Summary:
https://github.com/facebook/react-native/issues/27264 changed stylesheet validation to avoid enumerating properties on the prototype of a style. It introduces a secondary behavior change, where null/undefined styles used to be tolerated but now lead to an exception. This is because `for in undefined` will noop where `for of Object.keys(undefined)` will throw.
This scenario of undefined/null styles seems to actually show up in practice and was previously well tolerated. E.g. `Button.js` has code that looks like this:
```jsx
const styles = StyleSheet.create({
button: Platform.select({
ios: {},
android: {
elevation: 4,
// Material design blue from https://material.google.com/style/color.html#color-color-palette
backgroundColor: '#2196F3',
borderRadius: 2,
},
}),
```
For non ios/Android platforms, that creates a style object which looks like:
```js
{
button: undefined,
...
}
```
This previously meant that the component would be unstyled if created, but now means out-of-tree platforms throw if the builtin Button component is required.
This change restores the previous `for in` loop but adds a `hasOwnProperty` check to avoid properties on prototypes.
## Changelog
[General] [Fixed] - Restore Previous Behavior for StyleSheet Validation of Null/Undefined Styles
Pull Request resolved: https://github.com/facebook/react-native/pull/29171
Test Plan: Validated that importing Buttons will no longer cause an exception, and that invalid properties are still caught.
Reviewed By: JoshuaGross
Differential Revision: D22118379
Pulled By: TheSavior
fbshipit-source-id: 650c64b934ccd12a3dc1b75e95debc359925ad73
e75557b48f