Parcel 2: Using the same package in different packages result in multiple instance of said package

[Banou26]

🐛 bug report

Using the same package(e.g React) in multiple packages from a monorepo that are being aliased result in using different instances of React, even though they have the same versions

I also tried specifying them as peerDependency but same result

🌍 Your Environment

Software	Version(s)
Parcel	2.0.0-alpha.3.2

[mischnic]

When do you observe "result in using different instances of React", (in) which monorepo package are you building?

[Banou26]

https://github.com/Banou26/parcel-bug-3971

e.g, if you just have React as a peerDependency in the Foo package which is imported from Main, it'll throw

ℹ️ @parcel/package-manager: yarn: Saved lockfile.
ℹ️ @parcel/package-manager: yarn: Saved 26 new dependencies.
ℹ️ @parcel/package-manager: yarn: Direct dependencies
ℹ️ @parcel/package-manager: yarn: All dependencies
🚨 Error: Cannot find module 'react' from '/home/banou/dev/parcel-alias-dependency/packages/foo'
Error: Cannot find module 'react' from '/home/banou/dev/parcel-alias-dependency/packages/foo'
    at ResolverRunner.resolve (/home/banou/dev/parcel-alias-dependency/packages/main/node_modules/@parcel/core/lib/ResolverRunner.js:124:15)
    at async DepPathRequestRunner.runRequest (/home/banou/dev/parcel-alias-dependency/packages/main/node_modules/@parcel/core/lib/RequestTracker.js:356:43)
    at async Promise.all (index 2)
    at async AssetGraphBuilder.build (/home/banou/dev/parcel-alias-dependency/packages/main/node_modules/@parcel/core/lib/AssetGraphBuilder.js:175:7)
    at async Parcel.build (/home/banou/dev/parcel-alias-dependency/packages/main/node_modules/@parcel/core/lib/Parcel.js:370:11)
    at async Parcel.startNextBuild (/home/banou/dev/parcel-alias-dependency/packages/main/node_modules/@parcel/core/lib/Parcel.js:275:21)
    at async PromiseQueue._runFn (/home/banou/dev/parcel-alias-dependency/packages/main/node_modules/@parcel/utils/lib/PromiseQueue.js:98:7)
    at async PromiseQueue._next (/home/banou/dev/parcel-alias-dependency/packages/main/node_modules/@parcel/utils/lib/PromiseQueue.js:85:5)

And if you have React as dependency in both, the result in window is false for

foo.js

import React from 'react'

export default React

main.js

import React from 'react'
import FooReact from 'foo'

console.log(FooReact === React)

Which makes the use of features like React Hooks impossible across multiple packages since they need the same instance of React to work

[Banou26]

Also if I try to ls their version, everything seems fine from npm regarding my packages

[banou@banou-pc main]$ npm ls react
@parcel-alias-dependency/main@0.0.1 /home/banou/dev/parcel-alias-dependency/packages/main
├─┬ @parcel-alias-dependency/foo@0.0.1 -> /home/banou/dev/parcel-alias-dependency/packages/foo
│ └── react@16.12.0 
├─┬ parcel@2.0.0-alpha.3.2
│ ├─┬ @parcel/config-default@2.0.0-alpha.3.1
│ │ └─┬ @parcel/reporter-cli@2.0.0-alpha.3.1
│ │   └── react@16.12.0  deduped
│ └── react@16.12.0  deduped
└── react@16.12.0

[mischnic]

(peerDependencies aren't handled properly yet, though I'm not completely sure if that's the problem here: #3753)

[Banou26]

BTW, I get the same result from lerna's bootstrapping replacing the aliases

[Banou26]

Oh, wow, if I install parcel at the root of the monorepo and build the package via parcel ./packages/main/index.html, the react instance become shared
Banou26/parcel-issue-3971@ecec1fc

Is that the intended behavior ?
Doesn't seem right to me

Edit: Actually, after some more advanced tests, I get mixed results from building at the root, it's so weird, gonna post more soon

[Banou26]

https://github.com/Banou26/parcel-bug-3971

If we yarn install at the root and build from the root, the instance of React is shared between the packages.

If we yarn install at the root and either of the sub packages and build from the root, the instance isn't shared anymore.

If we build from a sub package, the instance isn't shared either.

Thing is, you have to yarn install in the sub packages, else parcel throws an error saying that it doesn't find the package.

This error weirdly doesn't happen for React, maybe because react is implicitly imported from parcel?

But yeah, this makes monorepos kind of unusable when working with a framework like react.

[Banou26]

I've been trying hard to make this work.

I couldn't find a way to make parcel work with this so I tried with webpack, to my big surprise, webpack had the same exact problem.

Luckily I found a nice little thread that had the same problem using webpack, the solution was this:
facebook/create-react-app#6027 (comment)

webpack.config.js

{
  resolve: {
    modules: [path.resolve('node_modules'), 'node_modules']
  }
}

Apparently there's some big problems related to this: facebook/create-react-app#6207

Any idea how i could make this work with parcel ?

[mischnic]

Thing is, you have to yarn install in the sub packages, else parcel throws an error saying that it doesn't find the package.

You should be using yarn's workspace feature: https://yarnpkg.com/lang/en/docs/workspaces/

If you used that and #3753 was merged, I think it should work by specifying react as a peer dependency in foo.

A workaround: react is bundled only once if you specify don't specify it as a devDependency but only as a peerDependency in foo and ignore the yarn warning.