ReturnType of Intersection Types

[christianalfoni]

TypeScript Version: 2.9.2 and 3.0.0-dev.20180705

Search Terms:
typescript intersection ReturnType

Code

const foo = () => ({
    foo: 'bar'
})

const bar = () => ({
    bar: 'baz'
})

type Test = typeof foo & typeof bar

type Test2 = ReturnType<Test>

Expected behavior:
I would expect Test2 to be a union of ReturnType of "foo" and "bar"

Actual behavior:
I only get the ReturnType of "bar"

Playground Link:
http://www.typescriptlang.org/play/#src=const%20foo%20%3D%20()%20%3D%3E%20(%7B%0D%0A%20%20%20%20foo%3A%20'bar'%0D%0A%7D)%0D%0A%0D%0Aconst%20bar%20%3D%20()%20%3D%3E%20(%7B%0D%0A%20%20%20%20bar%3A%20'baz'%0D%0A%7D)%0D%0A%0D%0Atype%20Test%20%3D%20typeof%20foo%20%26%20typeof%20bar%0D%0A%0D%0Atype%20Test2%20%3D%20ReturnType%3CTest%3E%0D%0A

You will have to excuse if I am being ignorant on this and it actually is expected behaviour :-)

[mhegazy]

ReturnType (and conditional types on signatures in general) only operate on single overload. For intersection, union or overloads, the first is always picked.

#6606 tracks adding call types.

[christianalfoni]

@mhegazy Aha, I see... thanks for the quick response!

[christianalfoni]

Okay, so this black magic TypeScript code actually solves it:

type UnionToIntersection<U> = 
  (U extends any ? (k: U)=>void : never) extends ((k: infer I)=>void) ? I : never

const foo = () => ({
    foo: 'bar'
})

const bar = () => ({
    bar: 'baz'
})

type Test = typeof foo | typeof bar // Notice the OR here

type Test2 = UnionToIntersection<ReturnType<Test>>