Restricting a generic parameter to a combination type in Typescript

Is there a method in Typescript to restrict a generic parameter to only accept a union type? To clarify my question, I wish that T extends UnionType would serve this purpose:

function doSomethingWithUnion<T extends UnionType>(val: T) {}

doSomethingWithUnion<number | string>(...) // valid
doSomethingWithUnion<number>(...) // should throw an error

I came across a solution in another discussion on SO (53953814) which demonstrates how to determine if a type is a union type:

type IsUnion<T> = [T] extends [UnionToIntersection<T>] ? false : true

type Foo = IsUnion<string | number> // true
type Bar = IsUnion<string> // false

This allows us to, for instance, ensure that a function will never return if the generic parameter is not a union type:

declare function doSomethingWithUnion<T>(val: T): [T] extends [UnionToIntersection<T>] ? never: boolean;

doSomethingWithUnion<number>(2) // never
doSomethingWithUnion<string|number>(2) // => boolean

(Link to playground)

However, I have not yet discovered a way to limit the type itself in a similar fashion.

Answer №1

When working with Typescript, it is possible to set an upper bound for a generic type parameter, but not a lower bound or any other constraint. Initially, it may seem impossible to achieve this, but surprisingly, it can be done by linking the upper bound of 'T' to 'T' itself.

function test<T extends (IsUnion<T> extends true ? any : never)>(arg: T): T {
  // ...
  return arg;
}

// Valid
test<string | number>('example');

// Error: Type 'string' does not meet the criteria of 'never'.
test<string>('example');

// Error: Argument of type 'string' cannot be assigned to parameter of type 'never'.
test('example');

A point to consider is that even if 'T' can be 'never', which is not a union, it still works because 'never' encompasses everything and adheres to any potential upper limit imposed on the type variable. In practical scenarios, if 'T' is 'never', the function can only be called with an argument of type 'never' (essentially rendering it unusable), making this issue negligible.

If a different upper limit is needed for 'T', it can be specified instead of using 'any'.

Playground Link

Answer №2

I gave this a try and it appears to be effective. I'm curious if there's a way to streamline it further by utilizing a union type instead of having two types that are then combined into a union.

function onlyUnions<Type1, Type2>(result: Type1 | Type2) {
    return result;
}

onlyUnions<string, number>("string");

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