Typescript ReturnType of generic function

Question

The new ReturnType in TypeScript 2.8 is a really useful feature that lets you extract the return type of a particular function.

function foo(e:          


        

Answer 1

This is my currently working solution for extracting un-exported internal types of imported libraries (like knex):

// foo is an imported function that I have no control over
function foo(e: T): InternalType {
    return e;
}

interface Wrapper {
  // wrapped has no explicit return type so we can infer it
  wrapped(e: T) {
    return foo(e)
  }
}

type FooInternalType = ReturnType['wrapped']>
type Y = FooInternalType
// Y === InternalType