Is it possible to annotate exclusive union in TypeScript?

August 24, 2022

I’m trying to find a type for generic error handling that does something like this:

function request(): GenericError<{body:string}> {
  if (/* Some condition */) {
    return { error: "An error occurred" };
  }

  return { body: "Things went well" };
}

The goal is to infer that body exists if error doesn’t, i.e:

const response = request();

if (response.error) return;

/* TypeScript should infer that body can't be undefined here */

I don’t know if this is possible in TypeScript and this question might be a duplicate, but I don’t know how to word the problem exactly. I tried annotating the function with the answers in this question but none of them could guarantee body to exist if error didn’t, as far as I could see.

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>Solution :

A union { error: string } | { body: string } + narrowing will do it :

function request(): { error: string } | { body: string } {
    if (/* Some condition */) {
        return { error: "An error occurred" };
    }

    return { body: "Things went well" };
}

const response = request();

if ('error' in response) { // narrowing
    response.error // OK
    return 
}

response.body // OK

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