Add support for transpiled modules

[Victorystick]

Don't merge yet

This PR tracks how best to handle CommonJS modules that have already been transpiled by Babel, TypeScript or something similar. These CommonJS modules define a __esModule property on their exports object, and export their default export as a default property.

We should:

1. remove the __esModule property
2. use the default property for the default export instead of the module.exports object

Fixes #10

[Rich-Harris]

Would it be appropriate to warn users in this situation? It may be that we're dealing with external packages, in which case the user may have limited control, but it might help people understand that their Babel config is messed up etc

[flying-sheep]

messed up? does this mean it’s somehow not working as intended? why?

[Rich-Harris]

@flying-sheep Messed up as in Babel is being used to convert ES6 modules to CommonJS modules, which means Rollup has to convert them back to ES6 modules so that it can bundle them (quite possibly in order to generate a CommonJS bundle). Keeping everything as ES6 until you need to run it is the name of the game.

[flying-sheep]

ok, but how? jsnext:main should point to a “distributable” package, not a ES2015 package… should we really introduce another field and copy of the lib for an ES2015 version?

i’ll comment in rollup/rollup#337 about it.

for this issue and plugin, i think we should simply detect the most common ways stuff is distributed and be practical instead of idealistic. this is a practical plugin, written for a world of commonjs modules. it should “get” as many of them as possible.

[Rich-Harris]

Should have been more specific – I meant keeping everything as ES6 modules is the goal, since that's the only thing that's getting translated back and forth otherwise.

[flying-sheep]

makes sense. this is a frequent language confusion in the discussions here.

but still: both should be supported as we can’t hope to convince everyone to use jsnext:main (which means adding build configs and doubling package size) and therefore should be practical.

1. let’s help redux with fixing their use of jsnext:main
2. let’s finish and pull this PR

then we can actually work productively with rollup as node_modules bundler

[Victorystick]

Looks good to me. 👍

[Rich-Harris]

Have released this as 2.0.0 (since it's technically a breaking change) – it doesn't include warnings about already-transpiled modules, but we could revisit that in future if we choose

[fregante]

Sorry, I think this change is indeed breaking support of rollup modules in browserify/babel. If you export default but not __esModule, browserify/babel won't pick that up. I had left a comment here too: rollup/rollup#496 (comment)