Add option to represent non-required properties as required fields in TS with | undefined

[Gelio]

Description

Proposal

I suggest adding an option that would determine whether non-required object properties get turned into:

  someProperty?: string;

or

  someProperty: string | undefined;

The latter seems safer to work with as a producer of objects that have to match the generated TypeScript types. When adding a new non-required property to an object, I want TypeScript to point out to me all the places in which I should consider adding/skipping that property.

Right now after adding a new non-required property, TypeScript will happily accept existing code, since those existing objects are still valid, because the property is optional. If the property was declared as required, but undefinable (| undefined), I would have to manually go through all the places where that property should appear and either set it to a valid value, or set it to someProperty: undefined (if it should be omitted).

I understand this is a breaking change, which is why I suggest making it a CLI option, rather than changing the default behavior.

For reference, ts-proto has a useOptionals option that decides whether optional properties are generated using ? or | undefined. They recommend | undefined as the default behavior (the behavior I want also from openapi-typescript).

Checklist

• I’m willing to open a PR for this (see CONTRIBUTING.md)