Property 'x' does not exist on type 'never'

Question

In the following Typescript code, the compiler says that the doit property does not exist in the type 'never'. Could this be a compiler error?

class X { public foo(): void { if (this instanceof Y) { } else { this.doit(); } } private doit(): void { } } class Y extends X { } 

I found the following workaround:

 const temp = (this instanceof Y); if (temp) { } else { this.doit(); } 

The compiler has no problems with this equivalent code, which again makes me suspect that there is a compiler error.

Answer 1

Yes, it seems to be a mistake: InstanceOf is narrowed incorrectly when two types extend the same base class .

But no matter what is the point of what you are doing?
If you want foo to behave differently in cases of Y , then why not redefine it in Y :

 class Y extends X { public foo(): void { ... } } 

And if doit is only required in instances of Y , it should be in Y , if necessary, both can be protected in X