JavaScript is a class-free, object-oriented language, and as such,
it uses prototypal inheritance instead of classical inheritance. This can be
puzzling to programmers trained in conventional object-oriented languages
like C++ and Java. JavaScript’s prototypal inheritance has more expressive
power than classical inheritance, as we will see presently.

Java JavaScript
Strongly-typed Loosely-typed
Static Dynamic
Classical Prototypal
Classes Functions
Constructors Functions
Methods Functions

But first, why do we care about inheritance at all? There are primarily two
reasons. The first is type convenience. We want the language system to
automatically cast references of similar classes. Little type-safety is
obtained from a type system which requires the routine explicit casting of
object references. This is of critical importance in strongly-typed languages,
but it is irrelevant in loosely-typed languages like JavaScript, where object
references never need casting.

The second reason is code reuse. It is very common to have a quantity of objects
all implementing exactly the same methods. Classes make it possible to create
them all from a single set of definitions. It is also common to have objects
that are similar to some other objects, but differing only in the addition or
modification of a small number of methods. Classical inheritance is useful for
this but prototypal inheritance is even more useful.

To demonstrate this, we will introduce a little sugar
which will let us write in a style that resembles a conventional classical language.
We will then show useful patterns which are not available in classical languages.
Then finally, we will explain the sugar.

Classical Inheritance

First, we will make a Parenizor class that will have set and get
methods for its value, and a toString method that will wrap
the value in parens.

function Parenizor(value) {
    this.setValue(value);
}

Parenizor.method('setValue', function (value) {
    this.value = value;
    return this;
});

Parenizor.method('getValue', function () {
    return this.value;
});

Parenizor.method('toString', function () {
    return '(' + this.getValue() + ')';
});

The syntax is a little unusual, but it is easy to recognize the classical
pattern in it. The method method takes a method name and a function,
adding them to the class as a public method.

So now we can write

myParenizor = new Parenizor(0);
myString = myParenizor.toString();

As you would expect, myString is "(0)".

Now we will make another class which will inherit from Parenizor,
which is the same except that its toString method will
produce "-0-" if the value is zero or empty.

function ZParenizor(value) {
    this.setValue(value);
}

ZParenizor.inherits(Parenizor);

ZParenizor.method('toString', function () {
    if (this.getValue()) {
        return this.uber('toString');
    }
    return "-0-";
});

The inherits method is similar to Java’s extends.
The uber method is similar to Java’s super. It lets a method
call a method of the parent class. (The names have been changed to avoid
reserved word restrictions.)

So now we can write

myZParenizor = new ZParenizor(0);
myString = myZParenizor.toString();

This time, myString is "-0-".

JavaScript does not have classes, but we can program as though it does.

Multiple Inheritance

By manipulating a function’s prototype object, we can implement
multiple inheritance, allowing us to make a class built from the methods of
multiple classes. Promiscuous multiple inheritance can be difficult to
implement and can potentially suffer from method name collisions. We could
implement promiscuous multiple inheritance in JavaScript, but for this example
we will use a more disciplined form
called Swiss
Inheritance
.

Suppose there is a NumberValue class that has a setValue
method that checks that the value is a number in a certain range,
throwing an exception if necessary. We only want
its setValue and setRange methods for our ZParenizor.
We certainly don’t want its toString method. So, we write

ZParenizor.swiss(NumberValue, 'setValue', 'setRange');

This adds only the requested methods to our class.

Parasitic Inheritance

There is another way to write ZParenizor. Instead of inheriting
from Parenizor, we write a constructor that calls
the Parenizor constructor, passing off the result as its own.
And instead of adding public methods, the constructor adds privileged methods.

function ZParenizor2(value) {
    var that = new Parenizor(value);
    that.toString = function () {
        if (this.getValue()) {
            return this.uber('toString');
        }
        return "-0-"
    };
    return that;
}

Classical inheritance is about the is-a relationship, and parasitic
inheritance is about the was-a-but-now’s-a relationship. The constructor
has a larger role in the construction of the object. Notice that the uber
née super method is still available to the privileged methods.

Class Augmentation

JavaScript’s dynamism allows us to add or replace methods of an existing class.
We can call the method method at any time, and all present and future
instances of the class will have that method. We can literally extend a class
at any time. Inheritance works retroactively. We call this Class Augmentation
to avoid confusion with Java’s extends, which means something else.

Object Augmentation

In the static object-oriented languages, if you want an object which is slightly
different than another object, you need to define a new class. In JavaScript,
you can add methods to individual objects without the need for additional classes.
This has enormous power because you can write far fewer classes and the classes
you do write can be much simpler. Recall that JavaScript objects are like hashtables.
You can add new values at any time. If the value is a function, then it becomes
a method.

So in the example above, I didn’t need a ZParenizor class at all.
I could have simply modified my instance.

myParenizor = new Parenizor(0);
myParenizor.toString = function () {
    if (this.getValue()) {
        return this.uber('toString');
    }
    return "-0-";
};
myString = myParenizor.toString();

We added a toString method to our myParenizor instance without
using any form of inheritance. We can evolve individual instances because the
language is class-free.

Sugar

To make the examples above work, I wrote four sugar
methods. First, the method method, which adds an instance method to
a class.

Function.prototype.method = function (name, func) {
    this.prototype[name] = func;
    return this;
};

This adds a public method to the Function.prototype, so all
functions get it by Class Augmentation. It takes a name and a function, and
adds them to a function’s prototype object.

It returns this. When I write a method that doesn’t need to return
a value, I usually have it return this. It allows for a cascade-style
of programming.

Next comes the inherits method, which indicates that one class inherits
from another. It should be called after both classes are defined, but before
the inheriting class’s methods are added.

Function.method('inherits', function (parent) {
    this.prototype = new parent();
    var d = {}, 
        p = this.prototype;
    this.prototype.constructor = parent; 
    this.method('uber', function uber(name) {
        if (!(name in d)) {
            d[name] = 0;
        }        
        var f, r, t = d[name], v = parent.prototype;
        if (t) {
            while (t) {
                v = v.constructor.prototype;
                t -= 1;
            }
            f = v[name];
        } else {
            f = p[name];
            if (f == this[name]) {
                f = v[name];
            }
        }
        d[name] += 1;
        r = f.apply(this, Array.prototype.slice.apply(arguments, [1]));
        d[name] -= 1;
        return r;
    });
    return this;
});

Again, we augment Function. We make an instance of the
parent class and use it as the new prototype. We also
correct the constructor field, and we add the uber method to
the prototype as well.

The uber method looks for the named method in its own prototype.
This is the function to invoke in the case of Parasitic Inheritance or Object
Augmentation. If we are doing Classical Inheritance, then we need to find the
function in the parent‘s prototype. The return statement
uses the function’s apply method to invoke the function, explicitly
setting this and passing an array of parameters. The parameters (if
any) are obtained from the arguments array. Unfortunately, the
arguments
array is not a true array, so we have to use apply again to invoke
the array slice method.

Finally, the swiss method.

Function.method('swiss', function (parent) {
    for (var i = 1; i < arguments.length; i += 1) {
        var name = arguments[i];
        this.prototype[name] = parent.prototype[name];
    }
    return this;
});

The swiss method loops through the arguments.
For each name, it copies a member from the parent‘s
prototype to the new class’s prototype.

By Enrico

My greatest passion is technology. I am interested in multiple fields and I have a lot of experience in software design and development. I started professional development when I was 6 years. Today I am a strong full-stack .NET developer (C#, Xamarin, Azure)

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.