Advanced TypeScript: Unions With Never

• TypeScript lets us union a type with never, but it has no effect. For example, number | never is just number, and string | never is just string.

• >

  function returnStringOrNever(): string | never {
    return 'Amir';
  }
  const name2: string = returnStringOrNever();
  name2;

  Result:

  'Amir'

• >

  function returnNumberOrNever(): number | never {
    return 1;
  }
  
  const n: number = returnNumberOrNever();
  n;

  Result:

  1

• This makes sense if we think about what the never type actually means. A never means "this can never actually have a value at runtime."

• The type number | never means "it's either a number, or (a thing that can never actually have a value at runtime)". If the never part can never have a value, then there's no need for it to be part of the union. The TypeScript compiler knows that it can discard that possibility. It's kind of like how n + 0 is always just n, no matter what value n has.

• This may remind you of something that we saw in an earlier lesson: if types X and Y don't share any possible values, then X&Y gives us never.

• These are two separate rules about never. First, impossible intersections give us the type never. Second, never can be removed from any union.

• On their own, unions with never aren't useful. For now, the only thing to know is that the never part of a union is always thrown away. However, in future lessons on more advanced types, unions with never are going to become very important!