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You Don't Know JS: this & Object Prototypes

Appendix A: ES6 class

If there's any take-away message from the second half of this book (Chapters 4-6), it's that classes are an optional design pattern for code (not a necessary given), and that furthermore they are often quite awkward to implement in a [[Prototype]] language like JavaScript.

This awkwardness is not just about syntax, although that's a big part of it. Chapters 4 and 5 examined quite a bit of syntactic ugliness, from verbosity of .prototype references cluttering the code, to explicit pseudo-polymorphism (see Chapter 4) when you give methods the same name at different levels of the chain and try to implement a polymorphic reference from a lower-level method to a higher-level method. .constructor being wrongly interpreted as "was constructed by" and yet being unreliable for that definition is yet another syntactic ugly.

But the problems with class design are much deeper. Chapter 4 points out that classes in traditional class-oriented languages actually produce a copy action from parent to child to instance, whereas in [[Prototype]], the action is not a copy, but rather the opposite -- a delegation link.

When compared to the simplicity of OLOO-style code and behavior delegation (see Chapter 6), which embrace [[Prototype]] rather than hide from it, classes stand out as a sore thumb in JS.

class

But we don't need to re-argue that case again. I re-mention those issues briefly only so that you keep them fresh in your mind now that we turn our attention to the ES6 class mechanism. We'll demonstrate here how it works, and look at whether or not class does anything substantial to address any of those "class" concerns.

Let's revisit the Widget / Button example from Chapter 6:

class Widget {
	constructor(width,height) {
		this.width = width || 50;
		this.height = height || 50;
		this.$elem = null;
	}
	render($where){
		if (this.$elem) {
			this.$elem.css( {
				width: this.width + "px",
				height: this.height + "px"
			} ).appendTo( $where );
		}
	}
}

class Button extends Widget {
	constructor(width,height,label) {
		super( width, height );
		this.label = label || "Default";
		this.$elem = $( "<button>" ).text( this.label );
	}
	render($where) {
		super.render( $where );
		this.$elem.click( this.onClick.bind( this ) );
	}
	onClick(evt) {
		console.log( "Button '" + this.label + "' clicked!" );
	}
}

Beyond this syntax looking nicer, what problems does ES6 solve?

  1. There's no more (well, sorta, see below!) references to .prototype cluttering the code.
  2. Button is declared directly to "inherit from" (aka extends) Widget, instead of needing to use Object.create(..) to replace a .prototype object that's linked, or having to set with .__proto__ or Object.setPrototypeOf(..).
  3. super(..) now gives us a very helpful relative polymorphism capability, so that any method at one level of the chain can refer relatively one level up the chain to a method of the same name. This includes a solution to the note from Chapter 4 about the weirdness of constructors not belonging to their class, and so being unrelated -- super() works inside constructors exactly as you'd expect.
  4. class literal syntax has no affordance for specifying properties (only methods). This might seem limiting to some, but it's expected that the vast majority of cases where a property (state) exists elsewhere but the end-chain "instances", this is usually a mistake and surprising (as it's state that's implicitly "shared" among all "instances"). So, one could say the class syntax is protecting you from mistakes.
  5. extends lets you extend even built-in object (sub)types, like Array or RegExp, in a very natural way. Doing so without class .. extends has long been an exceedingly complex and frustrating task, one that only the most adept of framework authors have ever been able to accurately tackle. Now, it will be rather trivial!

In all fairness, those are some substantial solutions to many of the most obvious (syntactic) issues and surprises people have with classical prototype-style code.

class Gotchas

It's not all bubblegum and roses, though. There are still some deep and profoundly troubling issues with using "classes" as a design pattern in JS.

Firstly, the class syntax may convince you a new "class" mechanism exists in JS as of ES6. Not so. class is, mostly, just syntactic sugar on top of the existing [[Prototype]] (delegation!) mechanism.

That means class is not actually copying definitions statically at declaration time the way it does in traditional class-oriented languages. If you change/replace a method (on purpose or by accident) on the parent "class", the child "class" and/or instances will still be "affected", in that they didn't get copies at declaration time, they are all still using the live-delegation model based on [[Prototype]]:

class C {
	constructor() {
		this.num = Math.random();
	}
	rand() {
		console.log( "Random: " + this.num );
	}
}

var c1 = new C();
c1.rand(); // "Random: 0.4324299..."

C.prototype.rand = function() {
	console.log( "Random: " + Math.round( this.num * 1000 ));
};

var c2 = new C();
c2.rand(); // "Random: 867"

c1.rand(); // "Random: 432" -- oops!!!

This only seems like reasonable behavior if you already know about the delegation nature of things, rather than expecting copies from "real classes". So the question to ask yourself is, why are you choosing class syntax for something fundamentally different from classes?

Doesn't the ES6 class syntax just make it harder to see and understand the difference between traditional classes and delegated objects?

class syntax does not provide a way to declare class member properties (only methods). So if you need to do that to track shared state among instances, then you end up going back to the ugly .prototype syntax, like this:

class C {
	constructor() {
		// make sure to modify the shared state,
		// not set a shadowed property on the
		// instances!
		C.prototype.count++;

		// here, `this.count` works as expected
		// via delegation
		console.log( "Hello: " + this.count );
	}
}

// add a property for shared state directly to
// prototype object
C.prototype.count = 0;

var c1 = new C();
// Hello: 1

var c2 = new C();
// Hello: 2

c1.count === 2; // true
c1.count === c2.count; // true

The biggest problem here is that it betrays the class syntax by exposing (leakage!) .prototype as an implementation detail.

But, we also still have the surprise gotcha that this.count++ would implicitly create a separate shadowed .count property on both c1 and c2 objects, rather than updating the shared state. class offers us no consolation from that issue, except (presumably) to imply by lack of syntactic support that you shouldn't be doing that at all.

Moreover, accidental shadowing is still a hazard:

class C {
	constructor(id) {
		// oops, gotcha, we're shadowing `id()` method
		// with a property value on the instance
		this.id = id;
	}
	id() {
		console.log( "Id: " + this.id );
	}
}

var c1 = new C( "c1" );
c1.id(); // TypeError -- `c1.id` is now the string "c1"

There's also some very subtle nuanced issues with how super works. You might assume that super would be bound in an analogous way to how this gets bound (see Chapter 2), which is that super would always be bound to one level higher than whatever the current method's position in the [[Prototype]] chain is.

However, for performance reasons (this binding is already expensive), super is not bound dynamically. It's bound sort of "statically", as declaration time. No big deal, right?

Ehh... maybe, maybe not. If you, like most JS devs, start assigning functions around to different objects (which came from class definitions), in various different ways, you probably won't be very aware that in all those cases, the super mechanism under the covers is having to be re-bound each time.

And depending on what sorts of syntactic approaches you take to these assignments, there may very well be cases where the super can't be properly bound (at least, not where you suspect), so you may (at time of writing, TC39 discussion is ongoing on the topic) have to manually bind super with toMethod(..) (kinda like you have to do bind(..) for this -- see Chapter 2).

You're used to being able to assign around methods to different objects to automatically take advantage of the dynamism of this via the implicit binding rule (see Chapter 2). But the same will likely not be true with methods that use super.

Consider what super should do here (against D and E):

class P {
	foo() { console.log( "P.foo" ); }
}

class C extends P {
	foo() {
		super();
	}
}

var c1 = new C();
c1.foo(); // "P.foo"

var D = {
	foo: function() { console.log( "D.foo" ); }
};

var E = {
	foo: C.prototype.foo
};

// Link E to D for delegation
Object.setPrototypeOf( E, D );

E.foo(); // "P.foo"

If you were thinking (quite reasonably!) that super would be bound dynamically at call-time, you might expect that super() would automatically recognize that E delegates to D, so E.foo() using super() should call to D.foo().

Not so. For performance pragmatism reasons, super is not late bound (aka, dynamically bound) like this is. Instead it's derived at call-time from [[HomeObject]].[[Prototype]], where [[HomeObject]] is statically bound at creation time.

In this particular case, super() is still resolving to P.foo(), since the method's [[HomeObject]] is still C and C.[[Prototype]] is P.

There will probably be ways to manually address such gotchas. Using toMethod(..) to bind/rebind a method's [[HomeObject]] (along with setting the [[Prototype]] of that object!) appears to work in this scenario:

var D = {
	foo: function() { console.log( "D.foo" ); }
};

// Link E to D for delegation
var E = Object.create( D );

// manually bind `foo`s `[[HomeObject]]` as
// `E`, and `E.[[Prototype]]` is `D`, so thus
// `super()` is `D.foo()`
E.foo = C.prototype.foo.toMethod( E, "foo" );

E.foo(); // "D.foo"

Note: toMethod(..) clones the method, and takes homeObject as its first parameter (which is why we pass E), and the second parameter (optionally) sets a name for the new method (which keep at "foo").

It remains to be seen if there are other corner case gotchas that devs will run into beyond this scenario. Regardless, you will have to be diligent and stay aware of which places the engine automatically figures out super for you, and which places you have to manually take care of it. Ugh!

Static > Dynamic?

But the biggest problem of all about ES6 class is that all these various gotchas mean class sorta opts you into a syntax which seems to imply (like traditional classes) that once you declare a class, it's a static definition of a (future instantiated) thing. You completely lose sight of the fact that C is an object, a concrete thing, which you can directly interact with.

In traditional class-oriented languages, you never adjust the definition of a class later, so the class design pattern doesn't suggest such capabilities. But one of the most powerful parts of JS is that it is dynamic, and the definition of any object is (unless you make it immutable) a fluid and mutable thing.

class seems to imply you shouldn't do such things, by forcing you into the uglier .prototype syntax to do so, or forcing you to think about super gotchas, etc. It also offers very little support for any of the pitfalls that this dynamism can bring.

In other words, it's as if class is telling you: "dynamic is too hard, so it's probably not a good idea. Here's a static-looking syntax, so code your stuff statically."

What a sad commentary on JavaScript: dynamic is too hard, let's pretend to be (but not actually be!) static.

These are the reasons why ES6 class is masquerading as a nice solution to syntactic headaches, but it's actually muddying the waters further and making things worse for JS and for clear and concise understanding.

Note: If you use the .bind(..) utility to make a hard-bound function (see Chapter 2), the function created is not subclassable with ES6 extend like normal functions are.

Review (TL;DR)

class does a very good job of pretending to fix the problems with the class/inheritance design pattern in JS. But it actually does the opposite: it hides many of the problems, and introduces other subtle but dangerous ones.

class contributes to the ongoing confusion of "class" in JavaScript which has plagued the language for nearly two decades. In some respects, it asks more questions than it answers, and it feels in totality like a very unnatural fit on top of the elegant simplicity of the [[Prototype]] mechanism.

Bottom line: if ES6 class makes it harder to robustly leverage [[Prototype]], and hides the most important nature of the JS object mechanism -- the live delegation links between objects -- shouldn't we see class as creating more troubles than it solves, and just relegate it to an anti-pattern?

I can't really answer that question for you. But I hope this book has fully explored the issue at a deeper level than you've ever gone before, and has given you the information you need to answer it yourself.