How to throw compilation error if subclass wants to add a new property?

[glass furnace]

@misty crystal Here's a shortened URL of your playground link! You can remove the full link from your message.

german.jablo#0

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Preview:ts class A { propA = "A" methodA() { console.log("A") } // [key: string]: keyof A extends typeof key ? unknown : never; // Doesn't do anything. Doesn't prvent subclasses to add new methods or properties // [key: string]: typeof key extends keyof this ? this[typeof key] : never; // Type instantiation is excessively deep and possibly infinite.(2 ...

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[raven kraken]

why do you want this? subtypes can always have additional properties; that's just what subtyping means in a structural type system

[misty crystal]

I want the subclasses to add all their properties inside the “data” property, to simplify the toJSON serialization by making it automatic.

I know I could use other patterns, and I have explored several, but each one has its tradeoffs. This one I think could be a good option

[raven kraken]

is it a requirement to use "normal" class X {} syntax? you could maybe pull this off using a validator type if you create the classes via a function instead

but that might feel pretty weird

what's stopping the toJSON method from serializing top-level properties?

[misty crystal]

yes, my library provides the superclass, users have to define the subclass. I don't want them to have to worry about serialization, so I do:

  toJSON() {
    return {
      // ... here I parse my props
      data: JSON.stringify(this.data),
    };
  }

[raven kraken]

right, but can't that be return JSON.stringify(this) (or something morally equivalent to it)?

(probably need to change the method name in the base class to avoid infinite recursion, since JSON.stringify will look for a method named toJSON)

perhaps i'm missing something—i might need to know what // ... here I parse my props looks like