fs: ensure return value type of options.filter in cpSync consistent with doc

[kylo5aby]

Ensure return type of options.filter in cpsync matches doc.
according to the documentation, the return value of options.filter has boolean type.

[lpinca]

Can you add a test?

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/58349/

[kylo5aby]

Can you add a test?

Hi, I have added related test

[targos]

I have the feeling this was done on purpose, to support truthy values, and there's a Promise check to prevent mistakes.

[targos]

#39372 (review)

[kylo5aby]

Thanks for your feedback. and I have a question, for

opt.filter = (value) => {
 //
}

its return type contradicts the documentation, which states Returns: <boolean>, but it wont throw an error. Should here use a more precise description, such as the type that is coercible to boolean type?

[targos]

/cc @nodejs/fs @bcoe @aduh95

[aduh95]

I'm fine with either (changing the code or the docs); if we change the code, let's land this as semver-major.

[jasnell]

I think I might prefer changing the docs to changing the code here.