Refer: Medium - Get Return Type

Describe

Implement the built-in ReturnType<T> generic without using it.

For example

code snippetCopytypescript
const fn = (v: boolean) => {
  if (v) return 1
  else return 2
}

type a = MyReturnType<typeof fn> // should be "1 | 2"

Test Cases

code snippetCopytypescript
import { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<string, MyReturnType<() => string>>>,
  Expect<Equal<123, MyReturnType<() => 123>>>,
  Expect<Equal<ComplexObject, MyReturnType<() => ComplexObject>>>,
  Expect<Equal<Promise<boolean>, MyReturnType<() => Promise<boolean>>>>,
  Expect<Equal<() => 'foo', MyReturnType<() => () => 'foo'>>>,
  Expect<Equal<1 | 2, MyReturnType<typeof fn>>>,
  Expect<Equal<1 | 2, MyReturnType<typeof fn1>>>,
]

type ComplexObject = {
  a: [12, 'foo']
  bar: 'hello'
  prev(): number
}

const fn = (v: boolean) => (v ? 1 : 2)
const fn1 = (v: boolean, w: any) => (v ? 1 : 2)

Solution

code snippetCopytypescript
type MyReturnType<T extends (...args: any[]) => any> = T extends (...args: any[]) => infer P ? P : never