Flow: Given "type T = ?{…}" how can I get "type U = {…}", i.e., "type U = T \setminus null \setminus undefined"?

[omerzach]

Relay generates nice Flow types for me like

type T = {| user: ?{ firstName: string } |};

.

I'd like to be able to say a component accepts { firstName: string } without hard-coding the firstName: string in. I can use type U = $PropertyType<T, "user"> to dynamically get type U = ?{ firstName: string }, but would like another operator to let me require that the object is not null or undefined? Is there anything like this? It looks like $Diff<A, B> almost does this, but only works when A and B are both Objects.

[samwgoldman]

$NonMaybeType<$PropertyType<T,"user">> should give you what you want.

[omerzach]

Perfect!

[madbence]

i used this before i've heard about $NonMaybeType :smiley_cat: :

type _Definitely<T, U: ?T> = T;
type Definitely<T> = _Definitely<*, T>;

type T = {| user: ?{ firstName: string } |};
type U = Definitely<$PropertyType<T, 'user'>>;

({ firstName: 'foo' }: U);

[omerzach]

Should I add that to https://flow.org/en/docs/types/utilities/ or is it documented somewhere else?