HTML5GameTemplate & JavaScript: The Good Parts

Purpose of this session

• Demonstrate principles and practices that allow ThoughtWorks to use JavaScript as a first class language.
• Describe JavaScript's differences (especially the good and bad parts)
• Show how the 'Functional Object Creation Pattern' allows us to better use healthy OO design patterns
• Introduce JS unit testing in Jasmine
• Practice these new concepts by writing a simple game

JavaScript as a First Class Language

JavaScript was rapidly adopted as the language of web client behavior when Java Applets failed to be a viable solution. The vast major of web developers treat it as a second (or third) class citizen in their development ecosystem. Most code snippets do not incorporate good design. Many developers just Copy/Paste the JavaScript that they need without understanding it.

This approach worked well enough when:

• we only wanted minor behavior in our web clients
• the bits of behavior were largely independent
• there was no abstraction beyond the DOM.

As we started building 'thick' web clients these practices weren't sustainable, but they are how almost everyone was doing it and how most people are doing it today.

ThoughtWorks takes JavaScript more seriously and we use the same rigor with it as we do with our 'server' languages. We initially use good software design practices, we unit test our code, and we refactor it.

JavaScript: The Good Parts

We're just going to hit some of the high points of this book. There is a lot more to it and reading the entire book carefully will be a good investment of your time.

JavaScript has some hideously Bad Parts

Programming model based on global variables

Use namespace objects to get out of the global namespace

var game = game || {};

game.runner = function(spec){
    ...
};

Scope: uses block scope syntax but variables are function scoped

Note that control blocks do not create a new scope. You can also reference variables before you declare them. The book recommends declaring all of your variables at the top of each code block. This is safer but makes your code more difficult to refactor than declaring and instantiation variables at the same time.

In practice, neither solution is a good solution.

Here's an example of what can go wrong if you don't declare variables at the top of the function.

var stuff = 'stuff';
var hoist = function() {
    console.log(stuff);
    var stuff = 'other stuff';
    console.log(stuff);
};

hoist();

The first console.log outputs undefined because the var stuff in the function was hoisted to the top of the function.

var stuff = 'stuff';
var hoist = function() {
    var stuff; //has not been defined
    console.log(stuff);
    stuff = 'other stuff'; //defined here
    console.log(stuff);
};

hoist();

Semi-colon insertion

It sometimes inserts semicolons in places where they are not welcome. Consider the consequences of semicolon insertion on the return statement. If a return statement returns a value, that value expression must begin on the same line as the return:

var expects_to_return_map = function() {
    return
    {
        status: true;
    }
};

expects_to_return_map();

This appears to return an object containing a status member. Unfortunately, semicolon insertion turns it into a statement that returns undefined.

var expects_to_return_map = function() {
    return // <--- This semi-colon is automatically inserted by the interpreter
    {
        status: true;
    }
};

expects_to_return_map();

Always, always, always use K&R style braces in JavaScript:

var actually_returns_a_map = function() {
    return {
        status: true
    }
};

actually_returns_a_map();

Phony arrays

Arrays are implemented as maps in JavaScript. This has huge performance implications.

Lots of falsy values

Value	Type
0	Number
NaN (not a number)	Number
'' (empty string)	String
false	Boolean
null	Object
undefined	Undefined

== vs ===

== and != are evil because they force the type of the second operand to be coerced to the type of the first. This leads to the following strange cases;

'' == '0'          // false
0 == ''            // true
0 == '0'           // true

false == 'false'   // false
false == '0'       // true

false == undefined // false
false == null      // false
null == undefined  // true

' \t\r\n ' == 0    // true

We use JavaScript because:

It's more or less mandatory for building modern Web Apps

It has many Beautiful Features:

• Functions as first class objects
• “Dynamic objects with prototypal inheritance”
• Powerful object literal notation

Some important aspects of JavaScript:

Uses prototypal inheritance instead of class inheritance

Excerpt From: Douglas Crockford. “JavaScript, The_Good_Parts"

JavaScript is a prototypal inheritance language. That means that objects can inherit properties directly from other objects. The language is class-free.

If a function is invoked with the new prefix, then a new object will be created with a hidden link to the value of the function's prototype member, and this will be bound to that new object.”

// Create a constructor function called Quo.
// It makes an object with a status property.

var Quo = function (string) {
    this.status = string;
};

// Give all instances of Quo a public method
// called get_status.

Quo.prototype.get_status = function (  ) {
    return this.status;
};

// Make an instance of Quo.

var myQuo = new Quo("confused");

document.writeln(myQuo.get_status(  ));  // confused

Loose/weak Typing

What do you expect these lines to display.

console.log(true + 5 + "10");
console.log(true + "10");

Type these into a JS console and see if you were right.

Duck Typing

If it walks like a duck and talks like a duck... JavaScript doesn't care what type things are. It's happy to call a method on any object that has that method.

Functional Object Creation Pattern

The Challenges with 'new'

Constructor Methods are dangerous to call

If you accidentally call the constructor function directly, terrible things can happen to variables in the global namespace.

Instance variables and methods are public unless you use the Functional Object Creation Pattern

You can't have encapsulation if all of your instance variables are public.

An Alternative

Use a namespaced creation function that makes a new object (without calling 'new') that has methods which are a closures over the shared variables that are hidden from others because they are scoped to the creation function.

Make a function that creates an object

var game = game || {};
game.entity2d = function() {
    var that = {};
    return that;    
}

Add a method

var game = game || {};
game.entity2d = function() {
    var that = {};
    
    that.update = function() {
    };
    
    return that;    
}

Add some behavior to the method (a closure)

var game = game || {};
game.entity2d = function() {
    var that = {};

    
    var position = game.vector2d({x: 200, y: 100});
    var velocity = game.vector2d({x: 1, y: -2});
    var acceleration = game.vector2d({x: 0, y: 0.02});
    
    that.update = function() {
        velocity.add(acceleration);
        position.add(velocity);
    };
    
    return that;    
}

Inject dependencies into the object creation function using a map

var game = game || {};
game.entity2d = function(spec) {
    var that = {};

    var position = spec.position;
    var velocity = spec.velocity;
    var acceleration = spec.acceleration;
    
    that.update = function() {
        velocity.add(acceleration);
        position.add(velocity);
    };
    
    return that;    
}

Add more behavior to your class

var game = game || {};
game.entity2d = function(spec) {
    var that = {};

    var position = spec.position;
    var velocity = spec.velocity;
    var acceleration = spec.acceleration;
    
    that.update = function() {
        velocity.add(acceleration);
        position.add(velocity);
    };
    
    that.accelerate = function(delta) {
        acceleration.add(delta);
    }
    
    return that;    
}

Exercise

Implement these user stories by creating a new object using the functional object creation pattern

User Stories

1. Draw a box that is positioned so that the invader moves into the top of it if no action is taken
2. Reset the invader to its starting position whenever it overlaps the box
3. Reset the players score whenever the invader is reset
4. Place more boxes that work like the original box to make a game

Jasmine Unit Tests

This is a good intro & reference: http://jasmine.github.io/2.0/introduction.html

Exercise

Redo the previous user stories using TDD

What does the template code give you

HTML5 Boilerplate is a simple example of Test Driven game-like behavior implemented in Javascript using a functional object creation pattern (as described in 'Javascript: the Good Parts, Chapter 5).

The sample contains examples of:

• Keyboard Input
• Sprite drawing
• Simple object motion (using an Euler integrator)