JavaScript Classes

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(): 

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 

let myCar1 = new Car("Ford", 2014);
let 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:

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() { ... }
}

Example

Create a Class method named "age", that returns the Car age: 
class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
  age() {
    let date = new Date();
    return date.getFullYear() - this.year;
  }
}
let myCar = new Car("Ford", 2014);
document.getElementById("demo").innerHTML =
"My car is " + myCar.age() + " years old.";

Example

You can send parameters to Class methods: 
class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
  age(x) {
    return x - this.year;
  }
}

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

let 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
    let date = new Date(); // This will work
    return date.getFullYear() - this.year;
  }
}
Learn more about "strict mode" in: JS Strict Mode.