`abstract` modifier cannot be used with private a identifier

[lilnasy]

🔎 Search Terms

abstract, private fields, classes

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Classes

⏯ Playground Link

https://www.typescriptlang.org/play/?ssl=17&ssc=22&pln=1&pc=1#code/IYIwzgLgTsDGEAJYBthjAgggg3gKAQVEhngQGIAPALgQDsBXAWxAFMoCFOBzViADQAUASlydCUPgyh0EEABYBLMADoqnAL55OkYBEWwEAIQQBeJKnTGErShFZ0AJhmz5ChWAHs6JBvE9QImLu7mAMAA7sIuLuCspqlGYIAKwxWoRaWnhePojASXSsAO5YKkbZ3mCeyKwqyJ7cgsAqvAIiwkA

💻 Code

abstract class A {
  abstract #x: number
  
  getX() {
    return this.#x
  }

  static B = class B extends A {
    constructor() {
      super()
      this.#x = 5
    }
  }
}

const a = new A.B
console.log(a.getX())

This code works as you would expect at runtime (chrome devtools screenshot), since nested static classes can access the same private fields as their containing class.

🙁 Actual behavior

TypeScript errors on line 2 with this message:

'abstract' modifier cannot be used with a private identifier.(18019)

... requiring a ts-expect-error directive.

🙂 Expected behavior

The abstract keyword is allowed.

Additional information about the issue

No response

[whzx5byb]

Wait... this should be a corrrect behavior and I'm very surprised that it was tagged as "Bug", since an abstract propety will be erased in runtime so you can never access this.#x in class A. And, how can you ever "implement" a private abstract property? They belong to different classes.

[lilnasy]

@whzx5byb Yeah, I guess I got that wrong. I didn't know abstract fields are elided. I would need the abstract private field to remain in the compiled JS so maybe the abstract modifier is not what I want in the first place, even though it is what I want conceptually.

As for implementing an abstract private field, it is possible if the subclass is authored as a static property of the superclass declaring the private field. See the linked screenshot of it working in pure JS in modern chrome. It's kind of wild and arcane (sorry), and I didn't think it would work until I tried it.

Seems like private fields don't belong to classes but rather the class block they are declared in. Different classes can share the same private fields if they share the same class block, which classes declared as static fields of other classes do.

[RyanCavanaugh]

I agree with #60627 (comment) - this isn't something that can actually work in practice

[lilnasy]

Yeah, @whzx5byb brings up a convincing counter-example there.