Will/does Webpack support "module" field?

[TehShrike]

To allow importing of ES6 modules with an ES5 module in the package.json's "main" field.

As proposed by rollup.

[IngwiePhoenix]

I +1 this.

Proposed:

• jsnext:main should be within the default list ( https://github.com/webpack/webpack/blob/master/lib/WebpackOptionsDefaulter.js#L83-L88 ) in WebPack2, since it understands ES6 OOTB.

However... Should jsnext:main be treatened in a way that it can't be ignored? Just a thought.

Kind regards,
Ingwie.

[SimenB]

#1612 (comment)

[taion]

Coming from ReactTraining/react-router#3333, I'd really like if there were a way to specify the location of the ES module build for webpack users.

I'm a little bit frightened that people are importing from the subdirectory containing the ES modules directly.

[rosskevin]

+1 to be a default as @IngwiePhoenix says.

Is there a current way to plugin a resolver to webpack 2 to prefer jsnext:main and otherwise fallback to main? I can currently alter resolve.mainFields (until it is in the default list) but don't we still have to either include that dynamically as part of the src loading?

[TheLarkInn]

@sokra I could take a stab at this if this is as easy as it appears to be.

[taion]

Why do you need a plugin for this? Just set e.g.

mainFields: ['jsnext:main', 'browser', 'main']

in your webpack config.

[rosskevin]

@taion - The problem with this approach is transitive dependencies (or so it seemed when I tried it). I don't think it is as simple as it looks to simply configure or plug-in, it needs to be considered in the primary resolution so that we can take advantage of tree-shaking etc.

[taion]

It works just fine transitively.

[rosskevin]

That is my error in that case; if you have figured it out I'm sure all the participants here would like to see an example of this running.

[taion]

In webpack.config.js, just do e.g.

resolve: {
  mainFields: ['jsnext:main', 'browser', 'main'],
},

That's all you need.

[rosskevin]

hmm, that didn't work for me when I tried it and I had multiple transitive dependency failures. Several projects including react-router provide these sources yet my imports failed. I was sure to allow sources to be pulled (not excluded) from node_modules.

[TheLarkInn]

I'd assume this would be a desired default with fallback for main. No?

[adamdicarlo]

Besides adding 'jsnext:main' to mainFields, don't you need to somehow teach Babel "you actually need to process some stuff in my node_modules folder"?

[umidbekk]

@adamdicarlo

That works for me

{
  test: /\.js$/,
  loader: 'babel',
  exclude: /node_modules\/(?!(([^\/]+?\/){1,2}(src|es6)))/,
}

Tested it here

[adamdicarlo]

that matches /node_modules/react-router/node_modules/foo/es6/index.js. but anyway, I don't think checking for a folder named src or es6 is a long term/general solution.

Webpack supports callbacks for exclude/include; in the past, I've set up logic to do this:

1. figure out which package the file is a part of (by recursively scanning folder & parent folders for a package.json), and
2. inspect that package.json to figure out whether its source needs transpilation

but if Webpack will support jsnext:main out of the box, it seems pretty silly to manually need to set up this kind of mechanism for every project...

[taion]

Babel doesn't need to process anything in node_modules. By convention, an ES module build is the same as the CJS build, with the only exception being the module system. The rest of the ESNext features should still be transpiled.

[taion]

Take a look at e.g. how Redux and React Router do this.

[adamdicarlo]

Hmm, right. That's maybe good enough in general. There are downsides, though: your host project's babel plugins do not get applied to the (e.g.) React components you npm installed. So you can't, for instance, use babel-plugin-rewire to mock a package in node_modules. You can't apply babel-plugin-react-transform to those components either.

And when npm linking packages and working on them in the context of a host app—which is already a pain, of course—you'd have to have the linked package's build watcher running, too.

[adamdicarlo]

Another thought: What about the (not too distant?) future where we want to ship ES2015 code to browsers? Surely packages will mostly be written in ES2016 (or ES2017) then — so they'd need to provide ES5 and ES2015 (and eventually ES2016) builds. ¯_(ツ)_/¯ (WebAssembly support can't come fast enough IMO...)

[taion]

wasm isn't going to fix anything because if for some bizarre reason you decide to compile JS to wasm, you're effectively opting out of the bulk of things that the JIT can do.

Yes, it's true that the "projects ship transpiled code" has some set of disadvantages – but that's orthogonal to the question of the choice of module system used for distribution.

[umidbekk]

@adamdicarlo

I don't think checking for a folder named src or es6 is a long term/general solution.

Webpack 2 still in beta, so it's just temporary workaround

@taion

Babel doesn't need to process anything in node_modules. By convention, an ES module build is the same as the CJS build, with the only exception being the module system. The rest of the ESNext features should still be transpired.

Yep, but modules like redux-form or moment use /src dir in jsnext:main

[scopsy]

@hccampos Hey Hugo,
I'm currently running in an issue where a library I'm using is dependent on a package that uses "module" field in its package.json file.
When compiling ng build --aot --prod I'm getting an error:

Unexpected token: name (CalendarVietnamese) [0.a821737870a81bd01043.chunk.js:8779,6]

When I removed the "module" from that package compilation works fine.

Any suggestions?

[hccampos]

@scopsy the only solutions I know of at the moment are:

1. Eject the webpack config and fix it manually.
2. Fork the library, the update package.json and use the fork in your project.

We are using option 2 and just maintaining forks of the original repos for libraries that don't work with the Angular CLI

[scopsy]

@hccampos When ejecting the webpack config what needs to be changed there?

[gaearon]

Since we are using the Angular CLI which has no transpiling other than Typescript, any modules which specify a module field will be imported as es2015 modules. And a lot of times things will only really break when you run it in an older browser that doesn't support class, something which you normally don't do everyday when developing.

I think we might be conflating two different things here. The module entry point should point to code with ES6 modules but not necessarily other ES6 features. People who want their libraries to be in ESM format can/should still precompile other ES6 features so that older browsers don't break.

[donaldpipowitch]

People who want their libraries to be in ESM format can/should still precompile other ES6 features so that older browsers don't break.

I asked for this as a feature request to support this as a native webpack feature a while ago, if someone is interested in this topic: #2933.

[hccampos]

@gaearon absolutely! That is what MobX does, for example. However, more and more libraries seem to be popping up which don't transpile the rest to es5, breaking builds which run without babel/bubble. The worst part is that you only notice the breakage when testing on older browsers, which usually happens on a CI environment or manually after deploy.