OOP in JS - 2/2

• JS classes are like syntactic sugar, not the same as classes of other strongly typed languages.
• Only adds syntax-wrapping to make it familiar for devs coming from other languages.
• classes are a special type of functions behind the scenes, hence can be written as class expression as well as class declaration.

## class expression:
const Person = class {
}

## class declaration:
class Person {
  constructor(fName, bYear){
   this.fName = fName;
   this.bYear = bYear;
  }
  calcAge(){
   console.log(2024 - this.bYear);
  }
}

- constructor is a method of this class. Pass values for properties to have in objects created using this fn.
- then set the properties of the object using this.xxx = xxx;
- On using 'new' operator, this constructor will be called automatically and return a new object which will be stored in LHS variable as shown below.
Ex. const ronald = new Person('ronald',1975); // Person { fName: 'ronald', bYear: 1975 }
- Methods are written outside the constructor fn and will be added to the prototype property of the object which can be verified using devConsole.
Ex. ronald.calcAge(); // 49

ronald.__proto__ === Person.prototype; // true

- No commas need to be added while adding multiple methods below the constructor fn inside the class.

## Hence, the above syntax works same as constructor fn syntax but with a familiar syntax of strongly typed class based languages.

## Adding a fn explicitly to the prototype:
Person.prototype.greet = function(){
console.log(`Hey ${this.fName}`);
}
ronald.greet(); // 'Hey ronald'

Impt Points:

• Fn declarations are hoisted while Class declarations are NOT hoisted.
• Also first class citizen just like Fns i.e can be passed-to & returned-from fns.
• Body of class is always executed in strict mode whether we activate the strict mode or not.
• Classes make the code look cleaner, with reduced character noise provided you know how its implemented under the hood. ** To become an expert in JS, you need to understand the intricate language implementation details like with classes.

Accessor Properties: Getters & Setters i.e fns that get & set the value. But on the outside, they still look like regular properties.

Normal Properties are called Data Properties.

• Getters & Settters are common to all objects in JS i.e every object can have getter and setter property. These getter-setter are called as accessor properties, while normal properties are called as data properties.

- Getter & setter are fns that get and set a value, from outside look like normal properties.

const account = {
  owner: 'jonas',
  movements: [200,300,100,500],
  get latest(){
    // will return an array with last value. Hence, use pop to get the value.
    return this.movements.slice(-1).pop();
  },
  set latest(mov){
    this.movements.push(mov);
  }
}

account.latest; // 500
account.latest = 50; 
account.latest; // 50

Just like above, classes also support the getter-setter methods but acccessed like using a property syntax.

These are very useful for data validation.

Static Methods

Ex. Array.from() = Converts array-like structure to array.
Array.from(document.querySelector('h1'));
Array.from(document.querySelectorAll('h1'));

Ex. .from is attached to the Array construcutor, not to the prototype property of the constructor. Hence all arrays don't inherit this fn.
[1,2,3].from(); // .from is not a function

Ex. Number.parseFloat(12) is a static method on Number constructor, not available on number variables.

Creating a static method.

// Static methods are not inherited. They are not added to prototype.
className.fnName = function(){
  console.log(this); // Entire constructor() which is calling the method
  console.log("JS is awesome")
};
className.fnName();

// Rule =  whatever object is calling the method, 'this' points to that object inside the fn. Hence its simply the entire constructor() above.

//Inside class, we need to use static keyword for adding a static method.
static fnName = function(){
  console.log(this); // can point to the entire class defn
  console.log("JS is awesome")
};

// Static methods and instance methods will be different from each other.
// instance methods will be prototype, hence all instances can have access to them

Object.create():

Manually used to set prototype of our object to any object that we want.
Will be used to implement inheritance b/w classes.
Prototypal Inheritance Implemented using this fn.
Object.create returns an empty object.
Works in a different way in which constructor fns and classes work.
Still there is idea of prototypal inheritance, even without involvement of 'prototype', 'constructor()', 'new' operator.

const PersonProto = {
  // This method will be looked up using __proto__ link
  calcAge(){
    console.log(2024 - this.bYear);
  }
};

// baba will be created, with its prototype set to PersonProto object.
const baba = Object.create(PersonProto);
baba;

baba.name = 'Roger';
baba.bYear = '2000';
baba.calcAge();

Constructor Fn --(.prototype)--> Person.prototype
Object Instance --(proto)--> Person.prototype

Works just like it worked for fn constructors or in classes
No need of constructor() or .prototype property to achieve this goal.

const PersonProto = {
  // This method will be looked up using __proto__ link
  calcAge(){
    console.log(2024 - this.bYear);
  },
  // Noting special with init name, its a normal fn here.
  // This has nothing to with ES6 constructor()
  // Manual way of initialzing an object.
  init(fName, bYear){
    this.fName = fName;
    this.bYear = bYear;
  }
};

// baba will be created, with its prototype set to PersonProto object.
const baba = Object.create(PersonProto);
baba;

baba.name = 'Roger';
baba.bYear = '2000';
baba.calcAge();

baba.__proto__;    // { calcAge: [Function: calcAge] }
baba.__proto__ === PersonProto; //true


const alice = Object.create(PersonProto);
alice.init("alice", 2000);
alice;   // { fName: 'alice', bYear: 2000 }  

Ways to create Prototypal Inhertitance:
Constructor Fn
ES6 Classes
Object.create

Inheritance between Classes using constructor():

All of these techniques allow object to lookup for methods on its prototype.
Real classes do not exist in JS.

const Person = function(firstName, bYear){
  this.firstName = firstName;
  this.bYear = bYear;
};

Person.prototype.calcAge = function(){
  console.log(2024 - this.bYear);
};

const Student = function(firstName, bYear, course){
  // This is the duplicate code, any change in Person won't be reflected here.
  this.firstName = firstName;
  this.bYear = bYear;
  this.course = course;
};

Student.prototype.introduce = function(){
  console.log(`My name is ${this.firstName} and I study ${this.course}`);
}

const matt = new Student("Matt", 2000, "CSE");
matt.introduce(); //  'My name is Matt and I study CSE'

Removing redundant code from above example:

const Person = function(firstName, bYear){
  this.firstName = firstName;
  this.bYear = bYear;
};

Person.prototype.calcAge = function(){
  console.log(2024 - this.bYear);
};

const Student = function(firstName, bYear, course){
  // Person(firstName, bYear); -> This doesn't work because we are calling it as a regular fn call. 'new' has to be used to call this fn constructor. This fn call is simply a regular fn call, in which 'this' is set 'undefined'. Hence, an error as it cannot set firstName on undefined.
  // We want to set the 'this' inside this fn to be same as inside Person above.
  Person.call(this, firstName, bYear);
  this.course = course;
};

Student.prototype.introduce = function(){
  console.log(`My name is ${this.firstName} and I study ${this.course}`);
}

const matt = new Student("Matt", 2000, "CSE");
matt.introduce(); //  'My name is Matt and I study CSE'

'new' makes a link automatically between object instance and its prototype via proto
Whole idea of inheritance is that child class can share behavior from parent classes up the prototype chain.
Prototype[Object.prototype] = null; // Sits on top of prototype chain.

const Person = function(firstName, bYear){
  this.firstName = firstName;
  this.bYear = bYear;
};

Person.prototype.calcAge = function(){
  console.log(2024 - this.bYear);
};

const Student = function(firstName, bYear, course){
  Person.call(this, firstName, bYear);
  this.course = course;
};

// Student.prototype = Person.prototype; => This doesn't work because we won't get the prototype chain, rather we will get 
// Constructor fn[i.e Person()]    --------------> Person.prototype
// Constructor fn[i.e Student()]   --------------> Person.prototype
// Object [Matt] __proto__: Student.prototype ---> Person.prototype

// Student.prototype manually linked for lookup to Person.prototype.
// This has to be done here and not after else Object.create will overwrite any of the existing methods like introduce() on it.
Student.prototype = Object.create(Person.prototype);

Student.prototype.introduce = function(){
  console.log(`My name is ${this.firstName} and I study ${this.course}`);
}

const matt = new Student("Matt", 2000, "CSE");
matt.introduce(); //  'My name is Matt and I study CSE'
matt.calcAge();    // 24

matt.__proto__;                   // Person { introduce: [Function (anonymous)] }
matt.__proto__.__proto__;        // { calcAge: [Function (anonymous)] }
matt.__proto__.__proto__.__proto__;   // [Object: null prototype] {}

Student.prototype.constructor = Student;   // [Function: Student]

matt instanceof Student; // true
matt instanceof Person; // true
matt instanceof Object; // true

class syntax hides a lot of behind the scene details.
Its just a layer of abstraction over constructor fns.
Mainly extend, super are needed to implement the same functionality of inheritance using classes which was established above using constructor fns.

extends - inheritance between classes, automatically sets prototype chain for us.

public field - similar to property defined in the constructor, available on created object

private field - not accessible outside of class, perfect for implementation of data privacy & encapsulation

static public field - available only on class

constructor() - automatically called by new operator whenever a new object from the class is created. Mandatory in parent class, can be omitted in child class if we want to have exact same name & same no of arguments in child class.

super():
its the constructor fn of the parent class, just pass-in the required arguments.
Always need to happen first, as this call is responsible for creating 'this' keyword in child-class.
Without doing this, we won't be able to access the 'this.course = course'
call to parent(super) class (necessary with extend). Needs to happen before accessing 'this' keyword in the constructor.

instance property - available on each created object, set based on input data of the constructor. fields are common for all the object, while these properties are unique for each object.

getter method - get a value out of an object by simply writing a property instead of a method.

setter method - use '_' to set a property with name as method, and also add getter.

static method - available only on class, cannot access instance properties nor methods, only static ones. Ex. only static public fields will be accessible inside static methods.

NOTE: Classes are:

1. syyntactic sugar over constructor fns.
2. not hoisted
3. first class citizens just like fns.
4. class body is always executed in strict mode