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12.1 JS 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".
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 1: JS Classes - Using a Class

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:
- 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 2: JS Class Methods part 1

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.";

You can send parameters to Class methods:

Example 3: JS Class Methods part 2

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 4: JS Classes uses "strict mode"

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;
  }
}

JavaScript Fundamentals course home
Module 12. JS Classes

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Example files created in this module:
JS Classes - Using a Class JS Class Methods part 1 JS Class Methods part 2 JS Classes uses "strict mode" JS Class Inheritance JS Class Getter/Setter part 1 JS Class Getter/Setter part 2 JS Class Getter/Setter part 3 JS Classes are not Hoisted JS Class Static Methods part 1 JS Class Static Methods part 2