Allow Jest to preprocess non-root node_modules

[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.

[gaearon]

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.

This is not intentional, no. We include instead of excluding so that npm link still works. I didn’t however realize somebody would create another node_modules inside src.

Can you please explain your reasoning for doing it like this? Are you trying to avoid relative paths by doing so?

[modernserf]

Avoiding relative paths is the immediate goal; the long-term goal is to allow apps to be structured more like monorepos.

[gaearon]

This is interesting. I’m not sure this is the approach we would take but we need to provide support for something like this.

[gaearon]

This will be a part of #741.

[cecilemuller]

It still would be so useful if Jest had the option to not ignore non-root node_modules instead of having to use a workaround (given projects that use local modules aren't likely to change structure just to use Jest).

[gaearon]

Oh, to clarify, I’m happy to review a PR that adds preprocessorIgnorePatterns: ["<rootDir>/node_modules"] to our Jest configuration.

[cecilemuller]

The trouble is, testPathIgnorePatterns seems to add to the defaults list instead of replacing the value?

For example, even with the following config, tests insides /src/node_modules aren't called:

"jest": {
    "testPathDirs": ["<rootDir>/src"],
    "testPathIgnorePatterns": ["<rootDir>/node_modules"]
}

But that's an issue with Jest itself, not CRA.

[gaearon]

Please file it in Jest repo, and we'll discuss it there.

[gaearon]

I've changed my mind. Let's just make src/node_modules work well and document it. People can always use symlinks to make it prettier.