I need to have an input variable be determined by a second boolean variable as directive to be used later on.

type TypeWithAge = {
  name: string,
  age: number
}

type TypeWithoutAge = MakeOptional<TypeWithAge, "age">;

type TypeWithAgeConditional<withName> = withName extends true ? TypeWithoutAge : TypeWithAge;

Now I need to have the function change the input param based on withName.

async function upsertFunction<withName extends boolean>(
   upsertItem: TypeWithAgeConditional<withName>, 
   includeName: withName
): boolean {
  
  if (includeName){
    upsertItem = { ...upsertItem, name: 'someValue' }
  }

  await writeItemToDb(upsertItem) // TS error since input of writeItemToDb expects TypeWithAge and cannot be changed in my case
  
  return true
}

The only workaround I have thus far is using

await writeItemToDb(upsertItem as TypeWithAge)

Logically it makes sense since the upsertItem will always correctly be of type TypeWithAge but using as xxx feels like a hardcoded pitfall waiting to happen.

How do I get upsertItem to also be of type TypeWithAgeConditional and get writeItemToDb to understand it will always be of correct type TypeWithAge?

There are other questions that use this to determine the output of a function but I have solved that in my code already and still can’t get this to work.