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Class Intro

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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 #

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 #

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 #

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.

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