Bug: Nothing returned from renderer when returning undefined

[raRaRa]

React version: 17.0.0

Steps To Reproduce

1. return undefined from a React component

Link to code example: https://codesandbox.io/s/blazing-rain-jtdzy?file=/src/App.js

The current behavior

Nothing was returned from render. This usually means a return statement is missing. Or, to render nothing, return null.

The expected behavior

Nothing should be rendered like in earlier React versions.

[raRaRa]

The simple fix is of course to return null instead of return undefined, but I never saw this breaking change documented for React 17.0.0. Apologies if this is by design.

[kachkaev]

This is intentional. See https://github.com/facebook/react/issues/19548, which was closed by https://github.com/facebook/react/pull/19550

Here is a blog post section on this behaviour: https://reactjs.org/blog/2020/08/10/react-v17-rc.html#consistent-errors-for-returning-undefined

Your codesandbox example would produce a warning in React 16 too.

[raRaRa]

@kachkaev Thanks. I've never noticed this warning during the development of my app in React 16, very odd. I'll close this issue since this is by design.

Thanks again.