How to use JavaScript Classes
In 2015 the ECMAScript 6 (ES6) standard introduced classes. Learn all about them
In 2015 the ECMAScript 6 (ES6) standard introduced classes.
JavaScript has a quite uncommon way to implement inheritance: prototypical inheritance. Prototypal inheritance, while in my opinion great, is unlike most other popular programming language’s implementation of inheritance, which is class-based.
People coming from Java or Python or other languages had a hard time understanding the intricacies of prototypal inheritance, so the ECMAScript committee decided to sprinkle syntactic sugar on top of prototypical inheritance so that it resembles how class-based inheritance works in other popular implementations.
This is important: JavaScript under the hood is still the same, and you can access an object prototype in the usual way.
A class definition
This is how a class looks.
class Person {
constructor(name) {
this.name = name
}
hello() {
return 'Hello, I am ' + this.name + '.'
}
}
A class has an identifier, which we can use to create new objects using new ClassIdentifier()
.
When the object is initialized, the constructor
method is called, with any parameters passed.
A class also has as many methods as it needs. In this case hello
is a method and can be called on all objects derived from this class:
const flavio = new Person('Flavio')
flavio.hello()
Class inheritance
A class can extend another class, and objects initialized using that class inherit all the methods of both classes.
If the inherited class has a method with the same name as one of the classes higher in the hierarchy, the closest method takes precedence:
class Programmer extends Person {
hello() {
return super.hello() + ' I am a programmer.'
}
}
const flavio = new Programmer('Flavio')
flavio.hello()
(the above program prints “Hello, I am Flavio. I am a programmer.“)
Classes do not have explicit class variable declarations, but you must initialize any variable in the constructor.
Inside a class, you can reference the parent class calling super()
.
Static methods
Normally methods are defined on the instance, not on the class.
Static methods are executed on the class instead:
class Person {
static genericHello() {
return 'Hello'
}
}
Person.genericHello() //Hello
Private methods
JavaScript does not have a built-in way to define private or protected methods.
There are workarounds, but I won’t describe them here.
Getters and setters
You can add methods prefixed with get
or set
to create a getter and setter, which are two different pieces of code that are executed based on what you are doing: accessing the variable, or modifying its value.
class Person {
constructor(name) {
this._name = name
}
set name(value) {
this._name = value
}
get name() {
return this._name
}
}
If you only have a getter, the property cannot be set, and any attempt at doing so (outside of the constructor, which sets the value when you initialize a new object with this class) will be ignored:
class Person {
constructor(name) {
this._name = name
}
get name() {
return this._name
}
}
If you only have a setter, you can change the value but not access it from the outside:
class Person {
constructor(name) {
this._name = name
}
set name(value) {
this._name = value
}
}
Getters and setters are very useful when you want to execute some code upon changing the property value, or if you want to create a “computed” property. You can alter the values you return by using a getter.
You can also run some code, like logging to the console or to a file when a value is changed.
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