Before getting started on the EventEmitter class, we need to take a look at another of the ES2015 features: classes. The JavaScript language has always had objects, and a concept of a class hierarchy, but nothing so formal as in other languages. The ES2015 class object builds on the existing prototype-based inheritance model, but with a syntax looking very much like class definitions in other languages.

For example, consider this class we'll be using later in the book:

class Note {
    constructor(key, title, body) {
        this._key = key;
        this._title = title;
        this._body = body;
    }
    get key() { return this._key; }
    get title() { return this._title; }
    set title(newTitle) { return this._title = newTitle; }
    get body() { return this._body; }
    set body(newBody) { return this._body = newBody; }
}

Once you've defined the class, you can export the class definition to other modules:

module.exports.Note = class Note { .. }   # in CommonJS modules
export class Note { .. }                  # in ES6 modules

The functions marked with get or set keywords are getters and setters, used like so:

var aNote = new Note("key", "The Rain in Spain", "Falls mainly on the plain");
var key = aNote.key;
var title = aNote.title;
aNote.title = "The Rain in Spain, which made me want to cry with joy";

New instances of a class are created with new. You access a getter or setter function as if it is a simple field on the object. Behind the scenes, the getter/setter function is invoked.

The preceding implementation is not the best because the _title and _body fields are publicly visible, and there is no data hiding or encapsulation. We'll go over a better implementation later.

One tests whether a given object is of a certain class by using the instanceof operator:

if (anotherNote instanceof Note) {
    ... it's a Note, so act on it as a Note
}

Finally, you declare a subclass using the extends operator, similar to what's done in other languages:

class LoveNote extends Note {
    constructor(key, title, body, heart) {
        super(key, title, body);
        this._heart = heart;
    }
    get heart() { return this._heart; }
    set heart(newHeart) { return this._heart = newHeart; }
}

In other words, the LoveNote class has all the fields of Note, plus this new field named heart.