Question about socket.io event hook up in the following code

In the node.js/socket.io 开发者_JAVA技巧code accompanying this article the following code is used to hook up events:

socket.on('message', function(message) {
    var handler = messageFactory[message.messageType];
    $chatMessages.append(handler(message));
  });

What is the logic/effect of referencing messageFactory as an array (or at least using [] symbols)? Does it create different handles for different messageType?

Thanks!

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What is the logic/effect of referencing messageFactory as an array (or at least using [] symbols)? Does it create different handles for different messageType?

messageFactory is an object with two methods chat and system.

I would assume message.messageType is either "chat" or "system"

So messageFactory[message.messageType] simply gets one of the two methods.

Then handler(message) calls that method.

This is becuase messageFactory.chat === messageFactory["chat"]

If you take a look at the server file ("Listing 5: The chatRoom module.") you will see methods returning

return {
  messageType: 'system',
  text: originalNick + ' changed nick to ' + newNick
};

So the server returns a message object with a messageType property thats read on the client, it appears that messageType is only "chat" or "system".

That code is basically the OO Command Design Pattern. It's running a different command based on the type of object returned