Private constructor type simplification produces incompatible type

🔎 Search Terms

private constructor incompatible parameter types removed

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about private constructor type simplification

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-beta#code/PTAEBUAsEsGdQMYBsCGt4BMD2BTWA7AcgBdQcAPOYgGlGlLlACsBXWUtnDUNHxLfADcc+aCNIB3FAE9QAMywAnUMWkAHaPgDmPfLORpYAOgCwAKAw4DinDwBG7RSgSkD6UAGEBjli5R2kWwBvc1Aw0DVFaEEUYlsEb2JFX2IlAAojTJRFLVgALlAWfABrfCwJfABtAF0ASgBucwBfc3MQUABBBATFDE0dVNAtERwnOO4jDCNiWFpiGHgkLCxi+ApnUlgUAFtbXi98Hz8AnHN-Rw3EVHcAcRGxrgOj4n9A0BCzcIiomLj+Q6SKXStXeLTMYPMFDUSlciXkRQQUFiXQQODUMw6eg813gAF5QGkDAAvIkFVRqHBYOSeRLJY6BEG4gB8oMaZihMP+7Hh+ERkGR3TRGL0d3wo1ijxxoHxhNQJLJ6kp1NF4vGT0B9JwjJZQSabPMbngAFlpAAFH4S7GGd6hcIAYki0QlADUUEgCnZloEUPg2V9Hb94rSgYo0ravmE1CwAtAEKAbChsPgkLIFFgCo5+tRwxGozG4wmkynQHZshmkv0cyCPhHwvM4EYHRa4q6kNKVMkcH7wmCwYbQCbzU64lb0AB1eiQAByWFN2R28Br9oDLrdHq9OB93cjzaDALpqVD1ZzX3rxibw5wrfbgK7Od7rTM7SgjEYEn5pHJeAQUXRoFgOB4DwpB2Cw0BINwxDQLstD0PAaiGP0ACE5hyAiSLECiQqwJi0ijrAaSDru+ENKh6Eflh6I4ViOKEWaxE4hO8wznOTjbLApFmG0YAvvAb4fioiqwD+0B-gBQHviIQz3BK3ByOBQHZLY0HQoo4xkEgAGSTYsGkHIKDgbAKFmGhvIYZRwrSCqDwYPhdFDoGJFsqZfICqiVG4dZsl2URl74Ux06zvO7ENEAA

💻 Code

// This class doesn't exist, it is just used as a convenient way for typing any class.
declare abstract class Constructable {
    private constructor(...args: unknown[]);
}

// According to generated .d.ts, this looks exact same as Constructable
abstract class GeneratedConstructable {
    private constructor() {}
}

export const funcThatAcceptsAnyClass = (clazz: typeof Constructable) => {};
export const funcThatAcceptsAnyGeneratedClass = (clazz: typeof GeneratedConstructable) => {};

class MyPrivateClass {
    #privateVal: boolean;
    private constructor(
        public readonly foo: string,
        public readonly bar: string
    ) {
        this.#privateVal = true;
    }
}
class MyPrivateClassWithNoParams {
    #privateVal: boolean;
    private constructor() {
        this.#privateVal = true;
    }
}

// This is what typescript sees at build time, its passing!
funcThatAcceptsAnyClass(MyPrivateClass);
funcThatAcceptsAnyClass(MyPrivateClassWithNoParams);

// This is what typescript sees when generated files are imported elsewhere, it fails!
funcThatAcceptsAnyGeneratedClass(MyPrivateClass);
funcThatAcceptsAnyGeneratedClass(MyPrivateClassWithNoParams);

🙁 Actual behavior

When a class contains a private constructor, the generated types file (.d.ts) strips the constructor down to the bare minimum, meaning any parameters are removed from the method.

This creates a type that is incompatible with the original type declaration.

Since it is only the result of generated code, it cannot be detected during build time. Instead it is caught when the types are referenced in a separate package.

🙂 Expected behavior

Either:

1. Parameters are not stripped from private class constructors
   • preferred
   • Less likely to be breaking change
   • Would produce slightly larger type files
2. Private constructors are treated as parameter-less during build time
   • Would not apply to "inside" the class (where private constructors can be legally called)
   • Would at least produce consistent type behavior

Additional information about the issue

I stumbled across this issue when trying to restrict the type of a value to anything with a class.

Essentially the use case is a map of Class -> Instance. Restricting to "is a class" is important for instanceof checks. (I understand JS can technically call new on any function, but its 2024 and I think it is fair to restrict "class" behavior to the class keyword)

The implementation was not actually responsible for doing the instantiation, so private or abstract constructors are totally acceptable.

I have various type checks in the package itself, that behave entirely as expected.
However when I started using this package in another, suddenly I am hit with:

Argument of type 'typeof MyPrivateClass' is not assignable to parameter of type 'typeof GeneratedConstructable'.
  Types of construct signatures are incompatible.
    Type 'new (foo: string, bar: string) => MyPrivateClass' is not assignable to type 'abstract new () => GeneratedConstructable'.
      Target signature provides too few arguments. Expected 2 or more, but got 0.

I would expect this kind of type behavior to either be caught during the original build time (second option in expected behavior) or to not implement this kind of constructor "simplifying" at all.

Perhaps it could be gated behind a compiler flag, because I can understand this is probably a fairly niche requirement. I understand in most cases, we use type comparisons against the instances, not the classes, and when using the classes a private constructor is an "internal" detail that usually does not expose it's functionality.

I would also accept an alternative to typeof Constructable.

The goal is to generate a type that effectively looks like private abstract new (...args: unknown[]) => any. However the private keyword is not allowed, and type matching fails on Cannot assign a 'private' constructor type to a 'public' constructor type

Start by reproducing the private-constructor example in the linked TypeScript Playground and compare the source declarations with the generated .d.ts output. Trace the declaration-generation path for private constructors, then make build-time and generated-type behavior consistent and add a regression test covering parameterized and parameterless private constructors.