Upgrading to Acorn 6

[marijnh]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

Description

This is to get the ball rolling up upgrading to Acorn 6, a new major version of the parser. There's two contentious parts:

• The plugin API in Acorn 6 works differently, which means the acornOptions.plugins option to the parse method will work differently after the upgrade. It'll work better, since plugins are now just values and easier to pass around, but code that uses this option in the old way will break.

• I had to add an ugly kludge to rollup.config.js to make the build go through. If you remove the return statement I added, the build fails with a 'default' is not exported warning, which points at node_modules/acorn/dist/acorn.mjs (which indeed does not export default), as apparently imported by its own rollup-commonjs generated code. I was unable to figure out why this warning came up (the old module also didn't export default), and I'm hoping someone here can help figure that out.

[lukastaegert]

Hi @marijnh, awesome to get this going. I also had a poke at this yesterday (since I read that dynamic imports are now acorn-6 compatible) so I have some insights into the "default is not exported" error.

This error is caused by rollup-plugin-commonjs and IMO can be fixed there (working on this plugin anyway right now). It is caused by the two acorn plugins we include by default which in turn require acorn. As rollup by default uses the .mjs version of acorn, these plugins are also redirected to use this version.

Now since those plugins are CJS and acorn is ESM, the CJS plugin will add a proxy wrapper around their import of acorn. Since the extension of acorn is .mjs, however, the CJS plugin will not check if the plugin has a default export but instead add a runtime check. This check is written as

import * as acorn from 'acorn';
export default ( acorn && acorn['default'] ) || acorn;

rollup again tries to simplify the namespace property access which fails because there is no default export (I believe the reason for using the computed property was to prevent this but this only worked in earlier versions of rollup).

I think the only safe way of handling this is to do default export detection for all files, not only those with a .js extension. Will look into this a little. At the same time I think I will try to improve the interop code to a more robust

import * as acorn from 'acorn';
export default acorn && 'default' in acorn ? acorn.default : acorn;

but I fear this will not solve the issue.

Update: Maybe it will work if I put the wrapper into a function as this will force Rollup to create a namespace object. A little more code but easy to achieve.

[lukastaegert]

I did the necessary updates to your branch, fixed the rollup-plugin-commonjs issue, types and tests. Please have a look!

[lukastaegert]

Since you correctly noted that this will be a breaking change due to the changes in which and how acorn plugins can be provided, I changed the base branch to the 1.0 release.

[adrianheine]

This allows to use acorn-export-ns-from, too :)

[guybedford]

@adrianheine probably worth creating a tracking issue for that, as it will likely be buggy :)