Type Aliases

Type Aliases

Type aliases create a new name for a type. Type aliases are sometimes similar to interfaces, but can name primitives, unions, tuples, and any other types that you’d otherwise have to write by hand.

type Name = string;
type NameResolver = () => string;
type NameOrResolver = Name | NameResolver;
function getName(n: NameOrResolver): Name {
  if (typeof n === "string") {
    return n;
  }
  else {
    return n();
  }
}

Aliasing doesn’t actually create a new type - it creates a new name to refer to that type. Aliasing a primitive is not terribly useful, though it can be used as a form of documentation.

Just like interfaces, type aliases can also be generic - we can just add type parameters and use them on the right side of the alias declaration:

type Container<T> = { value: T };

We can also have a type alias refer to itself in a property:

type Tree<T> = {
  value: T;
  left: Tree<T>;
  right: Tree<T>;
}

Together with intersection types, we can make some pretty mind-bending types:

type LinkedList<T> = T & { next: LinkedList<T> };

interface Person {
  name: string;
}

var people: LinkedList<Person>;
var s = people.name;
var s = people.next.name;
var s = people.next.next.name;
var s = people.next.next.next.name;

However, it’s not possible for a type alias to appear anywhere else on the right side of the declaration:

type Yikes = Array<Yikes>; // error