Typescript：为什么 keyof Union 永远不会

Question

This is my Typescript code:

interface Todo {
  title: string;
  content: string
}

type Union = Omit<Todo, 'content'> | {
  name: string
};
type key = keyof Union; // never

My question is that why the type key is never?

Answer 1

Because extends works like intersection & .

interface Todo {
  title: string;
  content: string
}

// a bit simplified
type A =  Omit<Todo, 'content'> // { title: string }
type B = { name: string };

type Union = A | B

type key = keyof Union; // never

keyof operator checks if union type has any sharable property. In your case neither A nor B has not same property.

Take a look on next example:

type A = { name: string, age: number }
type B = { name: string };

type Union = A | B

type key = keyof Union; // name

Here, keyof will return "name". because this property exists in both A and B.

Answer 2

The problem lies in the | operator. If you substitute it with & , you will get the result you expect:

interface Todo {
  title: string;
  content: string
}

type Union = Omit<Todo, 'content'> & {
  name: string
};
type key = keyof Union; // "title" | "name"