Implement private fields proposal

[dead-claudia]

I think it would be nice to have the stage 1 private fields proposal implemented in TypeScript. It'll mostly supercede the current pseudo-private implementation, and a fully-correct implementation is only transpliable to ES6 using per-class WeakMaps. I'll note that the spec itself uses WeakMaps internally to model the private state, which may help in implementing the proposal.

Currently, the proposal only includes private properties, but methods are likely to follow. Also, the most important part of the proposal is that private properties are only available within the class itself.

[glen-84]

What does the TypeScript team think about the syntax and the use of a symbol/character prefix?

I personally think that it looks awful, but apparently there are technical limitations (the usages need to identify the visibility, for reasons that I don't fully comprehend).

It would be sad to see TypeScript losing the current private/protected syntax, which is much cleaner and consistent with a number of other languages. Is this likely to happen? Are there any alternatives?

To add to this, there are talks about possibly switching the # and @ symbols around (i.e. using # for decorators), which would obviously have an affect on TypeScript as well.

[kitsonk]

I think it is "awful" too. I think like the :: bind operator, which didn't make it actually very far, it is worth holding off even trying to implement it until it gets a bit further down the path with T39. I highly suspect it will be a bit of a rough ride. Also, the down-emit to support it will be rather difficult I suspect, because of the need to rewrite every access site.

the usages need to identify the visibility, for reasons that I don't fully comprehend

Mainly because you need to identify at the usage site how to lookup the property, since it needs a different lookup algorithm so that descendent classes can re-use the same private names without needing to "know" about the ancestor private properties.

[littledan]

One advantage of the proposed syntax is that you can omit the this and just use #field directly. Further, the sigil may be swapped with decorators (tc39/proposal-private-fields#32) and instead @ would be used, so you'd use @field, just like Ruby. Do you find this syntax ugly still?

[dalexander01]

I don't think people will be happy with a non "private" keyword based solution.

[littledan]

I don't see a good way to do that while supporting general eval as JavaScript does--a concrete token allows some differentiation and makes it possible to support private state that's not based on a static type system as in TypeScript. We've discussed a number of syntax alternatives at tc39/proposal-private-fields#14 , and I don't see TypeScript's current syntax as something that can work properly all the time in a fully general JS environment.

[glen-84]

Mainly because you need to identify at the usage site how to lookup the property, since it needs a different lookup algorithm so that descendent classes can re-use the same private names without needing to "know" about the ancestor private properties.

I wish I knew more about how JavaScript works internally. I thought that it would work the way that it's described here and here, but apparently not. I still find it difficult to believe that that would have a significant impact on performance, but there are no numbers for comparison.

Do you find this syntax ugly still?

Essentially, yes. More so, I find it both inconsistent with the rest of the JavaScript syntax, as well as inconsistent with many other languages:

Language	Syntax	Notes
C#	private int x;
C++	private:     int x;
Java	private int x;
PHP	private $x;
Python	__x	"no comment"
Ruby	private     def method         # ...     end	It's a method, but fields are private by default I think.
TypeScript	private x;
Visual Basic	Private x As Integer

All but one of the above languages use the private keyword, and it seems that Python doesn't really have "proper" private state anyway.

With the exception of Python (again), none of the languages format field declarations or field accesses differently depending on the visibility, and they all allow for new visibility modifiers if that is ever required.

I also feel that both # and @ work better for things like decorators, that are (in Kevin's words) "syntactically 'outside' of the imperative flow".

I don't see a good way to do that while supporting general eval as JavaScript does

Would it be possible to explain this issue in layman's terms? (maybe in tc39/proposal-private-fields#14)

It would be nice to have a list of these issues with examples in one place, so that there is something to refer to.

[kitsonk]

I still find it difficult to believe that that would have a significant impact on performance, but there are no numbers for comparison.

It would, if you want it to be really private versus just a convention like it is in TypeScript. Whenever you look up a property in JavaScript, the process is essentially like this:

1. Look at instance for own property.
2. If no own property, look at prototype for own property.
3. Ascend prototype chain until no property found.
4. If found deal with property descriptor (writable, get, set, configurable)
5. If not found and read, return undefined or write, set value as own property if not sealed or frozen.

Now with privates with no pattern in their name, you wouldn't have own properties, you would have something like own private properties which wouldn't ascend a prototype chain. You would then have to look for them there as well for every operation that could possible access the private properties, because you can't be sure which one is being referred to. Looking it up in the normal way and then only if it is not found looking in privates could be very dangerous, because what if you found something in the prototype chain, but there is also a private version?

The biggest challenge is that JavaScript classes are still a bit of a misnomer. They are essentially syntactic sugar for prototypical inheritance constructor functions. There is one thought that occured to me which I will try to add to tc39/proposal-private-fields#14.

Would it be possible to explain this issue in layman's terms?

If it requires re-writing the call site, then you would never be able to support things like:

class Foo {
    private foobar: string;
    baz() {
        const p = 'foo' + 'bar';
        console.log(this[p]);
    }
}

Or any usage of eval like structures. You could choose not to support things like index access for privates or no eval support, but then "why bother". Pretty much all of this has been pointed out in one way or another in the proposal there but people still are "but I like private" 😁.

[glen-84]

you wouldn't have own properties, you would have something like own private properties

If you had a single set of properties with a visibility item in the descriptor, I guess the issue is that, if the property did not exist on the current object, you wouldn't know whether or not to ascend the prototype chain.

If it requires re-writing the call site, then you would never be able to support things like ...

AFAIK, that won't be supported anyway (ref).

I still don't really follow the eval case. I thought that the property checks would happen at runtime, after property names had been calculated.

but people still are "but I like private"

I'm trying not to be one of those people, and to understand the issues, but the proposed syntax just makes me uncomfortable. =)

[glen-84]

Okay, I think I understand the eval/rewrite case now. Usage sites within the class would be rewritten to indicate the visibility based on the declared properties, and that wouldn't be possible unless they are simple lookups.

[glen-84]

If you had a single set of properties with a visibility item in the descriptor, I guess the issue is that, if the property did not exist on the current object, you wouldn't know whether or not to ascend the prototype chain.

Hmm. But if it doesn't exist in the current class, then isn't it by definition non-private? If so, it would have to move up the prototype chain anyway (as with regular public property lookups). There would be no additional work for private access.

(I'm possibly missing something obvious)

[kitsonk]

But if it doesn't exist in the current class, then isn't it by definition non-private? If so, it would have to move up the prototype chain anyway (as with regular public property lookups).

No, you don't want to ascend. Private labels aren't inherited and subclasses can re-use privates without worrying about masking/name collisions.

[littledan]

@glen-84 Both of those posts that you mentioned indicate problems with un-sigil'd private state. Do you have ideas for solutions to those problems? I think complicating the lookup chain would be risky compatibility-wise, make JavaScript harder to implement with good performance (possibly making existing programs slower), be basically impossible to square with Proxies, and generally, significantly complicate the mental model of the language (which already has a relatively complex object system).

In the cross-language comparison, you mention Ruby. I think Ruby is a good example of private state with a sigil--@. You can call getter and setter methods without a sigil.

[glen-84]

No, you don't want to ascend. Private labels aren't inherited and subclasses can re-use privates without worrying about masking/name collisions.

I meant move up if the property wasn't on the current class, to look for a public or protected property.