Dynamic object key + discriminated union + typeof could have a better narrowing type

Search Terms

dynamic object key, discriminated union, typeof, type inference, narrowing type

Suggestion (or is it a bug report?)

We could write a discriminated union with one of the cases is a dynamic object key. For example...

type TExample = (
  {
    [key in string]: {
      foo: number
    }
  }
  | {
    errorCode: string
  }
)

So, if the key is errorCode it could be a { foo: number } or a string. If the key is any string that isn't errorCode, it should be a { foo: number }.

Then let's check that.

const func: () => TExample = () => ... // get value from somewhere

const value = func()

if ('errorCode' in value && typeof value.errorCode === 'string') {
  value // what's the type of value here?
}

So, makes sense that the value's type should be { errorCode: string }, right?
But the type still is TExample! I think that we could have a better narrowing type, because value only shoud be { errorCode: string } on this case.

Similarly, would be nice to have that:

if ('errorCode' in value && typeof value.errorCode === 'object') {
  value // should be { [key in string]: { foo: number } }
}

As well as...

if ('errorCode' in value && typeof value.errorCode === 'number') {
    value // should be never
}

Full code on playground

Edit

I just noticed that it happens even when we are not using a dynamic object key...

type TExample = (
  {
    aaa: {
        foo: number
    }
  }
  | {
    errorCode: string
  }
)

const func: () => TExample = () => ... // get value from somewhere

const value = func()

if ('errorCode' in value && ((typeof value.errorCode) === 'number')) {
    value // type is "{ errorCode: string }" ......  what!? I think that the expected is to be "never"
}

Playground

Use Cases

I'm opening this issue because I had a problem because of this limitation.

I'm developing a client for an API and, for convention, all errors is returned as { errorCode: string }.
And on an endpoint, the json can be a { [key in string]: TComplexObject } on success case, or be a { errorCode: string } on fail case.
So normally I'm checking if I had an error using if ('errorCode' in result) {, but on this endpoint it isn't enough since we have this limitation on TS. Then I don't have a good type inference on this case, needing to write a more complex code.

Examples

type TExample = (
  {
    [key in string]: {
      foo: number
    }
  }
  | {
    errorCode: string
  }
)

const func: () => TExample = () => ({ blah: { foo: 1 } })

const value = func()

if ('errorCode' in value && typeof value.errorCode === 'string') {
  value // should be { errorCode: string }
}

if ('errorCode' in value && typeof value.errorCode === 'object') {
  value // should be { [key in string]: { foo: number } }
}

if ('errorCode' in value && typeof value.errorCode === 'number') {
  value // should be never
}

Beginne mit den verlinkten TypeScript Playground-Beispielen und vergleiche die aktuellen Narrowing-Ergebnisse mit den gewünschten Ergebnissen für string-, object- und unmögliche typeof-Prüfungen. Erledigt ist die Aufgabe, wenn der Wert der discriminated union auf das passende Mitglied oder im unmöglichen Fall auf never eingeengt wird, einschließlich der Beispiele mit dynamischem Schlüssel und expliziter Eigenschaft.