Allow Jest to preprocess non-root node_modules

Created by: modernserf

Update: @modernserf is working on a PR to this issue. Please don't submit other PRs.

Currently, create-react-app allows you to structure your app like this:

node_modules/
└─ react/
src/
├─ __tests__/
├─ index.js
└─ node_modules/
   └─ @myapp/
      ├─ foo/
      └─ bar/

So you can import 3rd party modules with import "react" or local modules with import "@myapp/foo". Webpack will preprocess the files in src/node_modules, es6 features (including imports) are converted with babel. I imagine this behavior is intentional, because typical babel-loader configurations ignore all node_modules directories.

However, although Webpack will preprocess these files through Babel, Jest will not; if your tests imports one of these local modules, Jest will choke on the un-transpiled import statements.

I think all one needs to do to enable this is add preprocessorIgnorePatterns: ["<rootDir>/node_modules"] to the Jest config -- this will continue to not preprocess regular node_modules, but will preprocess internal node_modules, so it should have no impact on existing apps.

Update: @modernserf is working on a PR to this issue. Please don't submit other PRs.