[eslint-plugin-react-hooks] only allow capitalized component names

[kassens]

Summary

A name like _useFoo was previously recognized as a component name ("starting with non-lowercase"). This would misleadingly identify this code as valid (see the first commit in this PR):

function Component(props) {
  if (cond) {
    _useHook();
  }
}
function _useHook() {
  useState(null);
}

The main change in this PR is that components now have to start with 0 or more underscores followed by a capital letter. _component, $Component, _useFoo are no longer valid component names.

This means the above code would now be invalid because of the useState call in the non-hook/non-component function _useHook.

How did you test this change?

Updated unit tests.

[kassens]

We could disallow this by only allowing the leading underscore for top level identifiers, but figured it might be nice to recognize Hook.__useSemiPrivateHook?

[gaearon]

Why not take a stronger stance and remove all support for the _ prefix?

[gaearon]

I think we should forbid the _ prefix completely instead. We never suggested the _Foo pattern in documentation, the only reason it was supported is because some www code used it, and we didn't want to touch that code. I suggest that we fix this forward on www and remove support for the pattern completely.

[kassens]

Sounds good to me!

[gaearon]

We need to remember not to do a release without tagging this as a major.

[acdlite]

We need to remember not to do a release without tagging this as a major.

You can update the version in ReactVersions:

react/ReactVersions.js

Line 28 in 8d1b057

'eslint-plugin-react-hooks': '4.7.0',

That will bump the version in the @next channel, too.

[sizebot]

Comparing: 36c908a...bd5763e

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	134.97 kB	134.97 kB	=	43.23 kB	43.23 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	141.64 kB	141.64 kB	=	45.20 kB	45.20 kB
facebook-www/ReactDOM-prod.classic.js	=	486.06 kB	486.06 kB	=	86.51 kB	86.51 kB
facebook-www/ReactDOM-prod.modern.js	=	471.34 kB	471.34 kB	=	84.29 kB	84.29 kB
facebook-www/ReactDOMForked-prod.classic.js	=	486.06 kB	486.06 kB	=	86.51 kB	86.51 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

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