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():
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.";
"use strict"
Classes syntax must be written following The "use strict" Directive.
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;
}
}
Try it Yourself »
