Bundling javascript modules

ab.webc:

<script type="module">
import { LitElement } from 'lit';
customElements.define('a-b', class extends LitElement {});
</script>

cd.webc:

<script type="module">
import { LitElement } from 'lit';
customElements.define('c-d', class extends LitElement {});
</script>

index.webc:

<script type="module" :src="getBundleFileUrl('js')" webc:keep></script>

Uncaught SyntaxError: redeclaration of import LitElement3k1axR86sr.js:3:9note: Previously declared at line 1, column 9

One way to fix this would be to add an async callback to the bundles option which provides an api to collect sources and write outputs, or to do so in the transforms option.

eleventyConfig.addPlugin(bundlerPlugin, {
  transforms: [
    async function(content, collect) {
          // this.type returns the bundle name.
      if (this.type === 'js' &&
          // proposal: this.attributes exposes the source element's attributes
          this.attributes.type === 'module') {
        collect(this, content);
        return false; // don't bundle, instead hold the item in memory
      }
    },
    async function(content) {
      if (this.type == bundlerPlugin.collection) { // a Symbol()
        const { build } = await import('esbuild');
        const result = await build({
          bundle: true,
          /* ... collected has source strings and metadata, etc */
          entryPoints: writeToTmpAndMakeEntryPointsConfig(collected),
        });
        const [{ path, contents }] = result.outputFiles;
        return {
           url: path,
           content: contents,
        }
      }
    }
  ],
});

above APIs are proposals for discussion only, salt to taste.

Start with the ab.webc, cd.webc, and index.webc examples and trace how the bundles and transforms options handle module scripts. Review the proposed collect callback and esbuild integration, then confirm the API design with maintainers. Done means module sources can be collected and bundled without duplicate import declarations, with documented behavior and tests.