React: Improve error when you've forgotten to extend React.Component

[jamiebuilds]

When you forget to extend React.Component you get the following error:

TypeError: Cannot call a class as a function
    at _classCallCheck (null.js:7:99)
    at Hello (null.js:11:5)
    ...react internals...

[[Example](http://react.jsbin.com/?html,js,console)]

It would be nice if in dev you could do a check for Component.prototype.render and if it exists, provide a better warning along the lines of:

Warning: Using a class with a render method as a function, did you forget to extend React.Component?

I believe it would happen in ReactCompositeComponent.js.

[jamiebuilds]

Note that the error is much worse when you're compiling classes without _classCallCheck because this is just undefined

[aweary]

Thanks for the feature request @thejameskyle! I agree that a better error here would provide a better developer experience. It also looks like Fiber will get caught in an infinite loop if you try to render a class that doesn't extend React.Component, so it might even be necessary.

Here's an example demonstrating that. Be warned, it will crash or freeze your browser/tab.

Click at your own risk

cc @gaearon @acdlite ^

ReactCompositeComponent might be the right place for this, but we'd need to do it outside the DEV block since it should throw an error and we want to maintain the same invariants for DEV and PROD.

[acdlite]

The infinite loop is a known issue with error handling.

[acdlite]

Will fix before release

[aweary]

Thanks, @acdlite! Do you know if the fix will address @thejameskyle's request? Or would a better error for non-extending classes be orthogonal to your error handing fix?

[acdlite]

Yeah, fixing the infinite loop bug would not address the lack of a helpful error message. Good candidate for an external PR. Should also account for React.PureComponent.

[sw-yx]

gonna give it a shot... first contribution

[sw-yx]

i am a bit confused. ReactCompositeComponent no longer seems to be there and only exists in tests. Was it renamed somehow?

[gaearon]

That logic is now in ReactFiberClassComponent. Old engine is gone.

[sw-yx]

Ok I think I have a good handle on this. Upon deeper inspection the error happens earlier in the lifecycle than in ReactFiberClassComponent so I am placing the invariant higher up than originally suggested.

(The error actually gets thrown in mountIndeterminateComponent and so doesn't even reach mountClassInstance from ReactFiberClassComponent several lines lower)

As for what to check, i'm going with simply !(workInProgress.type.prototype && workInProgress.type.prototype.render) and that seems to work (i.e. not throw) for both pure functional components as well as extended react components while throwing for plain JS classes which I think is exactly what we want.

[sw-yx]

for anyone else watching we made this into a warning instead as requested by Dan. also added a test

[gaearon]

Fixed in https://github.com/facebook/react/pull/11168.