TypeError when using React: Cannot read property 'firstChild' of undefined

Question

Sometimes, when using React libraries, such as react-router, I get this error:

Uncaught TypeError: Cannot read property \'firstChild\' of undefined

Answer 1

This error is commonly caused by two versions of React loaded alongside.

For example, if you npm install a package that requires a different React version and puts it into dependencies instead of peerDependencies, it might install a separate React into node_modules//node_modules/react.

Two different Reacts won't play nicely together (at least yet).

To fix it, just delete node_modules//node_modules/react.
If you see a library putting React in dependencies instead of peerDependencies, file an issue.