"module not found : Error: Cannot resolve module 'react/lib/ReactMount' "

None of the above solutions worked for me. Spent the rest of the day in a rabbit hole of github issues/comments, weighing the pros/cons/feasibility of various hacky workarounds.

The quickest, simplest, "I just want to work on the original problem I intended to work on today" fix that worked for me comes from: https://github.com/gaearon/react-hot-loader/issues/417#issuecomment-261548082

In your webpack config, add the following alias to the resolve section:

resolve: {
  alias: { 'react/lib/ReactMount': 'react-dom/lib/ReactMount' }
}

This is not a stable long term fix, this is a development only fix so you can continue developing without being forced to deal with upgrade issues out of the blue.

I'm still not 100% sure why I'm seeing this problem in my one app and not another, both were generated from fountain.js react-redux generator and have identical package.json.

This issue is related to the react-hot-loader package. You are using an old version that relies on the ReactMount.js file being present in the node_modules/react/lib folder.

There is no easy one way fix for that but you have a few options which are:

1. Try to follow the instructions here: https://github.com/gaearon/react-hot-loader/blob/v3.0.0-beta.6/docs/README.md#usage-with-external-react (but I have been unlucky so far)

2. Remove the hot reloader for react (in your webpack.config remove the 'react-hot' loader)

3. Update the react-hot-loader package to version 3 (here is how to do so: https://github.com/gaearon/redux-devtools/commit/64f58b7010a1b2a71ad16716eb37ac1031f93915). But note that this package has been in alpha for a while now...

4. Rollback your react version to one that includes the ReactMount.js in the lib folder (15.0.1 used to have this file not sure when it stopped).

Update: React Hot Loader 3 is now in beta with a more comprehensive upgrade guide: https://github.com/gaearon/react-hot-loader/tree/v3.0.0-beta.7/docs#migration-to-30

You are using an outdated react-hot-loader package that uses the internal react api throught react\lib\ReactMount. Now react doesn't allow this hence the problem.

Try updating it to the latest version:

$ npm install --save-dev react-hot-loader@latest

Thank you for all your answers. I have solved my problems.

" This issue is related to the react-hot-loader package. You are using an old version that relies on the ReactMount.js file being present in the node_modules/react/lib folder." said by cheesemacfly.

So here is the solution for me: 1) updating the react-hot-loader to the latest version

npm install --save-dev react-hot-loader@latest

but here is another problem linked with react-hot-loader

2) so I removed the react-hot-loader from 'cfg/dev.js' change the code

loader: 'react-hot!babel-loader'

into

loader: 'babel-loader'