames();
        }
    });

Answer 1

callback is the acknowledgement function

server

        socket.on('new user', 
          function(data, calback){
                  // incidentally(not needed in this case) send back data value true 
                  calback(true);
          }
         );

client

    socket.emit('new user', 
              data, 
              function(confirmation){
                       console.log(confirmation);
                      //value of confirmation == true if you call callback(true)
                      //value of confirmation == false if you call callback(false) 
              }
             );

Answer 2

socket.on listen event 'new_user' this event is triggered by this way:

    var data= "mydata"
    var callback=function(bool){
      if(bool){
        console.log('success')
      }else{
        console.log('error')
      }
    }

socket.trigger('new_user',[data,callback])

callback is just a function pass as parameter of the trigger