JavaScript Class Implementation

This implementation adds the capability for a class to have protected members that can be accessed by derivative class constructors.

This is a JavaScript module. It can be imported into your script like so: import Class from "Class.mjs"

Class.extend()

Creates a child class. This is a static method of Class and its derivative classes.

Syntax

Class.extend(init)Class.extend(init,call)

Parameters

init
An initializer function that is called as part of the child class's constructor. The name of the initializer is used as the name of the class.

calloptional
A handler function for when the class is called without using the new keyword. Default behavior is to throw a TypeError.

Return value

The new class constructor. It has its own static copy of the extend method.

Initializer

The signature of an initializer function is expected to be:

functionMyClassName($super, ...args){//... code that does not include `this`constprotectedMembers=$super(arg1,arg2, ...);//... code that may include `this`}

$super
A Proxy to the parent class's constructor, bound as the first argument of the initializer. It is to be used like the super keyword. It must be called exactly once during the execution of the initializer, before any reference to this.

protectedMembers
An object whose members are shared among all the initializers that are executed when a new instance of the class is created. This allows a protected value defined in the initializer of a class to be accessed and modified within the initializer of a derivative class directly, without needing static getters and setters.

Examples

Create a new class

constMyClass=Class.extend(functionRectangle($super,width,height){$super();this.dimensions=()=>width+" x "+height;});letr=newMyClass(2,3);console.log(MyClass.name);// Rectangleconsole.log(r.toString());// [object Rectangle]console.log(r.dimensions());// 2 x 3

Use a call handler

constRectangle=Class.extend(functionRectangle($super,width,height){$super();this.dimensions=()=>width+" x "+height;},(width,height)=>"area is "+(width*height)+" square units"//handler for when Rectangle() is called without using `new`);console.log(Rectangle(2,3));// area is 6 square units

Inherit from a superclass

constRectangle=Class.extend(functionRectangle($super,width,height){$super();this.dimensions=()=>width+" x "+height;});constSquare=Rectangle.extend(functionSquare($super,width){$super(width,width);//this.dimensions() is inherited from Rectangle});lets=newSquare(2);console.log(s.dimensions());// 2 x 2

Use static methods of the parent class

constRectangle=Class.extend(functionRectangle($super,width,height){$super();this.dimensions=()=>width+" x "+height;});Rectangle.area=function(width,height){returnwidth*height;};constSquare=Rectangle.extend(functionSquare($super,width){$super(width,width);this.area=function(){return$super.area(width,width);//here, using `$super` as an object is equivalent to using `Rectangle`};});lets=newSquare(2);console.log(Rectangle.area(2,2));// 4console.log(s.area());// 4

Use protected members

constRectangle=Class.extend(functionRectangle($super,width,height){constprot=$super();prot.width=width;prot.height=height;Object.defineProperty(this,"width",{enumerable: true,configurable: true,get(){returnprot.width;},set(width){returnprot.width=width;}});Object.defineProperty(this,"height",{enumerable: true,configurable: true,get(){returnprot.height;},set(height){returnprot.height=height;}});this.dimensions=()=>prot.width+" x "+prot.height;});constSquare=Rectangle.extend(functionSquare($super,width){constprot=$super(width,width);Object.defineProperty(this,"width",{enumerable: true,configurable: true,get(){returnprot.width;},set(width){returnprot.width=prot.height=width;}});Object.defineProperty(this,"height",{enumerable: true,configurable: true,get(){returnprot.height;},set(height){returnprot.height=prot.width=height;}});});lets=newSquare(2);console.log(s.dimensions());// 2 x 2s.height=3;console.log(s.dimensions());// 3 x 3