Resolving Create React App’s “Uncaught ReferenceError: process is not defined”

A recent change to Create React App, or more specifically react-scripts, has caused hot reloading to throw an error: Uncaught ReferenceError: process is not defined. This is due to its dependency, react-error-overlay, being referenced as ^6.0.9 while react-error-overlay’s latest patch version 6.0.10 is a breaking change. As a result, your installation likely uses 6.0.10 when it needs 6.0.9. To fix this, you unfortunately need to Frankenstein your yarn.lock file.

Find your react-error-overlay@^6.0.9 entry, which should have its resolution [incorrectly]set to 6.0.10:

"react-error-overlay@npm:^6.0.9":
  version: 6.0.10
  resolution: "react-error-overlay@npm:6.0.10"
  checksum: e7384f086a0162eecac8e081fe3c79b32f4ac8690c56bde35ab6b6380d10e6c8375bbb689a450902b6615261fcf6c95ea016fc0b200934667089ca83536bc4a7
  languageName: node
  linkType: hard

Replace this entry with the following, which resolves to 6.0.9:

"react-error-overlay@npm:^6.0.9":
  version: 6.0.9
  resolution: "react-error-overlay@npm:6.0.9"
  checksum: 695853bc885e798008a00c10d8d94e5ac91626e8130802fea37345f9c037f41b80104345db2ee87f225feb4a4ef71b0df572b17c378a6d397b6815f6d4a84293
  languageName: node
  linkType: hard

Once saved, run yarn install again to pull the correct version into your node_modules and/or .yarn/cache directory. Your React application should now hot reload correctly.

To track this error in an official capacity, you can view this issue on GitHub.