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Now that’s class, it’s an object! – Prototypal Inheritance in JavaScript

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JavaScript is an object oriented programming language like most programming languages nowadays. However, unlike most programming languages it does not use the concept of classes. Instead from classes objects inherit directly from other objects, their so called prototypes. If you’re familiar with a traditional object oriented programming language like Java or C++ and / or if you already have fundamental knowledge of JavaScript, then this article is directed at you.

In the beginning there was Object.prototype

To fully understand how object oriented JavaScript works, you will have to abandon any concept of object orientation you know. Yes, forget it. You won’t need it, and I will teach you anew. JavaScript object orientation works quite differently than in most other languages, you will only get confused about what is going on in JavaScript if you try to apply your old concepts and ideas of object orientation to JavaScript. So forget about it. Done? Good. So now that you forgot, let me introduce you to the object:

{};

This is what we call an object literal in JavaScript. It is the most basic form of an object and works similar to array literals in other languages. You may have noticed that there is no reference to a class. – Didn’t I tell you to forget everything you know about other object oriented languages?! That certainly includes classes! – Whatever, there are no classes in JavaScript. Objects in JavaScript do not inherit from classes, but from other objects, their so called prototypes. If an object does not have an explicit prototype, like in the case of an object literal, it will inherit from a special built-in object, which is accessible through Object.prototype.

Objects in JavaScript are dynamic hashmaps. A hashmap is a set of key/value pairs. For each value in an object, like an attribute or a method, there is a key, that identifies that value and allows access to it. You can access an object member with the dot operator followed by the literal key (dot notation) or by enclosing the key in string form in index brackets (bracket notation):

o.member; // dot notation
o['member']; // bracket notation

If you think the bracket notation looks a lot like accessing arrays then this is because arrays in JavaScript basically are nothing but objects with integer attribute keys.

Because objects are dynamic, you can add and remove any member at runtime, even after the object was created. You add a member by simply assigning to it and your remove it by using the delete operator on the member:

var o = {}; // create an empty object
o.member = 'value'; // create the attribute "member" by assigning 
                    // value "value" to it
o.method = function() { // create the method "method" by assigning 
                        // an anonymous function to it
  return this.member;
};
o.method(); // returns "value"
delete o.member; // delete the member "member"
o.method(); // returns undefined

Also, when you create an object using an object literal, you can add an arbitrary amount of attributes and methods to it, by listing key/value pairs, separated by comma and split by a colon:

var printer = {
  message: 'Hello, World!',
  count: 0,
  printMessage: function () {
    console.log(this.message);
    return ++this.count;
  }
};

Defining objects this way is very cool and singlehandedly eliminates the need for the singleton pattern, but it can be very tyring if you require multiple objects with the same attributes over and over again. What you want is a method that makes creating similar objects easy. Luckily JavaScript provides such a method.

Crock’ the Constructor

For easier object instantiation JavaScript provides the new keyword. Calling any function with a prepended new keyword will turn this function into a constructor function. Constructor functions basically work like normal functions with the difference that their this keyword will be bound to a newly created object and they will automatically return this newly created object if there is no explicit return statement. Although you could turn any arbitrary function into a constructor function using the new keyword, this does not make much sense in most cases. Usually you build special functions that will be used exclusively as constructor functions. By convention those functions start with a capital letter.

function Printer() {
  this.message = 'Hello, World!';
  this.count = 0;
  this.printMessage = function () {
    console.log(this.message);
    return ++this.count;
  };
}
var printer = new Printer();
printer.printMessage(); // prints "Hello, World!" to console
printer.count; // returns 1

What this constructor function does is implicitly create a new object and bind it to the functions this identifier. Then the function body is executed, which will create a message attribute, a count attribute and a printMessage method on the new object. Since there is no explicit return statement, the new object, augmented with it’s new members, will be returned implicitly.
Note though, that this construct is not a class. There are no classes is JavaScript. This is simply a function which adds members to a new object. The style of calling it might look a bit like a class constructor in other languages, but I told you: forget about what other languages do.

But there is a problem with this example. Not with constructor functions in general, but because we used a very naive variant. Each time we call this constructor function it will create a new object with a message and a count attribute. Up until here this is fine, but it will also create a new anonymous function and add it as a method to the object each time you call the constructor function. This means, that each object will get it’s own, separate function, which will require it’s own memory, it’s own compilation time, etc. While this >might> be what you want, it’s most likely not. What you actually want are several objects that all use the exact same function. Prototypes are what enables this in a very easy way.

Honor, guide me!

Prototypes are JavaScript’s way of inheritance. Each object in JavaScript has exactly one prototype. There are several ways to declare a newly created object’s prototype (and some implementations even allow the prototype to be changed after object instantiation), but let us look at how exactly prototypes work first.

When you read a member from an object, JavaScript will look for this member on the object itself first. If JavaScript cannot find the member on the object, it will go to the objects prototype and see if it can find the member there. If it fails again it will proceed to move along the prototype chain until it either finds the member on one of the prototype objects, in which case it will return it’s value or has reached the end of the chain without finding the member on any of the objects on the prototype chain. In that case undefined is returned.

If you write an object’s member by assigning to it, JavaScript will always set the objects own member. When assigning a member, JavaScript will never alter any object on the prototype chain. This is possible however, you will only need to do it explicitly. The following code will demonstrate what I just explained. Note that p is set as o‘s prototype.

var p = {
    key: 'value',
    method: function () {
      return this.key;
    }
  }, o = Object.create(p); // create an object with p as it's prototype
 
p.key; // returns 'value'
p.method(); // returns 'value'
 
o.key; // returns 'value'
o.method(); // calling a method is a read, returns 'value'
 
o.key = 'other'; // assign as o's own member!
o.key; // returns 'other'
p.key; // returns 'value'
 
o.method() // returns 'other', because the method
           // is evaluated with o bound to this, even
           // though the method actually is on p
 
delete o.key;
o.key; // returns 'value', after deletion the member
       // is no longer found on o, but still on p, when
       // traversing the prototype chain.
 
p.key = 'changed';
o.key; // returns 'changed', the prototype chain is
       // evaluated each time a property is read from.

I just introduced you to the first method of setting an objects prototype: Object.create(prototype);. Object.create returns an empty object with it’s prototype set to whatever object you pass in. Object.create can do more, but this is out of scope for this article.

When using an object literal you cannot specify a prototype for your object. The syntax simply doesn’t give you the tools to do so (although this might come in a future ECMAScript version). However, when using the constructor function pattern, you can specifiy the prototype of the created object. This is done by setting the prototype attribute of the constructor function. Whenever the function is called using the new keyword, the returned object will have it’s prototype set to whatever the value of the constructor functions prototype attribute was at the time:

var p = {
  key: 'value',
  method: function() {
    return this.key;
  }
};
function Thing() {
  // do nothing, implicitly return this
}
 
Thing.prototype = p; // make p the prototype of all objects created using Thing
Thing.prototype.constructor = Thing; // this is not required, but makes your 
                                     // objects pass the object.constructor 
                                     // === Thing test like you would expect.
 
var o = new Thing();
 
o.key; // returns 'value'
o.method(); // returns 'value'
 
p.key = 'changed'; // assign to the prototype
o.key; // returns 'changed'
 
o.key = 'other'; // assign o's own attribute
o.key; // retutns 'other'
 
// etc

With this pattern, you can now create an arbitrary amount of Things, which all inherit from p. All those Things share the exact same methods and attributes. Whenever you call a Things method method, it will call the exact same function p.method (that is as long as you do not explicitly set the Things members to something else). So in comparison to our first try with using the constructor function, we will save memory, compilation time, etc, because method exists only once, on the prototype p.

Busfactor

You now basically know everything you need to know about object oriented JavaScript. The rest is forging this knowledge into practically usable patterns. The following code is a small example how an object oriented system could be created using JavaScript prototypal inheritance. Be aware, that there are multiple patterns to do object orientation in JavaScript. Some embrace the prototypal nature of JavaScript, while others try to mimic what JavaScript people call classical object orientation.

Personally I don’t like classical patterns as much (especially those that try to mimic information hiding and private methods and attributes), since most will make your code slow and clunky. The following pattern is a good balance I think. It is aware of JavaScripts prototypal model, but people coming from a classical model will be able to grasp what it does.

function Vehicle() {
    this.passengers = 0;
}
 
Vehicle.prototype.seats = 0;
 
Vehicle.prototype.board = function() {
  if (this.seats > this.passengers) {
    this.passengers++;
  }
}
 
Vehicle.prototype.deboard = function() {
  if (this.passengers > 0) {
    this.passengers--;
  }
}
 
function Car() {
  Vehicle.apply(this, arguments);
}
 
Car.prototype = new Vehicle();
Car.prototype.constructor = Car;
Car.prototype.seats = 5;
 
function Bus(driver) {
  Vehicle.apply(this, arguments);
  this.driver = driver;
}
 
Bus.prototype = new Vehicle();
Bus.prototype.constructor = Bus;
Bus.prototype.seats = 20;
 
Bus.prototype.board = function (ticket) {
  if (ticket) {
    Vehicle.prototype.board.apply(this, arguments);
  }
}

JavaScript is all about functions and objects. That’s all what’s to it, even in it’s inheritance model. I hope you enjoyed this article. If you have any further questions or would like to comment, please do so below. If you want to learn more about JavaScript come back soon, to learn more about the secrets of closures, scope and functions. You can also subscribe to the feed, to stay tuned. Or, if you are in a hurry and want some good lecture, you should definitly check out Douglas Crockfords “JavaScript: The Good Parts” (Amazon Affiliate Link).

socket.io and Express. Tying it all together.

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NOTE: This article was written for Express 2.x.x. It might not work for Express 3 without modification.
Express is a great web development framework for node.js. It provides easy access to stuff like routing, requests and sessions. socket.io is an abstraction layer for Websockets, with Flash and XHR fallbacks, that runs in both node.js and the client.

The Basics

You can have socket.io run with Express easily. By simply invoking socket.io’s listen method and passing it the Express app as a parameter:

var io = require('socket.io'),
    express = require('express'),
    app = express.createServer();
 
app.configure(function () {
    app.use(express.cookieParser());
    app.use(express.session({secret: 'secret', key: 'express.sid'}));
    app.use(function (req, res) {
        res.end('<h2>Hello, your session id is ' + req.sessionID + '</h2>');
    });
});
 
app.listen();
var sio = io.listen(app);
 
sio.sockets.on('connection', function (socket) {
    console.log('A socket connected!');
});

However, socket.io will not be aware of Express – and the other way around. So if a socket connects, we do not know to which Express session it belongs, but in most scenarios this is an essential information. Since socket.io 0.7 however we can easily obtain this information through the handshake/authorization mechanism. Through this we can tell socket.io to invoke a user-defined function whenever a new Websocket connection is incoming. The important point is, that it is called, before the connection is completed. This provides multiple benefits. For one, we can accept or revoke the connection based on various conditions and for the other part it allows us to inspect the header information of the HTTP request that is trying to establish the Websocket connection – including the cookie. The cookie will contain the sessionID of our Express session.

var parseCookie = require('connect').utils.parseCookie;
 
sio.set('authorization', function (data, accept) {
    // check if there's a cookie header
    if (data.headers.cookie) {
        // if there is, parse the cookie
        data.cookie = parseCookie(data.headers.cookie);
        // note that you will need to use the same key to grad the
        // session id, as you specified in the Express setup.
        data.sessionID = data.cookie['express.sid'];
    } else {
       // if there isn't, turn down the connection with a message
       // and leave the function.
       return accept('No cookie transmitted.', false);
    }
    // accept the incoming connection
    accept(null, true);
});

All the attributes, that are assigned to the data object are now accessible through the handshake attribute of the socket.io connection object.

sio.sockets.on('connection', function (socket) {
    console.log('A socket with sessionID ' + socket.handshake.sessionID 
        + ' connected!');
});

Getting, serious.

But what I find much more interesting is that we can not only extract the sessionID from the cookie, but we can also load the actual session and use, modify or even destroy it. For that we need to get hold of the session store that Express uses to save our sessions. By default this is a MemoryStore and will be created by Express. Instead we can create our own and pass it to Express. That way we have a reference to that store for ourself.

var io = require('socket.io'),
    express = require('express'),
    MemoryStore = express.session.MemoryStore,
    app = express.createServer(),
    sessionStore = new MemoryStore();
 
app.configure(function () {
    app.use(express.cookieParser());
    app.use(express.session({store: sessionStore
        , secret: 'secret'
        , key: 'express.sid'}));
    app.use(function (req, res) {
        res.end('<h2>Hello, your session id is ' + req.sessionID + '</h2>');
    });
});

Now, in our handshake function, we can not only get the sessionID from the cookie, but actually load the session data from the session store.

sio.set('authorization', function (data, accept) {
    if (data.headers.cookie) {
        data.cookie = parseCookie(data.headers.cookie);
        data.sessionID = data.cookie['express.sid'];
        // (literally) get the session data from the session store
        sessionStore.get(data.sessionID, function (err, session) {
            if (err || !session) {
                // if we cannot grab a session, turn down the connection
                accept('Error', false);
            } else {
                // save the session data and accept the connection
                data.session = session;
                accept(null, true);
            }
        });
    } else {
       return accept('No cookie transmitted.', false);
    }
});

All the good stuff

Now we have access to all of the session’s data through socket.handshake.session. But to be able to change the session data we will need to create an actual Express Session object. The constructor for the Express session object requires the request associated with the session aswell as the session data. We just acquired the session data from the session store, so we have that. But we do not have access to the HTTP request that is associated to the session. socket.io does not expose it to us. If you look closer at the Session prototype though you see, that there are only 3 properties, that are required on the request object passed to the constructor: sessionID, session and sessionStore. This is good news, since our handshake data object already has the properties sessionID and session. Both (after we created the Session object) with the values that the Session prototype expects to be there. Well and the last property, sessionStore, we can easily add to the data object.

var Session = require('connect').middleware.session.Session;
sio.set('authorization', function (data, accept) {
    if (data.headers.cookie) {
        data.cookie = parseCookie(data.headers.cookie);
        data.sessionID = data.cookie['express.sid'];
        // save the session store to the data object 
        // (as required by the Session constructor)
        data.sessionStore = sessionStore;
        sessionStore.get(data.sessionID, function (err, session) {
            if (err || !session) {
                accept('Error', false);
            } else {
                // create a session object, passing data as request and our
                // just acquired session data
                data.session = new Session(data, session);
                accept(null, true);
            }
        });
    } else {
       return accept('No cookie transmitted.', false);
    }
});

Now we can use the session object to modify and even destroy the session. Eg. you could use Session’s touch() method to keep the session from timing out for as long as the Websocket connection remains

sio.sockets.on('connection', function (socket) {
    var hs = socket.handshake;
    console.log('A socket with sessionID ' + hs.sessionID 
        + ' connected!');
    // setup an inteval that will keep our session fresh
    var intervalID = setInterval(function () {
        // reload the session (just in case something changed,
        // we don't want to override anything, but the age)
        // reloading will also ensure we keep an up2date copy
        // of the session with our connection.
        hs.session.reload( function () { 
            // "touch" it (resetting maxAge and lastAccess)
            // and save it back again.
            hs.session.touch().save();
        });
    }, 60 * 1000);
    socket.on('disconnect', function () {
        console.log('A socket with sessionID ' + hs.sessionID 
            + ' disconnected!');
        // clear the socket interval to stop refreshing the session
        clearInterval(intervalID);
    });
 
});

Sum it all up

From this code base you can keep going. You have everything set up to make your Express/socket.io hybrid application run and interact with each of it’s components as you wish. If you have any comments or suggestions, please feel free to use the comment box below. If you would like to see socket.io and Express in action together, have a look at my demo application.