types-and-variables-can-share-names.md

Types and variables can share names

The Superstruct documentation on type inference includes this neat trick:

import { Infer } from "superstruct";

const User = object({
  id: number(),
  email: email(),
  name: string(),
});

type User = Infer<typeof User>;

In the context of Superstruct, this creates both a variable User that checks whether an object has a particular shape and a type User of that shape.

The cool thing here is that the type and the variable both have the same name. So you can import them both at the same time:

import { assert } from "superstruct";

// import both the variable and the type
import { User } from "./user";

// this line uses the type
async function getUser(): Promise<User> {
  const res = await fetch("https://example.com/api/fake-user-endpoint");
  const json = await res.json();

  // this line uses the variable
  assert(json, User);

  return user;
}

Note that this doesn’t work for all variables. Classes, for example, are already available as types, so if you write something like this TypeScript will tell you that Test is a duplicate identifier:

class Test {}

type Test = string;