Parcel2 ignores .babelrc files in monorepos.

[astegmaier]

🐛 bug report

If you have a yarn-workspaces-based monorepo with custom .babelrc configuration, parcel ignores them.

This is related to #3917 - I'm filing a separate issue because there are other ways to resolve the problem reported there (for example, by adding @babel/plugin-proposal-optional-chaining to the default babel config - which IMO would be a good idea regardless of how this issue is treated).

🎛 Configuration (.babelrc, package.json, cli command)

I have a project configured like this:

├── package.json // {"workspaces": ["app1"]}
├── app1
│   ├── .babelrc
                  /* {
                      "plugins": ["@babel/plugin-proposal-class-properties"],
                       "presets": [[ "@babel/preset-typescript", { "isTsx": true }]]
                  } */
│   ├── index.ts // uses class properties
│   └── package.json
└── yarn.lock

🤔 Expected Behavior

The .babelrc file in the app1 project should be respected, and parcel build should succeed.

😯 Current Behavior

parcel build fails with this error:

🚨 Build failed.
@parcel/transformer-babel: /Users/Andrew/Projects/parcel2-monorepo-babel-bug/app1/src/index.ts: Missing class properties transform.
  1 | class MyClassWithProperties {
> 2 |   private message = "Hello from a class!";
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  3 |   sayHello() {
  4 |     console.log(this.message);
  5 |   }
SyntaxError: /Users/Andrew/Projects/parcel2-monorepo-babel-bug/app1/src/index.ts: Missing class properties transform.
  1 | class MyClassWithProperties {
> 2 |   private message = "Hello from a class!";
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  3 |   sayHello() {
  4 |     console.log(this.message);
  5 |   }
    at File.buildCodeFrameError (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/core/lib/transformation/file/file.js:267:12)
    at NodePath.buildCodeFrameError (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/traverse/lib/path/index.js:144:21)
    at pushBody (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/plugin-transform-classes/lib/transformClass.js:156:20)
    at buildBody (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/plugin-transform-classes/lib/transformClass.js:130:5)
    at classTransformer (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/plugin-transform-classes/lib/transformClass.js:527:5)
    at transformClass (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/plugin-transform-classes/lib/transformClass.js:563:10)
    at PluginPass.ClassExpression (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/plugin-transform-classes/lib/index.js:63:54)
    at newFn (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/traverse/lib/visitors.js:179:21)
    at NodePath._call (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/traverse/lib/path/context.js:55:20)
    at NodePath.call (/Users/Andrew/Projects/parcel2-monorepo-babel-bug/node_modules/@babel/traverse/lib/path/context.js:42:17)

Interestingly, I get the same error, even when I move the .babelrc file to the root of the monorepo.

💁 Possible Solution

As @mischnic pointed out here, these lines return a partialConfig without a `.babelrc

parcel/packages/transformers/babel/src/config.js

Lines 36 to 40 in 92fd22f

	let partialConfig = babelCore.loadPartialConfig({
	filename: config.searchPath,
	cwd: path.dirname(config.searchPath),
	root: options.projectRoot,
	});

A possible solution would be to leverage the babelrcRoots option to pass in the package directory (in addition to the monorepo root that is already specified in the root parameter)

🔦 Context

I'm trying to set up a monorepo where the packages are built with parcel2, and I need to customize the babel configuration for some of the packages individually.

💻 Code Sample

You can reproduce the issue in master branch of this repository.

There is a working-example branch that shows the same app (with the same .babelrc) building correctly in the non-monorepo case.

🌍 Your Environment

Software	Version(s)
Parcel	2.0.0-nightly.99
Node	10.16.3
Yarn	1.21.1
Operating System	Mac OSX 10.15.3