Understanding JavaScript Inheritance

So someone shoulder-taps you and asks you to explain the concepts behind JavaScript Inheritance to them. In my eyes you’ve got a few options.

The Terminology Play

You mention that it’s prototypal inheritance, not prototypical and pretty much gloss over the rest, comfortable in your superiority in terminology. You may go as far as saying “Objects just come from other Objects because there aren’t any classes.” Then you just link to Crock’s Post on it, and try to seem busy for the next few days.

Many years later you find out that Prototypal and Prototypical are synonyms, but you choose to ignore this.

The Like-Classical-Inheritance-But-Different Play aka the Run-On Sentence Play

“So in Java, like, you have classes or whatever, right? Well so imagine that you don’t have those, but you still want to do that same type of thing or whatever, so then you just take another object instead of a class and you just kind of use it like it’s a class, but it’s not because it can change and it’s just a normal object, and if it changes and you don’t override the object, oh yea, so you can decide to override the parent object class thing, so if you dont do that and the parent changes the link is live…”

And so forth.

The Animal Play

This is a pretty popular one.

So let’s say we want to make an Animal class in our code. As is often necessary in production JavaScript applications.

First we make a “constructor function,” which acts kind of like a constructor method on the inside of a class in a classical language when it’s invoked with the new operator. Except this one is on the outside.

1 2 3 4 5function Animal (name) { this.name = name; } var myAnimal = new Animal('Annie');

Then we want to have actions that all animals can do.

1 2 3Animal.prototype.walk = function () { console.log(this.name + ' is walking.'); };

But then you want to define a more specific type of animal. Things start to get weird.

1 2 3 4 5 6 7 8 9 10 11// I think we need to define a new Animal type and extend from it somehow function Dog (name) { this.name = name; } // BUT HOW DO WE EXTEND // WITHOUT AN INSTANCE TO USE? Dog.prototype = Animal.prototype; // ?? I HAVE NO IDEA // Maybe that'll work for some stuff? // ProHint™: probably not much, once you start modifying one of them :D

Then you remember that Prototypal Inheritance doesn’t really do ‘classes’ so much. So you do something like this:

1 2 3 4 5 6 7 8 9 10var Dog = new Animal('Annie'); // ??? NO THATS NOT IT >:( // Maybe we can try Object.create? I hear it's prototypal-y var Dog = Object.create(Animal); // Maybe that worked? Let's see... var myDog = new Dog('Sparky'); // TypeError: object is not a function // Shucks

And you eventually simply converge on the…

The Father/Son Analogy Play

Here we go. Finally a real world example of ‘instances begetting instances.’ It’ll be a perfect analogy. It’s even an interview question some places. Let’s see how we might implement the relationship of a father and son (or a parent to its child) in JavaScript.

We’ll start out like we did before, with a Human constructor

1 2 3function Human( name ) { this.name = name; }

Then we’ll add in a common human shared action.

1 2 3Human.prototype.sayHi = function () { console.log("Hello, I'm " + this.name); };

So we’ll create my dad first.

1 2 3 4 5 6// Instantiate him var myDad = new Human('Bill Sexton'); // Greet him myDad.sayHi(); // "Hello, I'm Bill Sexton"

Score. Now let’s create me.

1 2 3 4// Let's use ES5 `object.create` in order to be as 'prototypal' as possible. var me = Object.create(myDad); me.sayHi(); // "Hello, I'm Bill Sexton"

It’s a start! Seems like I inherited a little too much from my dad, but I inherited, none the less.

Let’s try to smooth things out to make the analogy work better. So we’ll instantiate objects without a name and have a parent name them after they’re created.

1 2 3 4 5 6 7 8 9 10 11// Wrap it all together function makeBaby(parent, name) { // Instantiate a new object based on the parent var baby = Object.create(parent); // Set the name of the baby baby.name = name; // Give the baby away return baby; }

Perfect. Now the baby can sayHi on its own.

1 2 3 4var alex = makeBaby(myDad, 'Alex Sexton'); alex.sayHi(); // "Hello, I'm Alex Sexton"

Err. yipes. Babies can’t talk. And what’s this deal with a baby being made by one parent. Not to worry, we can fix all of this.

First we’ll probably want to try to take two parents into the makeBaby function (no giggles).

1 2 3function makeBaby(father, mother, name) { var baby = Object.create(...// fuuu }

Multiple Inheritance! How did you get here? Ugh. Fine. We’ll just simply mock the human chromosome pattern into our little inheritance example.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42// Let's take a set of 4 genes for ease of // example here. We'll put them in charge // a few things. function Human (name, genes_mom, genes_dad) { this.name = name; // Set the genes this.genes = { darkHair: this._selectGenes(genes_mom.darkHair, genes_dad.darkHair), smart: this._selectGenes(genes_mom.smart, genes_dad.smart), athletic: this._selectGenes(genes_mom.athletic, genes_dad.athletic), tall: this._selectGenes(genes_mom.tall, genes_dad.tall) }; // Since genes affect you since birth we can set these as actual attributes this.attributes = { darkHair: !!(~this.genes.darkHair.indexOf('D')), smart: !!(~this.genes.smart.indexOf('D')), athletic: !!(~this.genes.athletic.indexOf('D')), tall: !!(~this.genes.tall.indexOf('D')) }; } // You don't have access to your own gene selection // so we'll make this private (but in the javascript way) Human.prototype._selectGenes = function (gene1, gene2) { // Assume that a gene is a 2 length array of the following possibilities // DD, Dr, rD, rr -- the latter being the only non "dominant" result // Simple random gene selection return [ gene1[Math.random() > 0.5 ? 1 : 0], gene2[Math.random() > 0.5 ? 1 : 0] ] }; Human.prototype.sayHi = function () { console.log("Hello, I'm " + this.name); }; function makeBaby(name, mother, father) { // Send in the genes of each parent var baby = new Human(name, mother.genes, father.genes); return baby; }

Elementary. My only beef is that we no longer are using real prototypal inheritance. There is no live link between the parents and the child. If there was only one parent, we could use the __proto__ property to set the parent as the prototype after the baby was instantiated. However we have two parents…

So we’ll need to implement runtime getters that do a lookup for each parent via ES Proxies.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25function makeBaby(name, mother, father) { // Send in the genes of each parent var baby = new Human(name, mother.genes, father.genes); // Proxy the baby return new Proxy(baby, { get: function (proxy, prop) { // shortcut the lookup if (baby[prop]) { return baby[prop]; } // Default parent var parent = father; // Spice it up if (Math.random() > 0.5) { parent = mother; } // See if they have it return parent[prop]; } }); }

So now we support live lookups of parents, and, you know, some simplified genetics.

Isn’t that just a simple, well-defined, example of how straightforward inheritance can be in JavaScript?

Conclusion

Sometimes these analogies get pretty crazy in my head, and I start to think that maybe instead of trying to apply known examples in the outside world in order to help people understand, it’s often better to just let someone know why they might wanna use inheritance in their programs!

I personally find the best Prototypal Inheritance analogy to be:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20var defaults = { zero: 0, one: 1 }; var myOptions = Object.create(defaults); var yourOptions = Object.create(defaults); // When I want to change *just* my options myOptions.zero = 1000; // When you wanna change yours yourOptions.one = 42; // When we wanna change the **defaults** even after we've got our options // even **AFTER** we've already created our instances defaults.two = 2; myOptions.two; // 2 yourOptions.two; // 2

So stop making everything so confusing and go program cool stuff, and ignore my old presentations when I used these analogies.

Understanding JavaScript Inheritance