JavaScript 2022 - New Features - Part 2

Class State Initialization

The class state can be initialized during declaration

class Emp {
    eno=100;
    ename="ram";
    constructor() {
    }
};

let obj = new Emp();
console.log(obj.eno,obj.ename); // 100 ram

In the above code the state of Emp is initialized during declaration i.e fields eno and ename are initialized to default values. Any object created of Emp will have the state.

Static Class Fields

The static keyword can be used with fields. The fields can be initialized during declaration.

// static field 
class Myclass {
    static eno;
    static name="ram";    
    static show() {
        console.log(Myclass.eno, Myclass.name); // undefined ram                
    }
};
Myclass.show();
console.log(Myclass.eno, Myclass.name); // undefined ram

In the above code the static keyword is used with field and function. The static field is accessed using class name.The static field has common state for all the objects.

Private access

Using the symbol # private access can be enforced. The programmer can make the following private

• field

• function

The private access can also be enforced for static elements.

// Private access
class Myclass2 {
    #data=100;
    static #data2=200;
    #show() {
        console.log("private data", this.#data, Myclass2.#data2); // 100 200
        console.log("")

    }
    show2() {
        this.#show();
    }
};
let obj2 = new Myclass2();
obj2.show2();

In the above code the private accessibility is enforced using # symbol. The element has to be prefixed with # symbol to make it private.

Static fields and Inheritance behaviour

The static fields can be inherited if they are public.

// static in inheritance
class MC {
    static mcField = "Base";
    static show() {
        console.log(MC.mcField);
    }
  };

class CC extends MC {
    static ccField = "Subclass";
  };

  console.log(MC.mcField); // Base
  console.log(CC.mcField); // Base 
  console.log(CC.ccField); // Subclass
  MC.show();// Base

In the above code the fields of MC class can be accessed in the CC class.

Static Block

A class can contain static block which can be used for initializing static fields.

// static block 
class Myclass3 {
    static i; // cannot declare 2 variables
    static j;
    static {
        Myclass3.i=100;
        Myclass3.j=200;
        console.log("Static block - init");
    }
    static show() {
        console.log(Myclass3.i,Myclass3.j); // 100 200
    }
};

Myclass3.show();

In the above code the static block is used to initialize the static fields.

Complete Code Listing

class Emp {
    eno=100;
    ename="ram";
    constructor() {
    }
};

let obj = new Emp();
console.log(obj.eno,obj.ename); // 100 ram

// static field 
class Myclass {
    static eno;
    static name="ram";    
    static show() {
        console.log(Myclass.eno, Myclass.name); // undefined ram                
    }
};
Myclass.show();
console.log(Myclass.eno, Myclass.name); // undefined ram


// Private access
class Myclass2 {
    #data=100;
    static #data2=200;
    #show() {
        console.log("private data", this.#data, Myclass2.#data2); // 100 200
        console.log("")

    }
    show2() {
        this.#show();
    }
};
let obj2 = new Myclass2();
obj2.show2();

// static in inheritance
class MC {
    static mcField = "Base";
    static show() {
        console.log(MC.mcField);
    }
  };

class CC extends MC {
    static ccField = "Subclass";
  };

  console.log(MC.mcField); // Base
  console.log(CC.mcField); // Base 
  console.log(CC.ccField); // Subclass
  MC.show();// Base

  // static block 
class Myclass3 {
    static i; // cannot declare 2 variables
    static j;
    static {
        Myclass3.i=100;
        Myclass3.j=200;
        console.log("Static block - init");
    }
    static show() {
        console.log(Myclass3.i,Myclass3.j); // 100 200
    }
};

Myclass3.show();