The Secret Life of Objects

Encapsulation

The core idea in object-oriented programming is to divide programs into smaller pieces and make each piece responsible for managing its own state.

Such program pieces are modeled using objects. Their interface consists of a specific set of methods and properties. Properties that are part of the interface are called public. The others, which outside code should not be touching, are called private.

Many languages provide a way to distinguish public and private properties and prevent outside code from accessing the private ones altogether. JavaScript, taking the minimalist approach, does not—not yet at least. There is work underway to add this to the language.

It is also common to put an underscore (_) character at the start of property names to indicate that those properties are private.

Methods

Methods are nothing more than properties that hold function values. This is a simple method:

let rabbit = {};
rabbit.speak = function(line) {
    console.log(`The rabbit says '${line}'`);

rabbit.speak("I'm alive.");
// → The rabbit says 'I'm alive.
};

Usually a method needs to do something with the object it was called on. When a function is called as a method—looked up as a property and immediately called, as in object.method()—the binding called this in its body automatically points at the object that it was called on.

function speak(line) {
    console.log(`The ${this.type} rabbit says '${line}'`);
}
let whiteRabbit = {type: "white", speak};
let hungryRabbit = {type: "hungry", speak};

whiteRabbit.speak("Oh my ears and whiskers, " + "how late it's getting!");
// → The white rabbit says 'Oh my ears and whiskers, how late it's getting!'
hungryRabbit.speak("I could use a carrot right now.");
// → The hungry rabbit says 'I could use a carrot right now.

Prototypes

A prototype is another object that is used as a fallback source of properties. When an object gets a request for a property that it does not have, its prototype will be searched for the property, then the prototype’s prototype, and so on.

So who is the prototype of that empty object? It is the great ancestral prototype, the entity behind almost all objects, Object.prototype.

console.log(Object.getPrototypeOf({}) == Object.prototype);
// → true
console.log(Object.getPrototypeOf(Object.prototype));
// → null

Many objects don’t directly have Object.prototype as their prototype but instead have another object that provides a different set of default properties. Functions derive from Function.prototype, and arrays derive from Array.prototype.

You can use Object.create to create an object with a specific prototype.

let obj = Object.create(null)

If you pass null to Object.create, the resulting object will not derive from Object.prototype

Classes

Prototypes are useful for defining properties for which all instances of a class share the same value, such as methods. Properties that differ per instance, such as our rabbits’ type property, need to be stored directly in the objects themselves.

So to create an instance of a given class, you have to make an object that derives from the proper prototype, but you also have to make sure it, itself, has the properties that instances of this class are supposed to have. This is what a constructor function does.

function Rabbit(type) {
    this.type = type;
}
Rabbit.prototype.speak = function(line) {
    console.log(`The ${this.type} rabbit says '${line}'`);
};
  
let weirdRabbit = new Rabbit("weird");

Class notation

The class keyword starts a class declaration, which allows us to define a constructor and a set of methods all in a single place. Any number of methods may be written inside the declaration’s braces. The one named constructor is treated specially. It provides the actual constructor function, which will be bound to the name Rabbit. The others are packaged into that constructor’s prototype. Thus, the earlier class declaration is equivalent to the constructor definition from the previous section. It just looks nicer.

Class declarations currently allow only methods—properties that hold functions—to be added to the prototype.

let object = new class { getWord() { return "hello"; } };
console.log(object.getWord());
// → hello 

Polymorphism

When you call the String function (which converts a value to a string) on an object, it will call the toString method on that object to try to create a meaningful string from it. Some of the standard prototypes define their own version of toString so they can create a string that contains more useful information than [object Object]. You can also do that yourself.

Class declarations currently allow only methods—properties that hold functions—to be added to the prototype.

Rabbit.prototype.toString = function() {
    return `a ${this.type} rabbit`;
};

console.log(String(blackRabbit));
// → a black rabbit

This is a simple instance of a powerful idea. When a piece of code is written to work with objects that have a certain interface—in this case, a toString method—any kind of object that happens to support this interface can be plugged into the code, and it will just work.

This technique is called polymorphism. Polymorphic code can work with values of different shapes, as long as they support the interface it expects.

The iterator interface

The object given to a for/of loop is expected to be iterable. This means it has a method named with the Symbol.iterator symbol (a symbol value defined by the language, stored as a property of the Symbol function).

When called, that method should return an object that provides a second interface, iterator. This is the actual thing that iterates. It has a next method that returns the next result. That result should be an object with a value property that provides the next value, if there is one, and a done property, which should be true when there are no more results and false otherwise.

Note that the next, value, and done property names are plain strings, not symbols. Only Symbol.iterator, which is likely to be added to a lot of different objects, is an actual symbol.

We can directly use this interface ourselves.

let okIterator = "OK"[Symbol.iterator]();
console.log(okIterator.next());
// → {value: "O", done: false}
console.log(okIterator.next());
// → {value: "K", done: false}
console.log(okIterator.next());
// → {value: undefined, done: true} 

The instanceof operator

It is occasionally useful to know whether an object was derived from a specific class. For this, JavaScript provides a binary operator called instanceof.

console.log(
    new SymmetricMatrix(2) instanceof SymmetricMatrix);
// → true
console.log(new SymmetricMatrix(2) instanceof Matrix);
// → true
console.log(new Matrix(2, 2) instanceof SymmetricMatrix);
// → false
console.log([1] instanceof Array);
// → true

The operator will see through inherited types. The operator can also be applied to standard constructors like Array. Almost every object is an instance of Object.

Summary

So objects do more than just hold their own properties. They have prototypes, which are other objects. They’ll act as if they have properties they don’t have as long as their prototype has that property. Simple objects have Object.prototype as their prototype.

Constructors, which are functions whose names usually start with a capital letter, can be used with the new operator to create new objects. The new object’s prototype will be the object found in the prototype property of the constructor. You can make good use of this by putting the properties that all values of a given type share into their prototype. There’s a class notation that provides a clear way to define a constructor and its prototype.

You can define getters and setters to secretly call methods every time an object’s property is accessed. Static methods are methods stored in a class’s constructor, rather than its prototype.

The instanceof operator can, given an object and a constructor, tell you whether that object is an instance of that constructor.

One useful thing to do with objects is to specify an interface for them and tell everybody that they are supposed to talk to your object only through that interface. The rest of the details that make up your object are now encapsulated, hidden behind the interface.

More than one type may implement the same interface. Code written to use an interface automatically knows how to work with any number of different objects that provide the interface. This is called polymorphism.

When implementing multiple classes that differ in only some details, it can be helpful to write the new classes as subclasses of an existing class, inheriting part of its behavior.