Get Return Type

introduction#

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

For example

ts
const fn = (v: boolean) => {
  if (v) return 1;
  else return 2;
};
type a = MyReturnType<typeof fn>; // should be "1 | 2"

ts
const fn = (v: boolean) => {
  if (v) return 1;
  else return 2;
};
type a = MyReturnType<typeof fn>; // should be "1 | 2"

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start point#

ts
/* _____________ Your Code Here _____________ */
type MyReturnType<T> = any;
 
/* _____________ Test Cases _____________ */
type cases = [
  Expect<Equal<string, MyReturnType<() => string>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<123, MyReturnType<() => 123>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<ComplexObject, MyReturnType<() => ComplexObject>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<Promise<boolean>, MyReturnType<() => Promise<boolean>>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<() => 'foo', MyReturnType<() => () => 'foo'>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<1 | 2, MyReturnType<typeof fn>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<1 | 2, MyReturnType<typeof fn1>>>
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.];
 
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);Try

ts
/* _____________ Your Code Here _____________ */
type MyReturnType<T> = any;
 
/* _____________ Test Cases _____________ */
type cases = [
  Expect<Equal<string, MyReturnType<() => string>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<123, MyReturnType<() => 123>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<ComplexObject, MyReturnType<() => ComplexObject>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<Promise<boolean>, MyReturnType<() => Promise<boolean>>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<() => 'foo', MyReturnType<() => () => 'foo'>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<1 | 2, MyReturnType<typeof fn>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<1 | 2, MyReturnType<typeof fn1>>>
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.];
 
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);Try

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my solutions#

Spoiler warning // Click to reveal answer

ts
type MyReturnType<T> = T extends (...args: any[]) => infer R ? R : never;Try

ts
type MyReturnType<T> = T extends (...args: any[]) => infer R ? R : never;Try

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