Talking about ES15: classes

2015-09-023 Min Read — In code

Update: I have also recorded a video about classes, if you want to watch it in addition to reading this post.

JavaScript has always been an object-oriented language. It never had a class definition, but the same behavior could be achieved using constructor functions and the prototype object.

Although many people like that, there are a lot of people who don't. So, JavaScript libraries{:target="_blank"} were created to "solve this problem".

Now in ES15 we have the capability of creating classes, which under the hoods, use the same technique of constructor functions and prototype objects. Details you don't really have to know if all you're doing is ES15, but I think you should anyway. ;) So, to define a class in ES15 is really simple.

class MyClass {}
let c = new Class();

There you go, how to define a class and instantiate it. Now, classes may have constructors. Here's an example of a class with a constructor which expects a parameter name.

class Person {
    constructor(name) {
        this.name = name;
    }
}
let p = new Person("Thiago");
console.log(p.name);

In the given example a public field called name was added to the class. We can have getters and setters for that field.

class Person {
    constructor(name) {
        this._name = name;
    }

    get name() {
        return this._name;
    }

    set name(value) {
        this._name = value;
    }
}
let p = new Person("Thiago");
console.log(p.name); // Thiago
p.name = "Temple";
console.log(p.name); // Temple

The code is pretty straightforward, but one thing to notice is that inside the constructor, the value of name was assigned to a "private" variable \name_. That's necessary because otherwise the getter name would call itself until a stack overflow error would be thrown. And, when I say "private", what I mean is that under the hoods, the class is no more than a function and there's no such thing as a private variable inside a function in JavaScript. One could write the code below and would be perfectly valid.

let p = new Person("Thiago");
console.log(p._name); // Thiago

Besides that, you can imagine that getters and setters are useful if we'd like to make some validations when setting a value or getting it.

As you may imagine classes can have methods, but they also can have static methods.

class Person {
    constructor(name) {
        this.name = name;
    }
    sayHello() {
        console.log(`Hello, ${this.name}`);
    }

    static myMethod() {
        console.log("I'm a static method");
    }
}
new Person("Thiago").sayHello(); // Hello, Thiago
Person.myMethod(); // I'm a static method

Classes can inherit from other classes and for that we use the keyword extends.

class Person {
    constructor(name) {
        this.name = name;
    }
}

class User extends Person {
    constructor(name) {
        super(name);
    }

    sayHello() {
        console.log(`Hello, ${this.name}`);
    }
}

let u = new User("Thiago");
u.sayHello(); // Hello, Thiago
console.log(u instanceof Person); // true
console.log(u instanceof User); // true

Here we can see the class User extending the class Person. Also, we could see the use of the keyword super to pass parameters to the parent constructor. The keyword super can be used only inside a constructor and from a method in a child class, like so:

class Person {
    declare() {
        return "I'm a person";
    }
}
class User extends Person {
    declare() {
        return this.super() + " and a User";
    }
}
let u = new User();
console.log(u.declare()); // I'm a person and a User

When declaring classes we're not obligated to declare a constructor, but if we do in a derived class, we have to call super, otherwise a SyntaxError will be thrown.

class Person {}

class User extends Person {
    // valid
}

// SyntaxError
class SpecialPerson extends Person {
    constructor() {}
}

In a derived class, we also have to call super before using this.

class Person {}

class User extends Person {
    constructor() {
        super();
        this.myValue = "a value";
    }
}

class SpecialPerson extends Person {
    constructor() {
        this.myValue = "a value"; // SyntaxError
        super();
    }
}
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