Class

A class has an identifier, which we can use to create new objects using new ClassIdentifier().

Syntax

class Person {
  constructor(name) {
    this.name = name
  }
  
  hello() {
    return 'Hello, I am ' + this.name + '.'
  }
}

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 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
  }
}