Overview

The JavaScript language already supports object orientated programming, but the syntax is strange and inheritance is wonky. This library is provided as a means to create classes in a more classical sort of way, including support for static class members, proper constructors, inheritance, and mixins.

Usage

Use npm to install the module:

npm install pixl-class

Then use require() to load it in your code:

var Class = require('pixl-class');

Then call Class.create() to create classes. See the next section for details.

Creating Classes

Here is how you create a class using the framework. You'll notice this is dramatically different than the traditional JavaScript prototype syntax.

var Animal = Class.create({
	
	// class member variables
	nickname: '',
	color: '',
	
	// class constructor
	__construct: function(new_name, new_color) {
		this.nickname = new_name;
		this.color = new_color;
	},
	
	// methods
	getInfo: function() {
		return("Nickname: " + this.nickname + "\nColor: " + this.color);
	}
	
});

This defines a class called Animal, with two member variables, nickname and color, a constructor and a getInfo() method which returns the nickname and color. You'll notice that to define the constructor method you use the keyword __construct which is exactly the same in PHP. Usage of this class is exactly what you would expect:

var dog = new Animal('Spot', 'Green');
console.log( dog.getInfo() );

Of course, you can also access the class member variables, as all members are public.

dog.nickname = 'Skippy';
dog.color = 'Blue';
console.log( dog.getInfo() );

Creating Subclasses

To create a subclass that inherits from a base class, use the following syntax:

var Bear = Class.create({
	
	// inherit from Animal
	__parent: Animal,
	
	// define a new member variable
	wants: 'Honey',
	
	// and a new method
	roar: function() {
		console.log("Roar!  Give me " + this.wants + "!");
	}
	
});

This defines a Bear class which inherits from the base Animal class, including its constructor. Notice that we passed the parent class as a reference, not a string. What we did is extend the base class by introducing a new member variable wants, and a new method roar(). Everything else from the base class will be present in subclass instances.

var grizzly = new Bear('Fred', 'Brown');
console.log( grizzly.getInfo() );

grizzly.wants = 'blood';
grizzly.roar();

Calling Superclass methods

You can also explicitly invoke a superclass method, in order to extend its functionality:

var Bear = Class.create({
	__parent: Animal,
	wants: 'Honey',
	roar: function() { console.log("Roar!  Give me " + this.wants + "!"); },
	
	// override base class method
	getInfo: function() {
		// first, get info from base class
		var info = Animal.prototype.getInfo.call(this);
		
		// append bear info and return combined info
		info += "\nWants: " + this.wants;
		return info;
	}
	
});

So here we are overriding the base class getInfo() method, but the first thing we do is call the superclass method of the same name. This is done using the Animal.prototype syntax, which points to the parent class prototype object. The JavaScript call() method allows you to call a function in object context (hence we are passing in the this keyword to it).

Invoking a superclass constructor is even easier, as the class variable is the constructor:

var Bear = Class.create({
	
	__parent: Animal,
	wants: 'Honey',
	
	// override base class constructor
	__construct: function(new_name, new_color, new_wants) {
		// invoke superclass constructor to set name and color
		Animal.call(this, new_name, new_color);
		this.wants = new_wants;
	}
	
	// and a new method
	roar: function() {
		alert("Roar!  Give me " + this.wants + "!");
	}
	
});

We don't provide a fancy way to access the parent class, because that would dirty up instance objects. Instead, we went for the clean approach. Your instance objects are pure, and not littered with any special case properties.

Static Members

You can define static class members (variables or methods) by using the __static keyword. These members do not become part of class instances, but instead live inside the class reference object, and must be accessed that way too. Example class definition:

var Beer = Class.create({
	
	// static members
	__static: {
		types: ['Lager', 'Ale', 'Stout', 'Barleywine']
	},
	
	// class member variables
	nickname: '',
	type: '',
	
	// class constructor
	__construct: function(new_name, new_type) {
		this.nickname = new_name;
		
		if (!Beer.types.indexOf(new_type) == -1) throw("Type not known: " + new_type);
		this.type = new_type;
	},
	
	// methods
	getInfo: function() {
		return("Nickname: " + this.nickname + "\nType: " + this.type);
	}
	
});

Here we define a Beer class which has a static member defined in the __static element. Anything placed there will not be propagated to class instances, and must be accessed using the class reference variable instead (e.g. Beer in the above example). As you can see in the constructor, we are checking the new type against the types array which is declared static, so we are getting to the list by using the syntax: Beer.types rather than this.types.

If you were to change Beer.types later on, then all classes would see the changes instantly. The content is effectively shared.

Mixins

You can define "mixin" classes using the __mixins keyword. This will import all the variables, methods and static members from the specified classes, excluding constructors. Example:

var Liquid = Class.create({
	flavor: "sweet"
});

var Glass = Class.create({
	size: 8
});

var Soda = Class.create({
	
	__mixins: ['Liquid', 'Glass'],
	
	drink: function() {
		console.log("Yum, " + this.size + " oz of " + this.flavor + " drink!");
	}
	
});

So in this example, we are importing all the variables and methods of the Liquid and Glass classes into our Soda class. Then, they are accessible using the normal this keyword, as if they were defined in the class.

Note that mixin properties will only be imported if they aren't already defined in your class. Meaning, they will not clobber any existing class members.

EventEmitters

All classes generated with Class.create() are event emitters by default. Meaning, they all have methods such as on(), once(), and emit(). Basically they inherit all the methods from Node's EventEmitter class, and can use them directly, as shown in this example:

var Party = Class.create({
	
	start: function() {
		console.log("Let's get this party started!");
		this.emit('dance');
	}
	
});

var birthday = new Party();
birthday.on('dance', function() {
	console("I'm dancing!");
} );
birthday.start();

If you don't want your classes to inherit from EventEmitter, simply declare an __events property and set it to false.

var Party = Class.create({
	
	// do not inherit from EventEmitter
	__events: false,
	
	start: function() {
		console.log("Let's get this party started!");
	}
	
});

Async/Await

Node.js v8 introduced native support for the async/await pattern. If your class has callback-based methods that you want to auto-convert into promises for async/await, simply declare a __promisify property, and set it to true:

var Sleeper = Class.create({
	
	// promisify all methods
	__promisify: true,
	
	sleep: function(ms, callback) {
		// sleep for N milliseconds, then fire callback
		setTimeout( function() { callback(false); }, ms );
	}
	
});

This will wrap all your methods with Node's util.promisify, making them instantly ready for async/await. Example usage:

var snooze = new Sleeper();

async function main() {
	await snooze.sleep( 1000 ); // waits for 1 second here
	console.log("This happened 1 second later!");
};

main();

If you only want some of your methods to be promisified, set the __promisify property to an array containing all the method names. Example:

{
	// only promisify some methods
	__promisify: ["sleep"]
}

Note that in order for your methods to be promise-compatible, they must accept a callback as the final argument, and that callback must be called using the standard Node.js convention (i.e. (err) or (err, result)). The error must be the first argument sent to the callback (or false/undefined on success), and a result, if any, must be the second argument.

License

Copyright (c) 2015 - 2017 Joseph Huckaby

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.