TypeScript Version: 3.6.2

Search Terms:

• globalThis

Code

This code passed the type checker in TS 3.5:

interface MyWindow extends Window {
  foo: string;
}

(window as MyWindow).foo = 'bar';

but fails with this error in TS 3.6:

Conversion of type 'Window & typeof globalThis' to type 'MyWindow' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
  Property 'foo' is missing in type 'Window & typeof globalThis' but required in type 'MyWindow'.ts(2352)

Following the release notes, I can work around the issue by changing from interface to type and using an intersection:

type MyWindow = (typeof window) & {
  foo: string;
}

(window as MyWindow).foo = 'bar';

but this feels more obscure than the old way. Is this WAI? What's the rationale for the non-extendable Window?

Expected behavior:

An interface should be able to extend Window.

Actual behavior:

The error above.

Playground Link: 3.6 isn't on the playground yet.

Related Issues: