JOO

Simple class declaration function to write OO code in JavaScript

This library provides a simple class declaration function. Classes defined with this function behave exactly as same as normal JavaScript constructor functions. This program aims to be as just a syntax sugar to easily declare Classes, its inheritance, Mix-ins, namespace, statics etc. without any dependencies on other libraries nor any conflicts with them.

Table of contents

• Usage
• The API
• Features
  • Class declaration
  • Inheritance
  • Mix-ins
  • Static properties
  • Export to namespace
• Good Points

Usage

in Node.js

install joo with npm npm install joo, then require it as below.

var def = require('joo');

in HTML

load joo as a script <script type="text/javascript" src="/path/to/joo.js"></script>, then require it as below.

var def = require_joo();

Then, define your class with def().

function MyClass() {}
def(MyClass).
it.provides({
    doSomething: function() {}
});

The API

Here is the whole API of this program.

var def = require_joo();

def(function() {
    // This is the class constructor.
    // You can call SuperClass's constructor as below.
    this._super();
}).as('myapp.module.MyClass').
it.inherits(SuperClass).
it.borrows(MethodProvider, OtherProvider, AndMore).
it.hasStatic({
    STATIC_PROPERTY: [],
    STATIC_METHOD: function() {}
}).
it.provides({
    method: function() {
        // invocation of a method of the same name in SuperClass's implementation
        this._super();

        // below expressions refer the same static object.
        this._static.STATIC_PROPERTY;
        this.constructor.STATIC_PROPERTY;
        myapp.module.MyClass.STATIC_PROPERTY;
    }
});

The API provides .it as a syntactic ornament which makes class declaration look like natural sentences. Someone would claim that it causes unnecessary object reference, which influences performance. But I think it's acceptable cost unless you declare classes thouthands of times. Nevertheless, if you hate it, you can omit it.

def(MyClass).inherits(SuperClass).provides({...});

Features

Now, let's take a look at each functionalities in detail.

ClassDeclaration

First of all, you need to call require_joo(), which returns a function. To use the function, store it in any variable as you like, such as def, define, declare etc.

var def = require_joo();

// You can define constructor function in a standard way.
function BaseClass(name) {
    this.name = name;
}

// Then, you can define methods by chaining '.it.provides()' method.
def(BaseClass).it.provides({
    getName: function() {
        return this.name;
    }
});

That's it. You can use the class exactly as same as a standard JavaScript class.

var base = new BaseClass('base');

base instanceof BaseClass      // true
base.constructor === BaseClass // true
base.getName();                // 'base'

Inheritance

You can define class inheritance as below:

function ChildClass(name, age) {
    // You can call super class' constructor by 'this._super()'.
    this._super(name);
    this.age = age;
}
def(ChildClass).
it.inherits(BaseClass).
it.provides({
    getAge: function() {
        return this.age;
    }
});

The most exciting thing is that the scope chain of this._super() woks perfectly even in multiple level inheritances. This is difficult to write in a standard way.

function GrandChildClass(name, age) {
    this._super(name, age);
}
def(GrandChildClass).
it.inherits(ChildClass).
it.provides({
    getName: function() {
        // Even if the closest ancestor (in this case, ChildClass) doesn't implement getName() method explicitly,
        // this._super() method looks up to BaseClass's getName() method.
        // This is awesome, isn't it?
        return 'My name is ' + this._super();
    },
    getAge: function() {
        return 'I am ' + this._super();
    },
    toString: function() {
        // You can override built-in 'toString()' method in the same way.
        return this._super() + '(name: ' + this.name + ')'
    }
});

This works perfectly as we expect.

var g = new GrandChildClass('Hiroshi', 29);
g instanceof BaseClass;            // true
g instanceof ChildClass;           // true
g instanceof GrandChildClass;      // true
g.constructor == GrandChildClass;  // true
g.getName(); // 'My name is Hiroshi'
g.getAge();  // 'I am 29'
g + ''       // '[object Object](name: Hiroshi)'

Mix-ins

Our class can borrow methods from other classes or objects through .borrows() method.

// Mix-in provider can be an object;
var MethodProvider = {
    methodA: function() {}
};
// or Classes.
function OtherProvider() {}
OtherProvider.prototype.methodB = function() {};

// Our class can borrow methods from these providers.
function OurClass() {}
def(OurClass).
it.borrows(MethodProvider, OtherProvider).
it.provides({
    ownMethod: function() {}
});

Now OurClass has methodA, methodB, ownMethod, but an instance of the class won't be a Mix-in provider's instance.

var c = new OurClass();
c instanceof MethodProvider; // false
c instanceof OtherProvider;  // false
c.methodA();
c.methodB();
c.ownMethod();

Static Properties

Here is an example of static properties.

function MyClass() {}
def(MyClass).it.hasStatic({
    FOO: 0,
    BAR: 1,
    BLAH: {
        BAZ: true
    }
}).provides({
    method: function() {
        // You can refer to static properties through ._static property in an instance method.
        this._static.FOO == MyClass.FOO; // true
    }
});

MyClass.FOO      // 0
MyClass.BAR      // 1
MyClass.BLAH.BAZ // true

Export To Namespace

It's good habitat to declare your classes in your module scope, then export it to global scope under your namespace; To achieve this, use .as() method.

(function() {
    var def = require_joo();

    function ModuleLocalClass() {}
    def(ModuleLocalClass).as('myapp.module.foo.bar.Blah').
    it.provides({/*...*/});
})();

new ModuleLocalClass() // reference error.
var blah = new myapp.module.foo.bar.Blah();
blah.constructor // ModuleLocalClass

If you pass a second argument as a context object to .as() method, classes are exported under the context instead of a global scope.

(function() {
    var def = require_joo();
    var context = {};
    def(function() {}).as('myapp.module.Class', context);
    new context.myapp.module.Class();
})();

Good points

Some parts of the code, especially .this._super() method implementation was inspired by John Resig's Simple JavaScript Inheritance, but I further improved several points.

In this library,

• init method are not mandatory.

John Resig's implementation and many other libraries which provides original class system often requires us to declare init method as a constructor function. It is indeed convensional, but not standard. I wanted a simple syntax to use normal function as a class constructor. This is the main reason I wrote this code.

• you can easily implement Mix-in class with .borrow() method.
• this._super() method works appropriately both in constructors and in methods, even under multiple level inheritance.
• .constructor property correctly points to its constructor.
• declaration sentences look like a natural language, and has explicit semantics.

I hope this simple library would be useful to other developers.

define(MyClass).as('myapp.module.MyClass').
it.inherits(SuperClass).
it.borrows(Provider, OtherProvider).
it.provides({
    ownMethod: function() {
    }
});