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Access modifiers, as the name suggest, are an accessibility feature not a scope feature (like let). example.myVar = 500 should throw an access error. You don'

Access modifiers, as the name suggest, are an accessibility feature not a scope feature (like let). example.myVar = 500 should throw an access error. You don't need to distinguish because you shouldn't be shadowing private variables.

__ryan__ 8y ago
Not as far as the spec is concerned. They decided that private variables should be completely scoped to the class. "Having a private field named x must not prevent there from being a public field named x, so accessing a private field can't just be a normal lookup. This is only an issue in JavaScript because of its lack of static types. Statically typed languages use type declarations to distinguish the external-public/internal-private cases without the need of a sigil. But a dynamically typed language doesn't have enough static information to differentiate those cases." [0] There are also performance concerns to changing the lookup logic without the sigil. More info in the link. 0: https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md#why-isnt-access-thisx https://github.com/tc39/proposal-class-fields/blob/master/PR...
- dotancohen 8y ago
  I still don't see why this is an issue in Javascript. PHP, the canonical hate-it dynamically-typed language, does not have this issue. Sure, you say, that's because in PHP a public property and a private property cannot have the same name. Correct. Why is this a requirement? What problem is this trying to solve? In e.g. older Python we would use the convention of a leading underscore to indicate that a property is private, even if technically one _could_ access it publicly it was understood that is a bad idea to do in practice. It's not a security issue - in every language one could read the "private" properties by some method or another. Reflection, for instance. Or, back to PHP, by casting the object to an array (I did mention that PHP is the canonical hate-it dynamically-typed language).
  - Klathmon 8y ago
    I really recommend you read over the FAQ that is in the proposal at [0], it answers most of your questions. The problem is that it seems a "private by convention" system isn't private enough for many library authors, who are finding their private internal methods being used regardless, and then after some time becoming ossified leaving the maintainers in a tough situation. Support the "private" interface even though they shouldn't have to to avoid breaking a significant number of users, or break the "private" interface and leave many developers with code churn. Regardless of where fault lies (often times it's one library using the private interface of another library, and the developer gets caught in the middle), this is a bad thing, so people began workarounds like complicated closure systems or weird-looking interfaces which emulated true private fields without having support for them. This started becoming popular enough that it was decided that adding them to the official syntax would be better, and unlike in PHP or other languages, there is no way around it here. Serializing or casting the object doesn't expose private fields, and there is no reflection methods or anything that would allow it. They are truly private. I personally share your feelings that this isn't an issue that needs solving (i'm of the opinion that the lack of easy truly private properties had a non-zero impact on the rise of javascript, and the ability to tweak private interfaces in your code is a net benefit, even if it gives developers rope to hang themselves with), but the pain felt from it is real for many, and the workarounds they were using were not only slow and difficult to use and maintain, but also very hard to understand, and this alternative is a much better solution in my opinion. So while I most likely won't be using them, I reluctantly support them being added to the language. [0] https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md https://github.com/tc39/proposal-class-fields/blob/master/PR...
Klathmon 8y ago
If `example.myVar = 500` can throw errors, then is the private field really private? Since now it's existence can break the public interface, and changing it to `myOtherVar` during a refactor can break existing code. That removes a major benefit of private fields (which is to be able to change implementation details without breaking the public interface) I don't think anyone really loves the syntax, but there doesn't seem to be much of a better way of doing it.
- franciscop 8y ago
  What happens if you do `example.#myVar = 500`? Does it throw an error? Wouldn't it be the same situation?
  - Klathmon 8y ago
    as of right now, `example.#myVar` isn't valid syntax, which is the reason that the # sigil was chosen. It's not possible to have current code that works like that, so defining new properties for that code (that it can only be used inside the class as a private field) isn't breaking anything. In other words, outside of the class implementation, `example.#myVar` is always an error, regardless of the existence of a private field or not, and it will always be an error with or without this proposal. And inside the class it's only legal if the private field was defined in the class.
  - __ryan__ 8y ago
    The difference is that when you type example.myVar = 400 you are setting a property names "myVar" on the object to 400. When you type example.#myVar you are trying to set example's private field to 400. The intentions are distinct and it is not costly to determine at the call site what you're trying to do.
    - earthboundkid 8y ago
      What if I have private values I want to access programmatically? Ie. fields = ['#foo', '#baz', '#bar']; fields.forEach(field => { this[field] = frobnicate(field) }); ?

__ryan__ 8y ago

Not as far as the spec is concerned. They decided that private variables should be completely scoped to the class. "Having a private field named x must not prevent there from being a public field named x, so accessing a private field can't just be a normal lookup. This is only an issue in JavaScript because of its lack of static types. Statically typed languages use type declarations to distinguish the external-public/internal-private cases without the need of a sigil. But a dynamically typed language doesn't have enough static information to differentiate those cases." [0] There are also performance concerns to changing the lookup logic without the sigil. More info in the link. 0: https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md#why-isnt-access-thisx https://github.com/tc39/proposal-class-fields/blob/master/PR...

dotancohen 8y ago
I still don't see why this is an issue in Javascript. PHP, the canonical hate-it dynamically-typed language, does not have this issue. Sure, you say, that's because in PHP a public property and a private property cannot have the same name. Correct. Why is this a requirement? What problem is this trying to solve? In e.g. older Python we would use the convention of a leading underscore to indicate that a property is private, even if technically one _could_ access it publicly it was understood that is a bad idea to do in practice. It's not a security issue - in every language one could read the "private" properties by some method or another. Reflection, for instance. Or, back to PHP, by casting the object to an array (I did mention that PHP is the canonical hate-it dynamically-typed language).
- Klathmon 8y ago
  I really recommend you read over the FAQ that is in the proposal at [0], it answers most of your questions. The problem is that it seems a "private by convention" system isn't private enough for many library authors, who are finding their private internal methods being used regardless, and then after some time becoming ossified leaving the maintainers in a tough situation. Support the "private" interface even though they shouldn't have to to avoid breaking a significant number of users, or break the "private" interface and leave many developers with code churn. Regardless of where fault lies (often times it's one library using the private interface of another library, and the developer gets caught in the middle), this is a bad thing, so people began workarounds like complicated closure systems or weird-looking interfaces which emulated true private fields without having support for them. This started becoming popular enough that it was decided that adding them to the official syntax would be better, and unlike in PHP or other languages, there is no way around it here. Serializing or casting the object doesn't expose private fields, and there is no reflection methods or anything that would allow it. They are truly private. I personally share your feelings that this isn't an issue that needs solving (i'm of the opinion that the lack of easy truly private properties had a non-zero impact on the rise of javascript, and the ability to tweak private interfaces in your code is a net benefit, even if it gives developers rope to hang themselves with), but the pain felt from it is real for many, and the workarounds they were using were not only slow and difficult to use and maintain, but also very hard to understand, and this alternative is a much better solution in my opinion. So while I most likely won't be using them, I reluctantly support them being added to the language. [0] https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md https://github.com/tc39/proposal-class-fields/blob/master/PR...

Klathmon 8y ago

If `example.myVar = 500` can throw errors, then is the private field really private? Since now it's existence can break the public interface, and changing it to `myOtherVar` during a refactor can break existing code. That removes a major benefit of private fields (which is to be able to change implementation details without breaking the public interface) I don't think anyone really loves the syntax, but there doesn't seem to be much of a better way of doing it.

franciscop 8y ago
What happens if you do `example.#myVar = 500`? Does it throw an error? Wouldn't it be the same situation?
- Klathmon 8y ago
  as of right now, `example.#myVar` isn't valid syntax, which is the reason that the # sigil was chosen. It's not possible to have current code that works like that, so defining new properties for that code (that it can only be used inside the class as a private field) isn't breaking anything. In other words, outside of the class implementation, `example.#myVar` is always an error, regardless of the existence of a private field or not, and it will always be an error with or without this proposal. And inside the class it's only legal if the private field was defined in the class.
- __ryan__ 8y ago
  The difference is that when you type example.myVar = 400 you are setting a property names "myVar" on the object to 400. When you type example.#myVar you are trying to set example's private field to 400. The intentions are distinct and it is not costly to determine at the call site what you're trying to do.
  - earthboundkid 8y ago
    What if I have private values I want to access programmatically? Ie. fields = ['#foo', '#baz', '#bar']; fields.forEach(field => { this[field] = frobnicate(field) }); ?