Class-body directive prologue is incompatible with class fields

[domenic]

Discovered in TC39 today.

class Foo {
  "hide implementation";

   constructor() { }
   bar() { }
}

declares a public field named "hide implementation". Whee!

Solutions

Omit directive prologue in class bodies

We could just omit this capability. Then developers would have several more-awkward options, such as:

"hide implementation";

class Foo {
   constructor() { }
   bar() { }
}

// Anything else in the source file also gets implementation-hidden, which might be bad.

or

let Foo = (() => {
  "hide implementation";
  return class Foo {
     constructor() { }
     bar() { }
  };
})();

or

class Foo {
   constructor() {
     "hide implementation";
   }
   bar() {
     "hide implementation";
   }

   // repeat as needed
}

Don't use directive prologues

Basically #13.

Something else clever

TBD.

[nicolo-ribaudo]

class Foo extends Bar ("hide implementation") {
}

That is a function call

[ljharb]

lol oops, not so clever after all

[domenic]

In particular, if we're going to add any new syntax, I think it should be either:

• Backward-compatible (like the current proposal is), or
• Backward-incompatible and pretty (like Decorator-like syntax suggestion #13)

[rbuckton]

What about a different prologue for hiding class implementations, specified in the constructor:

class Foo {
  constructor() {
    "hide class implementation";
  }
}

[ljharb]

I like that - considering the constructor function is what class actually produces, this seems like a neat solution to the problem (and answers the question of what would happen if i put the pragma in the constructor with the current proposal)

[rbuckton]

I would imagine "hide implementation"; in the constructor would only hide the body of the constructor itself, while "hide class implementation"; would hide the body of the entire class.

[ljharb]

Although now that i think about it, it’d be weird if the constructor pragma impacted static methods :-/

[michaelficarra]

@rbuckton The constructor does not always appear first (or even early) in the class body. I wouldn't want to be unable to tell that we're in a special kind of context until the whole class body is read.

[rbuckton]

@ljharb: Possibly sounds like https://github.com/tc39/proposal-class-static-block#readme could be an option:

class Foo {
  static {
    "hide implementation";
  }
}

@michaelficarra: True, but there's still value in the simple solution.

[rbuckton]

Another option could be this:

class Foo {
  ("hide implementation");
}

This keeps the pragma at the top of the class and doesn't conflict with public fields, for the added cost of two characters.

[michaelficarra]

We're no longer interested in adding new directive placements.

[ljharb]

@michaelficarra so what's the plan for hiding the implementation of the constructor/class body? a pragma inside the constructor?

[michaelficarra]

@ljharb If you want to affect only the constructor, put it in the constructor. If you want it to apply to all of the methods of the class, wrap the class in an IIFE or just put the directive in the enclosing scope if you don't mind the over-application.

[ljharb]

Gotcha - i ask because the constructor’s implementation is the entire class body, so hiding it is hiding the class (altho not necessarily all its methods)