class ClassName {
constructor() { ... }
} Example #
class Car {
constructor(name, year) {
this.name = name;
this.year = year;
}
} 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.
