Is the .mjs file extension resolved by metro bundler?

[mweststrate]

Hi all, quick question as library maintainer, is the .mjs file extension supported by metro? In order words, will a package json with a module field like this work like with webpack (or is module ignored completely, which would be fine as well):

"module": "lib/mobx.mjs"

[mweststrate]

Had some collegues testing it, using .mjs extension caused the bundler to fall back to the "main" field containing the UMD bundle or our test package.

[mweststrate]

Update: seems that module or jsnext:main are not picked up by the bundler in the first place. A react-native field is picked up (deducted from source code, I couldn't find docs on that)

[mweststrate]

What are future plans on this? I have to admit it is quite unobvious as library author how a library should be published to make it suitable for RN, without falling back to UMD / commonjs

[mweststrate]

Closing for now, until .mjs has become a bit more mainstream. Still curious on thoughts around this though

[rafeca]

Hey @mweststrate! Thanks for bringing this up.

As you mention, metro-bundler does not take into account the module field on package.json, so it should be safe to use this field in a library to expose .mjs files as long as the package still exposes a .js file in the main field.

For now I think it's better to wait until .mjs gets more traction (and Node ships a version with support for it) to implement proper support for modules in metro.

In the meantime, RN users can customize the sourceExts config param (the same way you did for the default value in #60) to make metro parse these files.

If there is a lot of demand of users wanting to use files with the mjs extension, we could merge your PR, but for the time being I'd prefer to wait until the dust around ES6 modules settles.

[cpojer]

Please send a pull request to the resolution code in "ResolutionRequest", with tests, and we are happy to merge and support this functionality :)

[mweststrate]

Tnx! Closed for now, after diving into webpack a bit further, it is not even supported officially there, so probably it is better to wait until webpack has figured it out :).

For now just added a react-native field to the package.json of all the libraries, pointing to an ES5/ES module file with old fashioned .js extension. That works well with Metro.