Typescript type assertion for private Instance class variables

Bug Report

Unable to type assert a private class variable instance

🔎 Search Terms

assertion

⏯ Playground Link

Playground link with relevant code Playground link 2

💻 Code

export class A {
  private myArray: string[] | undefined;

  private constructor() {}

  private assertMyArray(): asserts this is this & {
    myArray: string[]
  } {
    if (!this.myArray) throw new Error('myArray is not defined');
  }

  private doSomething() {
    this.assertMyArray();
    const len = this.myArray.length; // <-- error here
  }
}

🙁 Actual behaviour

TS evaluates the type assertion assertMyArray as never

TS2339: Property 'myArray' does not exist on type 'never'.   
The intersection 'this & { myArray: string; }' was reduced to 'never' because property 'myArray' exists in multiple constituents and is private in some.

If changing from private to public signature everything will work as expected

🙂 Expected behaviour

I think the expected behaviour should be exactly as on public variables, so it would evaluate correctly and not resolve the assertion to never (while keeping the right this type)

[fatcerberus]

there’s no way to refer to the name of a private field in a way that creates the same kind of name as exists in the originating declaration.

To clarify on this potentially confusing point, what’s meant by this is that the “name” of a private property—for the purpose of structural typing—conceptually includes the identity of its containing class; in other words:

class A { private foo: string; }
class B { private foo: string; }

The usual typing model in TS is one where types are matched structurally according to the names of their properties, but under this model foo in A cannot be considered to have the same “name” as the foo in B because they originate from separate class definitions. Thus A and B are not assignable to each other.

The root of this issue is that there’s no way to construct a type on-the-fly that includes the requisite “foo originates from A” tag that would make the intersection method in the OP work.

[RyanCavanaugh]

Someone more clever might be able to figure this out, but AFAICT there is no way to do this because there’s no way to refer to the name of a private field in a way that creates the same kind of name as exists in the originating declaration.