Allow non-polymorphic ("private") extension of base classes

🔍 Search Terms

private non-polymorphic non-assignable inheritance extension subclass change method signature of base class

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

I would like to write an ES6 class that does not inherit the typing of its base class (and thus doesn't expose any properties or methods defined in the base class, unless they are overridden in the subclass). In C++ I would call this "private inheritance", but in short: I would like to be able to use the implementation of a class (by using the super keyword in a method) without declaring my instances to be assignment-compatible.

📃 Motivating Example

To the best of my knowledge, there are currently no ways to express this in TypeScript. In pure JavaScript, I can write the following:

export class MyTuple extends Array {
  constructor(...args) {
    super();
    this.push(...args);
    Object.freeze(this);
  }
}

Instances of the MyTuple class, despite extending the Array prototype, don't conform to the Array contract, as all of the mutation methods will throw a TypeError. Even worse, direct array element assignment will silently fail, as assignment to a read-only property is simply ignored. However, the only way to get TypeScript to output this JS code is to use a TypeScript class declaration, and these always propagate the base class typings.

In this particular case, the class is a better match for the ReadonlyArray interface, so ideally I could write the following in TypeScript:

export class MyTuple<T> extends private Array<T> implements ReadonlyArray<T> {
  constructor(...args: readonly T[]) {
    super();
    this.push(...args);
    Object.freeze(this);
  }
}

💻 Use Cases

1. What do you want to use this for?
   In my current project, I'm writing a variable-dimensionality geometry library in which the Point class is a specialization of Array<number>. Point is a generic class with the signature Point<Dims extends number>, so a 2D point is a Point<2>, a 3D point is a Point<3>, etc. Using Array as the base class provides a number of advantages, like being able to use .map() in order to create a new Point with the same dimensionality, based on the existing. However, most of the Array functionality shouldn't be exposed to consumers; in particular, anything that modifies the length of the array or creates an array of different length (push, slice, etc) would cause improper typing.
2. What shortcomings exist with current approaches?
   Well, first off, the return type of .map() is incorrect because of #10886; it and all the other copy-instantiating methods should return a Point but instead return number[]. I can't use declare to override the method signature directly because of #38008 and #48290, and TS won't allow me to override it as a property because the types don't match.
3. What workarounds are you using in the meantime?
   I'm removing the typing entirely by casting Array to a dummy constructor of Object:

   export class Point<Dims extends number = 2> extends (Array as new () => Object) implements PointLike<Dims> {
   

   Previously I'd been casting it to a constructor of readonly number[], but that didn't work because, as mentioned above, I need to declare map() as a method that returns a Point, and the existing number[] return signature can't be cast to a Point derived from readonly number[]. So now I'm casting it to Object, which comes with a fairly serious drawback: I can no longer use the super keyword in my methods, because TS now thinks that super is type Object, and I can't even correct it - the super keyword can't have a type assertion applied to it, as mentioned in #41034.

Now I go to write a lot of type assertions and // @ts-expect-error comments, because so far as I can tell, there is no other way around this.

動機となる MyTuple と Point の例から始め、関連する issue #10886、#38008、#48290、#41034 を確認してください。生成される JavaScript を変更せずに、private または非ポリモーフィックな基底型指定を行えるかどうかを判断してください。例が型チェックを通過し、代入不可能性と使用可能な super 呼び出しが維持されれば完了です。