fix(jsx): allow null, undefined, and boolean to be returned from function component

[usualoma]

fixes #3235
The deno converts an empty Fragment to null when the value of the "jsx" entry is "precompile", but "hono/jsx" was not able to handle the function component returning null, which caused the problem #3235.

What should be the returned value of an FC type?

In the current React type definition, FunctionComponent returns the ReactNode, and since ReactNode also includes boolean and undefined, it would be preferable if Hono's FC also included these (boolean and undefined).

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react/index.d.ts#L1114-L1122
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react/index.d.ts#L478-L489

However, the changes to add undefined and boolean have a considerable impact on existing code, so I have chosen not to do so in this pull request. I would like to discuss this at another time.

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code

[usualoma]

@yusukebe Would you please review?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.21%. Comparing base (e123546) to head (1a6d822).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3241   +/-   ##
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  Coverage   96.21%   96.21%           
=======================================
  Files         151      151           
  Lines       15102    15105    +3     
  Branches     2660     2664    +4     
=======================================
+ Hits        14530    14533    +3     
  Misses        572      572           

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[yusukebe]

@usualoma

Okay! I'll do it later.

[yusukebe]

Hi @usualoma

Looks good. I want to know:

In the current React type definition, FunctionComponent returns the ReactNode, and since ReactNode also includes boolean and undefined, it would be preferable if Hono's FC also included these (boolean and undefined).

Does this issue already exist before this PR (Or #3235)? As you said, I also think we should not include the change in this PR.

[usualoma]

@yusukebe

Thanks for the review!

The FC return type divergence was noticed during this work. I don't think there are any known issues.

I think it would be better to include undefined and boolean, but I think components that return undefined or boolean are very rare. Changing it would likely affect the result of <Component/>.toString(), so I don't think it should be changed unless there is some special motivation.

[yusukebe]

@usualoma

Thank you for your reply.

I think it would be better to include undefined and boolean, but I think components that return undefined or boolean are very rare. Changing it would likely affect the result of <Component/>.toString(), so I don't think it should be changed unless there is some special motivation.

I agree 👍

Now, let's merge this one!

[yusukebe]

LGTM!