Add hasDefaultExport to ModuleInfo

[lukastaegert]

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

List any relevant issue numbers:

Description

This is the second Plugin API extension I am working on at the moment. This time the goal is to ease to creation of proxy modules that just "reexport everything" from another module.
The problem here is that

export * from 'foo'

does NOT reexport everything from "foo"—it just reexports all NAMED exports. But if there was a default export, it will be lost (not sure why things were designed that way, but that is what the specs say and what Rollup does). So if you also want to reexport the default export as well, you have to know whether it exists before writing the proxy.

My suggestion (implemented in this PR) is to add a flag hasDefaultExport: boolean | null to ModuleInfo. The value null is used for external modules and before the module has been parsed.

When you const moduleInfo = await this.load({id}), then you can be sure that (if the module exists), moduleInfo.hasDefaultExport is the correct value.

@yyx990803, @patak-dev, @antfu, @marvinhagemeister not sure if this would be a problem to implement in Vite or WMR. My hope was that you already have some kind of information available, in the worst case you could probably do something with regular expressions executed when accessing a getter to figure this out.

Here is an example from the docs that shows how to use this:

export default function addProxyPlugin() {
  return {
    async resolveId(source, importer, options) {
      if (importer?.endsWith('?proxy')) {
        // Do not proxy ids used in proxies
        return null;
      }
      // We make sure to pass on any resolveId options to this.resolve to get the module id
      const resolution = await this.resolve(source, importer, { skipSelf: true, ...options });
      // We can only pre-load existing and non-external ids
      if (resolution && !resolution.external) {
        // we pass on the entire resolution information
        const moduleInfo = await this.load(resolution);
        if (moduleInfo.code.indexOf('/* use proxy */') >= 0) {
          return `${resolution.id}?proxy`;
        }
      }
      // As we already fully resolved the module, there is no reason to resolve it again
      return resolution;
    },
    load(id) {
      if (id.endsWith('?proxy')) {
        const importee = id.slice(0, -'?proxy'.length);
        // Note that namespace reexports do not reexport default exports
        let code = `console.log('proxy for ${importee}'); export * from ${JSON.stringify(
          importee
        )};`;
        // We know that while resolving the proxy, importee was already fully
        // loaded and parsed, so we can rely on hasDefaultExport
        if (this.getModuleInfo(importee).hasDefaultExport) {
          code += `export { default } from ${JSON.stringify(importee)};`;
        }
        return code;
      }
      return null;
    }
  };
}

I also considered if we should make the names of all exports available, but the problem here would be how to handle namespace reexports: If we do it right, we would need to merge the named exports of another module, and the logic would quickly get complicated and problematic, or invent a special syntax for that case, or ignore that case, none of which seemed right to me. So that is why I would go for this boolean for now.

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#module-info-exports

or load it into the REPL:
https://rollupjs.org/repl/?pr=4356

[codecov]

Codecov Report

Merging #4356 (9918ee1) into master (bebc50d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4356   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files         205      205           
  Lines        7331     7334    +3     
  Branches     2084     2086    +2     
=======================================
+ Hits         7235     7238    +3     
  Misses         36       36           
  Partials       60       60           

Impacted Files	Coverage Δ
src/ExternalModule.ts	100.00% <ø> (ø)
src/Module.ts	100.00% <100.00%> (ø)

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[patak-dev]

Looks good. I don't see an issue implementing this in the Vite dev server once we need to support it. Thanks for keeping us in the loop @lukastaegert!

[lukastaegert]

That is good. Turns out I would really love to have this quickly to fix an issue with rollup/plugins#1038 so I will merge this.