5 ms·

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
    I don't know what the spec's solution for that would be, but what you've typed would not work. Specifically because when I say `this.#foo` it is not a property on the object named "#foo". It is just the syntax for looking up a private field referenced by the literal #foo.
    - Klathmon 8y ago
      The spec's solution is to not solve that problem ;) Dynamic property access isn't allowed for private fields, so it kind of side steps that whole issue.
chacham15 8y ago
Correction: What happens if you do `example['#myVar'] = 500`? Does it throw an error? Wouldn't it be the same situation?
- Klathmon 8y ago
  dynamic access to private fields is considered a "non-goal" of sorts, so `example['#myVar']` will act the same as it does now, it will always refer to the public property of that name. https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md#why-doesnt-thisx-access-the-private-field-named-x-given-that-thisx-does https://github.com/tc39/proposal-class-fields/blob/master/PR...
  - franciscop 8y ago
    Then what about inside the class? What about `this['#myVar']`? Is it different from `this.#myVar`? If they are different it's very inconsistent and IMHO a lot worse. If they are the same, it's more confusing and still worse than the alternative suggested... what a mess. On one hand I follow a more functional programming style so this won't affect me but on the other I worry about trying to understand other people's code (especially very serious programming [tm]).
    - Klathmon 8y ago
      `this['#myVar']` will access the public field named `#myVar`, so it's the same as the `example` method. >If they are different it's very inconsistent and IMHO a lot worse. It is called out in the link I posted above as a "downside", and I agree that it sucks, but there aren't any other good options for this without breaking compatibility with current code. >I worry about trying to understand other people's code Luckily there is one big benefit to this that makes the impact of the weirdness surrounding the naming much less awful. Because of the "hard encapsulation" that the private fields have, they are literally and explicitly un-viewable, un-accessible, and un-usable outside of the class body. So the amount of complex meta-programming and many of the reasons you'd want dynamic property access in the first place (iterating, monkey-patching, using arbitrary strings as keys, computed keys, etc...) aren't applicable, and if you really want those features but also with privacy, you can nest a plain object as a private field and work with it that way just the same. So it may have some gotchas, but because the proposal is intentionally restricted as much as possible, and it explicitly disallows a lot of the metaprogramming that can be common and it only allows a very simple lookup syntax, the weirdness is very well contained, and at worst will be confined to between the opening and closing brace of a `class` statement.

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
  I don't know what the spec's solution for that would be, but what you've typed would not work. Specifically because when I say `this.#foo` it is not a property on the object named "#foo". It is just the syntax for looking up a private field referenced by the literal #foo.
  - Klathmon 8y ago
    The spec's solution is to not solve that problem ;) Dynamic property access isn't allowed for private fields, so it kind of side steps that whole issue.

chacham15 8y ago

Correction: What happens if you do `example['#myVar'] = 500`? Does it throw an error? Wouldn't it be the same situation?

Klathmon 8y ago
dynamic access to private fields is considered a "non-goal" of sorts, so `example['#myVar']` will act the same as it does now, it will always refer to the public property of that name. https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md#why-doesnt-thisx-access-the-private-field-named-x-given-that-thisx-does https://github.com/tc39/proposal-class-fields/blob/master/PR...
- franciscop 8y ago
  Then what about inside the class? What about `this['#myVar']`? Is it different from `this.#myVar`? If they are different it's very inconsistent and IMHO a lot worse. If they are the same, it's more confusing and still worse than the alternative suggested... what a mess. On one hand I follow a more functional programming style so this won't affect me but on the other I worry about trying to understand other people's code (especially very serious programming [tm]).
  - Klathmon 8y ago
    `this['#myVar']` will access the public field named `#myVar`, so it's the same as the `example` method. >If they are different it's very inconsistent and IMHO a lot worse. It is called out in the link I posted above as a "downside", and I agree that it sucks, but there aren't any other good options for this without breaking compatibility with current code. >I worry about trying to understand other people's code Luckily there is one big benefit to this that makes the impact of the weirdness surrounding the naming much less awful. Because of the "hard encapsulation" that the private fields have, they are literally and explicitly un-viewable, un-accessible, and un-usable outside of the class body. So the amount of complex meta-programming and many of the reasons you'd want dynamic property access in the first place (iterating, monkey-patching, using arbitrary strings as keys, computed keys, etc...) aren't applicable, and if you really want those features but also with privacy, you can nest a plain object as a private field and work with it that way just the same. So it may have some gotchas, but because the proposal is intentionally restricted as much as possible, and it explicitly disallows a lot of the metaprogramming that can be common and it only allows a very simple lookup syntax, the weirdness is very well contained, and at worst will be confined to between the opening and closing brace of a `class` statement.