Why is __typename possibly undefined?

[tippfelher]

Suppose I have a Query function that either returns TypeA or TypeB. I.e. something like this

type TypeA {
  something: String!
}

type TypeB {
  samethong: Number!
}

union TypeAorTypeB = TypeA | TypeB

type Query {
  getSomething: TypeAorTypeB!
}

On the client side I check whether the response was TypeA or TypeB like this:

if (data.__typename === 'TypeA') {
  // do something
}
else if (data.__typename === 'TypeB') {
  // samethong
}

The problem I have here is, that codegen generates all my types with an optional __typename, i.e. in the code above I need to introduce a check in the form of if (data.__typename === undefined) to cover all cases and I don't even know where the optionality ? comes from and why is it even neccessary?

Why exactly does codegen generate

export type TypeA= {
  __typename?: 'TypeA';
  something: Scalars['String'];
};

and not

export type TypeA = {
  __typename: 'TypeA';
  something: Scalars['String'];
};

?

Thank you in advance!

Edit 1: Not sure if it's important, but my resolvers always return in the form of

return ({
  __typename: 'TypeA';
  something: "somethingy";
})

so clearly __typename is never undefined.

Edit 2: Tried this option but it has no effect.

[tippfelher]

God damn I feel so f dumb. Added nonOptionalTypename: true, to the resolvers config.. (wrong place) Now it's working.

(Also found the relevant issues (link, link) covering my issue.)