.

[summer palm]

declare const fn: () => undefined | false

const a = fn()

!ts a

[zealous lava]

const a: false | undefined /* 3:7 */```

[summer palm]

its, not? unless you mean something else

unless the example you provided is exactly what you mean

screenshots aren't really helpful

playground examples and code snippets are much more useful

https://www.typescriptlang.org/play

there isn't enough information here to help you