Referencing `this` from a static class field

[rauschma]

TypeScript Version: Nightly Playground

The following code (Playground link) is legal in JavaScript, but produces a static error in TypeScript:

class C {
  static a = 'a';
  static b = this.a;
    // 'this' cannot be referenced in a static property initializer.
}
console.log(C.b); // 'a'

Practical use case – my library Enumify for enums:

class Color extends Enumify {
  static red = new Color();
  static orange = new Color();
  static yellow = new Color();
  static green = new Color();
  static blue = new Color();
  static purple = new Color();
  static _ = this.closeEnum(); // TypeScript: Color.closeEnum()
}

[DanielRosenwasser]

Sounds like you want #5863, but if I recall correctly, the problem is that we don't assume the static side of one class is substitutable for another because then every subclass construct signature would have to be assignable to the constructor in the superclass.

[rauschma]

AFAICT, this is simpler. In first example above, this is just a synonym for C.

Another example:

class A {
  // Legal in JS, not allowed in TS:
  static thisIsA = (this === A);
}
class B extends A {}

console.log(B.thisIsA); // true

[DanielRosenwasser]

So you want "Monomorphic this for static members" 😄

[rauschma]

Probably? 🤔

[robpalme]

Thank you for finding this @rauschma!

It's not just the type checking that's wrong. The emit is corrupting legit ESnext code. Playground

class C {
  static self = this;
}

...becomes...

class C {}
C.self = this;

[rauschma]

Right! Should be done via definition, not assignment.

[robpalme]

Should be done via definition, not assignment.

Actually that bit is handled fine via the option useDefineForClassFields. The badness comes from using this at top-level scope. The downlevel is just semantically wrong.

Interestingly this bug goes away if target == ESnext && useDefineForClassFields == true.

[RyanCavanaugh]

@sandersn can you look at the emit thing (described lower in comments) ?

[sandersn]

Note that the classic, non-spec-compliant emit has been wrong since at least 2.4: https://www.typescriptlang.org/play/?ts=2.4.1#code/MYGwhgzhAEDCD2J4CdoFMAeAXNA7AJjAKK4CuAtgJYBmAntAN4CwAUNNBFmFpcNMmnzQAvNFxoA7nEQoAFAEoA3K3aduvaCjC4A5mhFjJ0pMgXK2HLjz600IJFNHipCE2ZWX1fHQLwHnxnJKHmrW0ABGIKT6TkauQeaqVhoADqTIKSAxhi4ypsEWoRoA+gZYABaUEAB0oPAQaCQUZtAA9K3QACq0KWgAysDIlClYAFyByLVIDU3kCqwAvkA