optional property vs. nullability?

[mdecurtins]

I've been working with GraphQL for a few years and we've recently started integrating graphql-code-generator into our React projects at my suggestion. I'm digging a little bit more into how types are generated (also using the new typed-document-node for my Apollo hooks), and I'm a little bit confused about how codegen treats optionality vs nullability.

Forgive me if this is a silly question; I've done a bit of googling but haven't yet found an explanation.

In GraphQL, we might have a type definition like this:

type Foo {
  id: ID!
  name: String!
  bar: Bar
}

Which codegen would turn into:

export type Foo = {
  __typename?: 'Foo';
  id: Scalars['ID'];
  name: Scalars['String'];
  bar?: Maybe<Bar>;
};

Here's my question: why is it bar? and not bar? In other words, why does the generated type think that bar is an optional property on Foo, when the GraphQL schema merely declared that bar might be null?

Aren't these 2 different things? In Typescript, prop? indicates that prop might not exist on an object that is assignable to the type, but the GraphQL SDL by definition is saying, "I guarantee these fields will exist." After all, if a resolver doesn't return the fields specified by the type, GraphQL should error, because the resolver didn't fulfill the contract promised by the schema.

Can anyone explain why codegen does this? It looks like there's an avoidOptionals config for codegen (which I will definitely use), but I'm curious why the default is false when it results in types that seemingly confuse the optionality of a field with the potential for that field to be null.

Thanks!