Add Concept Exercise: Windowing System (Classes)

concepts/classes/.meta/config.json

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+{
+  "blurb": "JavaScript allows for object-oriented programming. Despite it having a \"class\" keyword, it is still a prototype-based language.",
+  "authors": ["junedev"],
+  "contributors": []
+}

concepts/classes/about.md

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+# About
+
+JavaScript includes the capabilities for object-oriented programming ([OOP][wiki-oop]).
+In OOP, you want to create objects (_instances_) from "templates" (_classes_) so that they include certain data and functionality.
+The data properties are called _fields_ in the OOP context, the function properties are called _methods_.
+
+JavaScript did not have classes at all before they were added to the language specification in 2015, but allowed for object-oriented programming using prototype-based inheritance.
+And even though a `class` keyword is available nowadays, JavaScript is still a _prototype-based_ language.
+
+To understand what it means to be a prototype-based language and how JavaScript actually works, we will go back to the time when there were no classes.
+
+## Prototype Syntax
+
+### Constructor Function
+
+In JavaScript, the template (class) is facilitated by a regular function.
+When a function is supposed to be used as such a template, it is called a _constructor function_ and the convention is that the function name should start with a capital letter.
+Instances (objects) are derived from the template using the `new` keyword when invoking the constructor function.
+
+```javascript
+function Car() {
+  // ...
+}
+
+const myCar = new Car();
+const yourCar = new Car();
+```
+
+It is important to note that in JavaScript, the instances and the constructor function keep a relationship to each other even after the instances were created.
+Every instance object includes a hidden, internal property referred to as `[[prototype]]` in the language specification.
+It holds a reference to the value of the `prototype` key of the constructor function.
+Yes, you read that correctly, a JavaScript function can have key/value pairs because it is also an object behind the scenes.
+
+Since 2015, `[[prototype]]` can be accessed via `Object.getPrototypeOf()`.
+Before that, it was accessible via the key `__proto__` in many environments.
+
+Do not confuse the prototype of an object (`[[prototype]]`) with the `prototype` property of the constructor function.
+
+```exercism/note
+To summarize:
+
+- Constructors in JavaScript are regular functions.
+- Constructing a new instance creates an object with a relation to its constructor called its _prototype_.
+- Functions are objects (callable objects) and therefore they can have properties.
+- The constructor's (function) `prototype` property will become the instance's _prototype_.
+```
+
+### Instance Fields
+
+Often, you want all the derived objects (instances) to include some fields and pass some initial values for those when the object is constructed.
+This can be facilitated via the [`this` keyword][mdn-this].
+Inside the constructor function, `this` represents the new object that will be created via `new`.
+`this` is automatically returned from the constructor function when it is called with `new`.
+
+That means we can add fields to the new instance by adding them to `this` in the constructor function.
+
+```javascript
+function Car(color, weight) {
+  this.color = color;
+  this.weight = weight;
+  this.engineRunning = false;
+}
+
+const myCar = new Car('red', '2mt');
+myCar.color;
+// => 'red'
+myCar.engineRunning;
+// => false
+```
+
+### Instance Methods
+
+Methods are added via the `prototype` property of the constructor function.
+Inside a method, you can access the fields of the instance via `this`.
+This works because of the following general rule.
+
+> When a function is called as a method of an object, its `this` is set to the object the method is called on. [^1]
+
+```javascript
+function Car() {
+  this.engineRunning = false;
+  // ...
+}
+
+Car.prototype.startEngine = function () {
+  this.engineRunning = true;
+};
+
+Car.prototype.addGas = function (litre) {
+  // ...
+};
+
+const myCar = new Car();
+myCar.startEngine();
+myCar.engineRunning;
+// => true
+```
+
+### The Prototype Chain
+
+`myCar` in the example above is a regular JavaScript object and if we would inspect it (e.g. in the browser console), we would not find a property `startEngine` with a function as value directly inside the `myCar` object.
+So how does the code above even work then?
+
+The secret here is called the _prototype chain_.
+When you try to access any property (field or method) of an object, JavaScript first checks whether the respective key can be found directly in the object itself.
+If not, it continues to look for the key in the object referenced by the `[[prototype]]` property of the original object.
+As mentioned before, in our example `[[prototype]]` points to the `prototype` property of the constructor function.
+That is where JavaScript would find the `startEngine` function because we added it there.
+
+```javascript
+function Car() {
+  // ...
+}
+
+Car.prototype.startEngine = function () {
+  // ...
+};
+```
+
+And the chain does not end there.
+The `[[prototype]]` property of `Car.prototype` (`myCar.[[prototype]].[[prototype]]`) references `Object.prototype` (the `prototype` property of the `Object` constructor function).
+It contains general methods that are available for all JavaScript objects, e.g. `toString()`.
+The `[[prototype]]` of `Object` is usually `null` so the prototype chain ends there.
+In conclusion, you can call `myCar.toString()` and that method will exist because JavaScript searches for that method throughout the whole prototype chain.
+You can find a detailed example in the [MDN article "Inheritance and the prototype chain"][mdn-prototype-chain-example].
+
+```exercism/caution
+Note that the prototype chain is only travelled when retrieving a value.
+Setting a property directly or deleting a property of an instance object only targets that specific instance.
+This might not be what you would expect when you are used to a language with class-based inheritance.
+```
+
+### Dynamic Methods (Adding Methods to All Existing Instances)
+
+JavaScript allows you to add methods to all existing instances even after they were created.
+
+We learned that every instance keeps a reference to the `prototype` property of the constructor function.
+That means if you add an entry to that `prototype` object, that new entry (e.g. a new method) is immediately available to all instances created from that constructor function.
+
+```javascript
+function Car() {
+  this.engineRunning = false;
+}
+
+const myCar = new Car();
+// Calling myCar.startEngine() here would result in "TypeError:
+// myCar.startEngine is not a function".
+
+Car.prototype.startEngine = function () {
+  this.engineRunning = true;
+};
+
+myCar.startEngine();
+// This works, even though myCar was created before the method
+// was added.
+```
+
+In theory, dynamic methods can even be used to extend the functionality of built-in objects like `Object` or `Array` by modifying their prototype.
+This is called _monkey patching_.
+Because this change affects the whole application, it should be avoided to prevent unintended side effects.
+The only reasonable use case is to provide a [polyfill][wiki-polyfill] for a missing method in older environments.
+
+## Class Syntax
+
+Nowadays, JavaScript supports defining classes with a `class` keyword.
+This was added to the language specification in 2015.
+On the one hand, this provides syntactic sugar that makes classes easier to read and write.
+The new syntax is more similar to how classes are written in languages like C++ or Java.
+Developers switching over from those languages have an easier time to adapt.
+On the other hand, class syntax paves the way for new language features that are not available in the prototype syntax.
+
+### Class Declarations
+
+With the new syntax, classes are defined with the `class` keyword, followed by the name of the class and the class body in curly brackets.
+The body contains the definition of the constructor function, i.e. a special method with the name `constructor`.
+This function works just like the constructor function in the prototype syntax.
+The class body also contains all method definitions.
+The syntax for the methods is similar to the shorthand notation we have seen for adding functions as values inside an object, see [Concept Objects][concept-objects].
+
+```javascript
+class Car {
+  constructor(color, weight) {
+    this.color = color;
+    this.weight = weight;
+    this.engineRunning = false;
+  }
+
+  startEngine() {
+    this.engineRunning = true;
+  }
+
+  addGas(litre) {
+    // ...
+  }
+}
+
+const myCar = new Car();
+myCar.startEngine();
+myCar.engineRunning;
+// => true
+```
+
+Similar to function declarations and function expressions, JavaScript also supports [class expressions][mdn-class-expression] in addition to the _class declaration_ shown above.
+
+Keep in mind that behind the scenes, JavaScript is still a prototype-based language.
+All the mechanisms we learned about in the "Prototype Syntax" section above still apply.
+
+### Private Fields, Getters and Setters
+
+By default, all instance fields are public in JavaScript.
+They can be directly accessed and assigned to.
+
+Adding actual private fields to the language specification is work in progress, see the [proposal document][proposal-private-fields] for details.
+
+In the meantime, you can make use of the established convention that fields and methods that start with an underscore should be treated as private.
+They should never be accessed directly from outside the class.
+
+Private fields are sometimes accompanied by [getters][mdn-get] and [setters][mdn-set].
+With the keywords `get` and `set` you can define functions that are executed when a property with the same name as the function is accessed or assigned to.
+
+```javascript
+class Car {
+  constructor() {
+    this._milage = 0;
+  }
+
+  get milage() {
+    return this._milage;
+  }
+
+  set milage(value) {
+    throw new Error(`Milage cannot be manipulated, ${value} is ignored.`);
+    // Just an example, usually you would not provide a setter in this case.
+  }
+}
+
+const myCar = new Car();
+myCar.milage;
+// => 0
+myCar.milage = 100;
+// => Error: Milage cannot be manipulated, 100 is ignored.
+```
+
+### Class Fields and Class Methods
+
+In OOP, you sometimes want to provide utility fields or methods that do not depend on the specific instance.
+Instead, they are defined for the class itself.
+This can be achieved with the `static` keyword.
+
+```javascript
+class Car {
+  static type = 'vehicle';
+
+  static isType(targetType) {
+    return targetType === 'vehicle';
+  }
+}
+
+Car.type;
+// => 'vehicle'
+
+Car.isType('road sign');
+// => false
+```
+
+### Class-Based Inheritance
+
+Besides the type of [inheritance][wiki-inheritance] along the prototype chain we saw earlier, you can also represent inheritance between classes in JavaScript.
+This is covered in the [Concept Inheritance][concept-inheritance].
+
+---
+
+[^1]: `this` Examples - As an object method, MDN. <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this#as_an_object_method> (referenced December 03, 2021)
+
+[wiki-oop]: https://en.wikipedia.org/wiki/Object-oriented_programming
+[mdn-prototype-chain-example]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain#inheritance_with_the_prototype_chain
+[concept-inheritance]: /tracks/javascript/concepts/inheritance
+[mdn-class-expression]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes#Class_expressions
+[wiki-inheritance]: https://en.wikipedia.org/wiki/Inheritance_(object-oriented_programming)
+[proposal-private-fields]: https://github.com/tc39/proposal-private-methods#private-methods-and-fields
+[mdn-get]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get
+[mdn-set]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/set
+[wiki-polyfill]: https://en.wikipedia.org/wiki/Polyfill_(programming)
+[mdn-this]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this
+[concept-objects]: /tracks/javascript/concepts/objects