Lots of Flow errors in React Native project - react-native

Question

I created a React Native project from Expo. Then I wanted to add Flow to it. I noticed in my node_modules/react-native folder there was a .flowconfig so I copied that to the root of my project. After running flow I got some warnings from files in node_modules/exponent so I added an ignore for that whole folder. Afterwards, I still get many errors when running flow. Here are a few:
node_modules/react-native/Libraries/Animated/src/AnimatedImplementation.js:227
227: /* $FlowFixMe */
^^^^^^^^^^^^^^^^ Error suppressing comment. Unused suppression
node_modules/react-native/Libraries/Animated/src/AnimatedImplementation.js:1120
1120: if (__DEV__) {
^^^^^^^ identifier `__DEV__`. Could not resolve name
node_modules/react-native/Libraries/Animated/src/AnimatedImplementation.js:2162
2162: if (__DEV__) {
^^^^^^^ identifier `__DEV__`. Could not resolve name
Expo SDK version: 14.0.0
Flow version: 0.37.0
React Native version: 0.41.2

Answer 1

You can suppress errors in .flowconfig file, in [options] section as follows:
[options]
suppress_type=$FlowFixMe
suppress_comment=\\(.\\|\n\\)*\\$FlowFixMe\\($\\|[^(]\\|(\\(>=0\\.\\(3[0-8]\\|[1-2][0-9]\\|[0-9]\\).[0-9]\\)? *\\(site=[a-z,_]*react_native[a-z,_]*\\)?)\\)
Then, in [libs] section, you should point so called library definition file, like:
[libs]
./libdefs.js
And in libdefs.js just declare:
declare var __DEV__:string;
These changes should resolve your errors. See:
https://flow.org/en/docs/config/options/#toc-suppress-comment-regex and
https://flow.org/en/docs/libdefs/creation/ for details.

Answer 2

Looks like it might have been an oversight on the flow types for that release version.
All of those errors look fairly harmless though so you could just ignore it. If you absolutely cannot ignore it, update to a later version of React Native that's locked to a different version of Flow? I've had good results personally (no errors) with React Native # 0.42 and Flow # 0.38.

Answer 3

Found this issue which recommended updating the flowconfig to use the latest version from create-react-native-app. Seems to have worked for me.