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NonNullable behaves differently when passing a property type from this #36061
Description
TypeScript Version: 3.7.2
Search Terms:
NonNullable this this[] this property nonnullable
Code
type SomeType = string | undefined
class Test {
prop: SomeType
func1 = (): this['prop'] => {
return this.prop
}
func2 = (): SomeType => {
return this.prop
}
func3 = (): NonNullable<this['prop']> => {
return this.prop!
}
func4 = (): NonNullable<SomeType> => {
return this.prop!
}
}Expected behavior:
This code compiles correctly, this['prop'] and SomeType point to the same type.
Actual behavior:
func3 is not happy with the return statement: Type 'string' is not assignable to type 'NonNullable<this["prop"]>'.
Playground Link: link
Related Issues: none
Activity
AnyhowStep commentedon Jan 7, 2020
Workaround,
Playground
RyanCavanaugh commentedon Jan 13, 2020
Wesley Wigham (@weswigham) can you link to the proper duplicate please
shilangyu commentedon Feb 23, 2020
Tried v3.8, still seems to not work. I also tried to find the duplicate issue but I couldn't (probably a broader topic)
weswigham commentedon Mar 4, 2020
There's another similar issue to this one to do with
thisandNonNullable<this>not being compatible, but in this case the issue is that when we do a property access onthis, we actually get the constraint of the access (so we report the "current" type rather thanthis["prop"]), which, whenNonNullable'd, then produces a (less precise) result.#26942 is a dupe with a slightly inaccurate answer (even adding a cast is not sufficient, because of what I described above).
JoniJnm commentedon Mar 10, 2023
Works since 4.8
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-8.html#improved-intersection-reduction-union-compatibility-and-narrowing
#49119