Typecript - How to send union strings as enum?

Question

The following code does not compile (https://www.typescriptlang.org/play):

// I can't change it to be Enum2
enum Enum1 {
  A = 'a',
  B = 'b',
}

// auto generated by third library tool - can't change it
enum Enum2 {
  A = 'a',
  B = 'b',
}

// auto generated by third library tool - can't change it
type Union = 'a' | 'b'

declare function f1(e: Enum1): void

// Argument of type 'Enum2.A' is not assignable to parameter of type 'Enum1'.(2345)
f1(Enum2.A) // does not compile
f1('a') // does not compile

Error:

TS2345: Argument of type 'HardhatPodStartedDto[]' is not assignable to parameter of type 'SetStateAction<HardhatPodStarted[]>'

What can I do to solve the compilation error?

I can't change Enum1, Enum2, Union.

I have a strong suspicion there are more constraints than you've listed. For instance, I'm guessing the code inside f1 does something with e, and that may well influence the suggestions you're given. But right now, we don't have a lot to work with. — T.J. Crowder
– T.J. Crowder, Commented May 1, 2022 at 13:15

T.J. Crowder · Accepted Answer · 2022-05-01 13:40:16Z

2

The only thing you haven't said you can't change is the function, so:

declare function f1(e: Union): void
//                     ^^^^^

Both of your examples then work: playground

If the implementaton of f1 needs to convert from Union to Enum1 or Enum2, there's really no shortcut for it. You can readily create a function, though:

const unionToEnum1 = (u: Union): Enum1 => {
    switch (u) {
        case "a":
            return Enum1.A;
        case "b":
            return Enum1.B;
        default:
            throw new Error(`Unexpected 'Union' value "${u}"`);
    }
};

And/or similar for Enum2.

And/or a compile-time conversion for those situations where one will work:

type UnionToEnum1<U extends Union> =
      U extends "a" ? Enum1.A
    : U extends "b" ? Enum1.B
    : never;

edited May 1, 2022 at 13:40

answered May 1, 2022 at 13:14

T.J. Crowder

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