• JS 类就像语法糖，与其他强类型语言的类不同。
• 仅添加语法包装以使来自其他语言的开发人员熟悉。
• 类是一种特殊类型的幕后函数，因此可以写为类表达式和类声明。

## 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'

导入点：

• Fn 声明被提升，而 Class 声明不被提升。
• 也是一等公民，就像 Fns 一样，即可以传递给 fns 并从 fns 返回。
• 无论我们是否激活严格模式，类的主体总是在严格模式下执行。
• 类使代码看起来更干净，如果您知道它是如何在幕后实现的，则可以减少字符噪音。 ** 要成为 JS 专家，您需要了解复杂的语言实现细节，例如类。

访问器属性：获取器和设置器，即获取和设置值的 fns。但从外观上看，它们仍然像普通属性。

普通属性称为数据属性。

• Getter 和 Settter 是 JS 中所有对象所共有的，即每个对象都可以有 getter 和 setter 属性。这些 getter-setter 称为访问器属性，而普通属性称为数据属性。

- Getter 和 setter 是获取和设置值的 fns，从外部看起来就像普通属性。

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.

静态方法

前任。 Array.from() = 将类似数组的结构转换为数组。
Array.from(document.querySelector('h1'));
Array.from(document.querySelectorAll('h1'));

前任。 .from 附加到数组构造函数，而不是构造函数的原型属性。因此，并非所有数组都继承此 fn.
[1,2,3].from(); // .from 不是函数

前任。 Number.parseFloat(12) 是 Number 构造函数上的静态方法，不适用于数字变量。

创建静态方法。

// 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

对象.create():

手动用于将对象的原型设置为我们想要的任何对象。
将用于实现继承黑白类。
使用此 fn 实现原型继承。
Object.create 返回一个空对象。
与构造函数 fns 和类的工作方式不同。
即使不涉及“prototype”、“constructor()”、“new”运算符，仍然存在原型继承的想法。

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();

构造函数 Fn --(.prototype)--> Person.prototype
对象实例 --(proto)--> Person.prototype

工作方式就像在 fn 构造函数或类中一样
不需要 constructor() 或 .prototype 属性来实现此目标。

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 }  

创建原型继承的方法：
构造函数 Fn
ES6 类
对象.create

使用constructor()在类之间进行继承：

所有这些技术都允许对象在其原型上查找方法。
JS 中不存在真正的类。

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'

删除上面示例中的冗余代码：

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' 通过 proto
在对象实例与其原型之间自动建立链接 继承的整个思想是子类可以共享原型链上父类的行为。
原型[Object.prototype] = null; // 位于原型链的顶部。

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