Convert an array of strings into string literal union type

Question

I'm trying to convert an array of strings from the value to the string union type inside a function. But can't achieve it.

Example:

const makeGet = (paths: string[]) => (path: typeof paths[number]) => paths.includes(path)
const makeGet2 =
  <T extends string>(paths: string[]) =>
  (path: T) =>
    paths.includes(path)

const routes = ['users', 'todos']
const readonlyRoutes = ['users', 'todos'] as const

const get = makeGet(routes)
const get2 = makeGet2<typeof readonlyRoutes[number]>(routes)

get('users') // no ts support
get2('users') // yes ts support

How should I refactor my makeGet function to be able to create a string union type from the passed routes array?

Playground

Andrea Simone Costa · Accepted Answer · 2022-04-04 14:20:01Z

1

This could be what you are looking for:

const makeGet =
  <T extends string>(paths: ReadonlyArray<T>) =>
  (path: T) =>
    paths.includes(path);

const routes = ["users", "todos"] as const;

const get = makeGet(routes);

get("users");
get("user"); // Argument of type '"user"' is not assignable to parameter of type '"users" | "todos"'

edited Apr 4, 2022 at 14:20

answered Apr 2, 2022 at 19:19

Andrea Simone Costa

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1 Comment

vladi_strilets Over a year ago

Thanks for the solution, it works fine. Is it possible to define the array of items outside the function? like in my example?

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