[Bug]: `react/prop-types` not working when prefixing function with underscore `_`

[KubiGR]

Is there an existing issue for this?

• I have searched the existing issues and my issue is unique
• My issue appears in the command-line and not only in the text editor

Description Overview

When naming function components with an underscore _ prefix, the react/prop-types rule does not report anything.
Other prefixes, like $ don't seem to create this problem.

function _EventsList({ eventDate }) {
    return (
        <div>
            {eventDate.events.map((event) => (
                <Event key={event.id} eventId={event.id} />
            ))}
        </div>
    );
}

Expected Behavior

I would expect the rule to appear regardless of the function name.

eslint-plugin-react version

v7.31.10

eslint version

v7.32.0

node version

v10.15.3

[ljharb]

Are you sure components can start with an underscore? Is <_EventsList /> even valid?

[ljharb]

(To be clear, the function name absolutely matters; anything that can't be used in jsx isn't a component)

[KubiGR]

Components can start with an underscore. It's true that it goes against standards because components should be in PascalCase.

Let me explain my reasoning and see if it makes sense. I am using a state library (mobx) that needs to wrap with HOCs all your components if you want to use it. I wanted to use the same name, so I could export as the wrapped but I also wanted to use the react/prop-types rule so I needed to create a separated named function. It's true that a prefix in PascalCase could work but even mobx in their docs use this example.

const _MyComponent = props => <div>hi</div>
export const MyComponent = observer(_MyComponent)

I fully understand that the component detection makes sense only in PascalCase but why is $ a prefix that works and _ a one that does not?

[ljharb]

Components shouldn't be arrow functions anyways - do this:

function MyComponent(props) { return <div>hi</div>; }
const observed = observer(MyComponent);
export { observed as MyComponent }

That said, if <_MyComponent /> works in React, then it's a bug that we don't support it. Can you confirm?

[KubiGR]

I don't know if that's what you are asking me to show but check it out https://codesandbox.io/s/relaxed-taussig-fivkl2?file=/src/App.js.

Yeah, arrow functions should not be used. I have seen you mention this a lot and I agree fully.
I like the export renaming. I will try that for sure.
Do you have any idea regarding the support with $ prefix and not the _ one? Because this was the intriguing thing for me.

[ljharb]

Since your codesandbox clearly shows that it works, it's thus an oversight/bug in this plugin :-)

[ljharb]

aha, an explicit check was added in 6b42731/#3335 due to #3334.

I think instead of ignoring things that start with _, we need to look for the next letter and see if it's capitalized.

[zisiszikos]

Components shouldn't be arrow functions anyways - do this:

function MyComponent(props) { return <div>hi</div>; }
const observed = observer(MyComponent);
export { observed as MyComponent }

That said, if <_MyComponent /> works in React, then it's a bug that we don't support it. Can you confirm?

I don't think that generally this is a good idea, because what if I wanted to export my original component function too? For example for using it at a testing file. I will get an error that MyComponent has already been exported.

[ljharb]

Obviously you can’t export two things under the same name, so my suggestion allows you to trivially rename either or both.

That said, exporting things only for testing is a very bad practice - if it’s never used in production without the wrapper, it should never be tested without the wrapper.