JavaScript Classes

ECMAScript2015, introduced classes.

A class is a type of function, but instead of using the keyword function to initiate it, we use the keyword class, and the properties are assigned inside a constructor() method.

Class Definition
Use the keyword class to create a class, and always add the constructor() method.

The constructor method is called each time the class object is initialized.

Example
A simple class definition for a class named "Car":

class Car {
  constructor(brand) {
    this.carname = brand;
  }
}
Now you can create objects using the Car class:

Example
Create an object called "mycar" based on the Car class:

class Car {
  constructor(brand) {
    this.carname = brand;
  }
}
mycar = new Car("Ford");

Note: The constructor method is called automatically when the object is initialized.

Methods
The constructor method is special, it is where you initialize properties, it is called automatically when a class is initiated, and it has to have the exact name "constructor", in fact, if you do not have a constructor method, JavaScript will add an invisible and empty constructor method.

You are also free to make your own methods, the syntax should be familiar:

Example
Create a method named "present":

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

mycar = new Car("Ford");
document.getElementById("demo").innerHTML = mycar.present();

As you can see in the example above, you call the method by referring to the object's method name followed by parentheses (any parameters would go inside the parentheses).

Example
Send a parameter to the "present()" method:

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

mycar = new Car("Ford");
document.getElementById("demo").innerHTML = mycar.present("Hello");

Static Methods
Static methods are defined on the class itself, and not on the prototype.

That means you cannot call a static method on the object (mycar), but on the class (Car):

Example
Create a static method and call it on the class:

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

mycar = new Car("Ford");

//Call 'hello()' on the class Car:
document.getElementById("demo").innerHTML = Car.hello();

//and NOT on the 'mycar' object:
//document.getElementById("demo").innerHTML = mycar.hello();
//this would raise an error.

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

Example
Send "mycar" as a parameter:

class Car {
  constructor(brand) {
    this.carname = brand;
  }
  static hello(x) {
    return "Hello " + x.carname;
  }
}

mycar = new Car("Ford");

document.getElementById("demo").innerHTML = Car.hello(mycar);

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;
  }
}

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;
  }
}

mycar = new Car("Ford");

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

Note: even if the getter is a method, you do not use parentheses when you want to get the property value.

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;
  }
}

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;
  }
}

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")
//This would raise an error.

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

//Now you can use the class:
mycar = new Car("Ford")
Note: For other declarations, like functions, you will NOT get an error when you try to use it before it is declared, because the default behavior of JavaScript declarations are hoisting (moving the declaration to the top).

"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(brand) {
    i = 0;
    this.carname = brand;
  }
}
var mycar = new Car("Ford");
Learn more about "strict mode" in our JavaScript Strict Mode chapter.