JavaScript Classes

Class Intro

ECMAScript 2015, also known as ES6, introduced JavaScript Classes.
JavaScript Classes are templates for JavaScript Objects.

JavaScript Class Syntax

Use the keyword class to create a class.
Always add a method named constructor():

Syntax

class ClassName {
  constructor() { ... }
}
        ...

Example

class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
}
        ...

The example above creates a class named "Car".

The class has two initial properties: "name" and "year".

Using a Class

When you have a class, you can use the class to create objects:

Example

const myCar1 = new Car("Ford", 2014);
const myCar2 = new Car("Audi", 2019);
        ...

The example above uses the Car class to create two Car objects.

The Constructor Method

The constructor method is a special method:

* It has to have the exact name "constructor"
* It is executed automatically when a new object is created
* It is used to initialize object properties

If you do not define a constructor method, JavaScript will add an empty constructor method.

Class Methods

Class methods are created with the same syntax as object methods.
Use the keyword class to create a class.
Always add a constructor() method.
Then add any number of methods.

Syntax

class ClassName {
  constructor() { ... }
  method_1() { ... }
  method_2() { ... }
  method_3() { ... }
}
        ...

Create a Class method named "age", that returns the Car age:

Example

class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
  age() {
    const date = new Date();
    return date.getFullYear() - this.year;
  }
}

const myCar = new Car("Ford", 2014);
document.getElementById("demo").innerHTML =
"My car is " + myCar.age() + " years old.";
        ...

You can send parameters to Class methods:

Example

class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
  age(x) {
    return x - this.year;
  }
}

const date = new Date();
let year = date.getFullYear();

const myCar = new Car("Ford", 2014);
document.getElementById("demo").innerHTML=
"My car is " + myCar.age(year) + " years old.";
        ...

use strict

The syntax in classes must be written in "strict mode".
You will get an error if you do not follow the "strict mode" rules.

Example

In "strict mode" you will get an error if you use a variable without declaring it:

class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
  age() {
    // date = new Date();  // This will not work
    const date = new Date(); // This will work
    return date.getFullYear() - this.year;
  }
}
        ...

Class Inheritance

To create a class inheritance, use the extends keyword.
A class created with a class inheritance inherits all the methods from another class:

Example

Create a class named "Model" which will inherit the methods from the "Car" class:

class Car {
  constructor(brand) {
    this.carname = brand;
  }
  present() {
    return 'I have a ' + this.carname;
  }
}

class Model extends Car {
  constructor(brand, mod) {
    super(brand);
    this.model = mod;
  }
  show() {
    return this.present() + ', it is a ' + this.model;
  }
}

let myCar = new Model("Ford", "Mustang");
document.getElementById("demo").innerHTML = myCar.show();
        ...

The super() method refers to the parent class.

By calling the super() method in the constructor method, we call the parent's constructor method and gets access to the parent's properties and methods.

Getters and Setters

Classes also allows you to use getters and setters.
It can be smart to use getters and setters for your properties, especially if you want to do something special with the value before returning them, or before you set them.
To add getters and setters in the class, use the get and set keywords.

Example

Create a getter and a setter for the "carname" property:

class Car {
  constructor(brand) {
    this.carname = brand;
  }
  get cnam() {
    return this.carname;
  }
  set cnam(x) {
    this.carname = x;
  }
}

const myCar = new Car("Ford");

document.getElementById("demo").innerHTML = myCar.cnam;
        ...

The name of the getter/setter method cannot be the same as the name of the property, in this case carname.

Many programmers use an underscore character _ before the property name to separate the getter/setter from the actual property:

Example

You can use the underscore character to separate the getter/setter from the actual property:

class Car {
  constructor(brand) {
    this._carname = brand;
  }
  get carname() {
    return this._carname;
  }
  set carname(x) {
    this._carname = x;
  }
}

const myCar = new Car("Ford");

document.getElementById("demo").innerHTML = myCar.carname;
        ...

To use a setter, use the same syntax as when you set a property value, without parentheses:

Example

Use a setter to change the carname to "Volvo":

class Car {
  constructor(brand) {
    this._carname = brand;
  }
  get carname() {
    return this._carname;
  }
  set carname(x) {
    this._carname = x;
  }
}

const myCar = new Car("Ford");
myCar.carname = "Volvo";
document.getElementById("demo").innerHTML = myCar.carname;
        ...

Hoisting

Unlike functions, and other JavaScript declarations, class declarations are not hoisted.
That means that you must declare a class before you can use it:

Example

//You cannot use the class yet.
//myCar = new Car("Ford") will raise an error.

class Car {
  constructor(brand) {
    this.carname = brand;
  }
}

//Now you can use the class:
const myCar = new Car("Ford")
        ...

JavaScript Static Methods

Static class methods are defined on the class itself.
You cannot call a static method on an object, only on an object class.

Example

class Car {
  constructor(name) {
    this.name = name;
  }
  static hello() {
    return "Hello!!";
  }
}

const myCar = new Car("Ford");

// You can call 'hello()' on the Car Class:
document.getElementById("demo").innerHTML = Car.hello();

// But NOT on a Car Object:
// document.getElementById("demo").innerHTML = myCar.hello();
// this will raise an error.
        ...

If you want to use the myCar object inside the static method, you can send it as a parameter:

Example

class Car {
  constructor(name) {
    this.name = name;
  }
  static hello(x) {
    return "Hello " + x.name;
  }
}
const myCar = new Car("Ford");
document.getElementById("demo").innerHTML = Car.hello(myCar);
        ...

References

* All the documentation in this page is taken from W3Schools
* All the documentation in this page is created by Nantungga Putra