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".

A JavaScript class is not an object.

It is a template for JavaScript objects.

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 is called automatically when a new object is created.

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.";

-

Browser Support

The following table defines the first browser version with full support for Classes in JavaScript:

Chrome 49	Edge 12	Firefox 45	Safari 9	Opera 36
Mar, 2016	Jul, 2015	Mar, 2016	Oct, 2015	Mar, 2016

"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;
  }
}

-

Learn more about "strict mode" in: JS Strict Mode.