Classes are Expressions
raganwald.com
raganwald.com
We know how to "solve" the problem of encapsulation with JavaScript. It's not with closures, Symbols, or some other arbitrary hack. We do it by enforcing specific idioms that only require a developer to recognize intent instead of learning 80 different ways to do the same thing. You simply put a single or double underscore in front of the property name.
I read what I thought was a clever anecdote by another developer the other day, that the underscore in "obj._varname" indicates "here be dragons." It's an extremely simple idiom to teach new JavaScript developers.
I get it, JavaScript is robust. So you can do fun things like this! These are great experiments. But really, what are you accomplishing? You've now just thrown in a closure and a bunch of lines of code (defining your symbols, etc.) that any developer new to your codebase is going to look at like "... what the... what is going on here?" You can solve the problem easily with one character.
Consistency. Convention. Creating useful, easy-to-understand, repeatable idioms. That's what we should be focusing on.
I prefer to mark up my code as private/protected/public for my own purposes both as built-in documentation and to have one less thing to think about after it's done. Plus underscores are ugly.
And, in some extent, it's actually freeing. I can reach in and change something big in a strict/compiled language and the compiler/IDE will tell me where I have to make other changes throughout the whole project. In a dynamic language, I either have to rely on unit testing, grep, or just having it break at runtime. But the real truth is, you just don't make those kind of changes when your language isn't strict. The pain isn't worth the payoff.
Heck, you could probably make a macro such that `this._foo` compiles to `this[privateProperties.foo]`, but I'd probably scream about how misleading that is…
1. expose a clean API surface so users don't care how it works (as long as it works :)
2. make it easier to change the innards without changing its public behavior
3. keep malicious programmers from delving into the innards of our precious code.
It seems like a lot of the discussions on encapsulation (over many years) focuses on #3. Especially in the C++ and Java world. But worrying about that leads to various forms of obfuscation that make the code worse. And who cares about folks who will hack our objects? That's their problem.
Oh wait, now I'm passing my object to a third party library that preforms infrastructure? I guess they better provide a hook that I can inject my own naming conventions.
Yes, underscore fields work, until they don't. Is a function that returns an object really that hard for beginners? I can answer that as I actually teach beginners - yes, it's hard for like two class sessions, after the third homework assignment they have no other issues. So yeah, closures are the way to solve this.
I wouldn't use what this article proposes unless I definitely needed classes, but for the 1% of the time that I do this is a pretty nifty solution to one of my biggest gripes.
var Person = function(first last) {
this.fullName = function() {
return first + " " + last;
};
this.rename = function(newFirst, newLast) {
first = newFirst;
last = newLast;
};
};
I don't get the relucatance to use the language's features rather than hacks (using naming standards and compilers to obfusticate names and then cross fingers) or other hacks (Symbol). Really? You Really want me to write code using Symbol? That's going to be awesome in the debugger. Let's add a watch to "weke30o304_first". Refresh, of shit it's now "gtgrf40r3fe_first", my watches all broke (T_T)
It also makes me sad the `class` spec didn't take that into account so you could use a closure with a `class`.
Symbol is a language feature. As is accessing a property with `[` and `]`. Using the language's features as they were designed is not a hack. And I think we may see your browser's tooling improve when it natively supports ECMAScript 2015, instead of whatever is transpiled today.
Now: Why do you think that properties named by symbols have this special property of being non-enumerable by default? Do you think it is just a coïncidence that it is useful for making a pseudo-private property?
I do not think this particular pattern is a "hack," I think it is one of the use cases that were considered when designing symbols in the first place.
But returning to the purpose of the article, whatever you or I may think of the performance of using closures to create private data, your pattern again supports the article's thesis: By making classes out of functions, objects, and prototypes, we can use the features of the language to extend a class's semantics.
If there was a full-featured monolithic class system like Ruby, we might find that the scoping for methods is not like the scoping for lambdas, and thus we can't just use a closure when we want one.
Your proposed solution leverages the exact same property of JavaScript as TFA.
var Person = function(first, last) {
var serializable = {
first: first,
last: last,
};
...
What, exactly, is the problem here? I don’t recall the article saying that symbols are the only way to do this, or the best way to do this out of all the options. I recall people saying similar things about closures for a while, until it caught on that closures and lexical scope are a natural part of JavaScript.
TFA’s point is that ECMAScript 2015’s choice to keep classes as first-class functions with prototypes that are objects is what allows us to use all of the techniques we already have for first-class values, including returning them from functions.
As I keep saying, using a closure (if that’s what you want to do) demonstrates the exact same thing, it works because a method is just a function.
Now that we have established that I am not claiming that symbols are better than closures, I will warn you about an edge case: When you use compact method syntax, you do get something that an ordinary function does not provide, the `super` keyword.
I do not know how to make that work in the case where you assign a function to an instance of an object. So if you want to override a function while calling the super-class’s implementation, it might be tricky to use the closure solution.
That being said... I wouldn’t stay up late worrying about it.
My reaction to the article's approach was largely: okay, cool, this seems approximately as good as the closure-based solution; is there any reason for me to switch? Or is this just a cool toy example to show off classes-as-expressions?
Btw, once a symbol is created, is it ever garbage collected? (They are not in Common Lisp for example).
Possibly on page reload (for javascript in browser) they would. But how about long-running javascript code?
What lessons have folks learned from trying these approaches in other languages? Which is more idiomatic, and why?
let Person = (() = > {
let firstNameProperty = Symbol('firstName'),
lastNameProperty = Symbol('lastName');
return class Person {
constructor (first, last) {
this[firstNameProperty] = first;
this[lastNameProperty] = last;
}
sameFirstName (otherPerson) {
return this[firstNameProperty] == otherPerson[firstNameProperty];
}
};
)();
var Person = (function() {
return function(firstName, lastName) {
var privateVars = {
firstName: "",
lastName: ""
};
this.getFirstName = function() {
return privateVars.firstName;
};
this.setFirstName = function(firstName) {
privateVars.firstName = firstName;
return this;
};
this.getLastName = function() {
return privateVars.lastName;
}
this.setLastName = function(lastName) {
privateVars.lastName = lastName;
return this;
};
this.sameFirstName = function(otherPerson) {
return this.getFirstName() == otherPerson.getFirstName();
}
this.setFirstName(firstName);
this.setLastName(lastName);
Object.freeze(this);
return this;
}
}());
var p = new Person("John", "Doe");
var q = new Person("John", "Smith");
var r = new Person("Jane", "Doe");
p.sameFirstName(q); // true
p.sameFirstName(r); // falseAnd it can, since the symbol is just a secret stored in a closure and there are other sorts of secrets. Aside from the "Math.random() + {enumerable: false}" suggestion found elsewhere, you can reproduce the precise[0] semantics of your example using something like:
// Please do not actually write code like this
var Person = (function() {
var names = [];
function Person(name) {
this.key = names.length;
names[this.key] = name;
}
Person.prototype.sameName = function(other) {
return names[other.key] === names[this.key];
};
return Person;
})();
class Person
names = []
constructor: (name)->
@key = names.length
names[@key] = name
sameName: (other)->
names[other.key] is names[@key]
Whether `names[this.key]` is as "nice" to work with as `this[nameKey]` is, uh, left up to the reader; personally, count me in the camp that doing either of those things is nuttypants compared to `this._name`.
[0]: Assuming that `otherPerson` in your constructor is a typo, of course ;P
Oops, yup. Fixed.
Your solution is pretty clever, though it leaks memory like a sieve. To get around that, you end up needing something like weak maps.
Once you have those, when you do the pattern you suggest, you are basically doing metaprogramming and reinventing the very idea of an "object" yourself from scratch.
That can be fun to do, but to me it starts to feel less like expressing "the same thing" and more like implementing a new language that lets you express the same thing. Just because I can implement a GC in C, that doesn't mean C has a GC. :)
And yet-- is the Symbol approach not metaprogramming? It's clearly a much saner way to shoehorn this feature in, but it's still a shoehorn. And the underlying magic in both cases is in fact simple closure scope, which is really what I wanted to demonstrate.
Edit: And I guess my broader point, to make this more constructive, is that in JS these kinds of idioms live on a continuum of metaprogramming, and for the sanity of your co-contributors you usually want to be doing as little of that as necessary. Like others, I'm wary of presenting something like your example as a "design pattern" without that context.
Edit 2: To illustrate what I mean, I swear to you that if this pattern is adopted widely you will find code like this in the wild:
... = > {
let accountNumberKey = Symbol('accountNumber');
return class Account {
constructor (accountNumber) {
this[accountNumberKey] = accountNumber;
this.accountNumberKey = accountNumberKey; // ??? but transaction log works now
...function Person(first, last){ var firstNameProperty = {}, // unique snowflake obj lastNameProperty = {} // yet another unique obj this[firstNameProperty] = first; this[lastNameProperty] = last; }
ES6 should be saving me from typing all the scoping hacks such as closure. not giving me syntax sugar for things i could do already without odd language constructs (now i have to run TWO loops to get my obj properties. which is what everyone is going to do anyway, since you can iterate on all the symbol properties anyway)
These types of imperative patterns make life extremely hard on tools, which now need to add in unreliable things like escape analysis to be able determine which classes are visible. Modules will help a little with explicit exports, so that the importing modules have some hope of tooling, but analysis within a module would suffer.
[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
But if you choose the wrong thing, one day your choice is obsolete. That is a risk, and it's prudent consider the consequences carefully.
If that troubles you, by all means write your code without any encapsulation, and wait for the language to acquire the feature in a few years. In the mean time, your code base will grow without that kind of encapsulation. Only you can determine whether this is a bad thing or not. YMMV, and all that.
This is a very common problem to ponder, we end up arguing about whether you should be an early adopter, a main streeter, or a laggard.
But really, the post isn't a screed in favour of using private properties, it's a suggestion that making classes out of first-class values in the language gives us the flexibility to build whatever we think we need.
Private properties is a particularly simple example. Traits and mixins are also useful, but would take even more space to articulate. But the general point is still that making new things ("classes") out of the existing materials ("functions and prototypes") is a win in general.
I don't particularly like this solution, because now I can't just write `this.rename()` in other Person functions. That's why I'd rather have a baked-in way to define private properties & functions instead of having to use (in my opinion) this weird syntax and later explain the whys and hows to other developers.
Your solution mostly works, but a simpler one takes advantage of combining computed property keys and compact method syntax:
(I wrote about them extensively years back here: http://mikegerwitz.com/papers/coope/coope.pdf.)
var name = 'name_' + Math.random() ; //assumed uniquenesshttps://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
Symbol tosses that in "for free."