make types nullable

[TimHaboldt]

Hey.
Was using your typescript compiler for a while and noticed there is no support for the nullable flag. I tried to implement a solution. I hope it satisfies you enough to make it into the main repository. Would be pleased to use this feature in future.
Ps. I implemented a solution that works for me. I cannot promise every possible nullable flag will work with those changed. The most important ones should be covered.
Supported are all basic types and unnamed enums. Named enums, objects and Interfaces shouldn`t be covered.

Regards

[bcherny]

Hey there, thanks for the contribution! I don't see nullable in the JSON-Schema spec -- mind linking to it in your summary?

[TimHaboldt]

Hey there, thanks for the contribution! I don't see nullable in the JSON-Schema spec -- mind linking to it in your summary?

I looked into it. It was a fault on my part.
It is a specification of the open API Schema (https://swagger.io/specification/) which we are currently using.
Therefore it opens the question if you would like to support the open API Schema too.
I would realize an open API flag you can set inside the options that would allow the open API Schema support.

[zmeyc]

@bcherny We also encountered this issue with nullable types. What do you think about supporting OpenAPI, are any changes needed for this PR or is it out of scope at all?

[bcherny]

Let's support this, but I'd suggest a slightly different implementation: instead of handling isNullable in the parser, generator, and IR, can you instead add a normalizer step that converts isNullable annotations to unions with null (ie. add it to the type array, and make type an array if it wasn't already)?

That way downstream code doesn't need to know anything about isNullable, and it's already handled by existing code that handles unions.

// normalizer.ts
rules.set('OpenAPI: Convert isNullable to union with null', schema => {
  // ...
}

This PR also needs a unit test for your new normalizer rule, as well as an e2e test.

[bcherny]

Thanks for the contribution! Let's take one more pass at this.

[zone117x]

Looks like nullable is also now supported in redoc@2.x. Would be great to have it supported here.
@bcherny I haven't looked into the code for this lib -- do you think implementing the changes you last suggested would be easy for a new contributor?

[TimHaboldt]

@zone117x I appreciate if someone else could take over. My workload does not allow me to work at anything else right now.

[zone117x]

I might be able to work on it if @bcherny is available for reviewing the PR and giving guidance where needed (for example how the tests should be structured, etc)

[bcherny]

Superseded by #411