Documentation Is:

• Missing
• Needed
• Confusing
• Not Sure?

Please Explain in Detail...

The documentation surrounding supporting multiple entries in the Javascript API is confusing.

The docs cover multiple entries/inputs briefly and suggest using a rollup.config.js that exports an array as opposed to an object, i.e:

export default [{
  input: 'main-a.js',
  output: {
    file: 'dist/bundle-a.js',
    format: 'cjs'
  }
}, {
  input: 'main-b.js',
  output: [
    {
      file: 'dist/bundle-b1.js',
      format: 'cjs'
    },
    {
      file: 'dist/bundle-b2.js',
      format: 'esm'
    }
  ]
}];

Alternatively see #2935 for a more in depth discussion surrounding this.

What is unclear is whether or not you can use this in the Javascript API, essentially I thought I would be able to call rollup.rollup and pass it an array similar to the one exported in the above example, however this results in the following error:

Unknown input option: 0. Allowed options: acorn, acornInjectPlugins, cache, chunkGroupingSize, context, experimentalCacheExpiry, experimentalOptimizeChunks, experimentalTopLevelAwait, external, inlineDynamicImports, input, manualChunks, moduleContext, onwarn, perf, plugins, preserveModules, preserveSymlinks, shimMissingExports, strictDeprecations, treeshake, watch
Error: You must supply options.input to rollup

I have since realized that rollup.rollup is only capable of accepting one config object and must be called multiple times with each config.

Based on this comment in #863 this is not possible (although from what I can see rollup.watch may accept multiple arguments and as mentioned in the referenced comment this may warrant it's own issue).

As a side note it is also unclear that rollup.watch does accept an array when rollup.rollup does not.

Your Proposal for Changes

If this is the intended behaviour and there is not a planned changed on the immediate horizon I think it would be good to explicitly highlight that the Javascript API does not support multiple entries.

I think it would also be beneficial to perhaps give a rudimentary example of calling rollup.rollup multiple times in the event that a user needs to bundle multiple entries/inputs.

something like:

const { rollup } = require('rollup');

const buildEntries = entries => {
    const pending = entries.map(async item => {
        const [inputOptions, outputOptions] = item;

        const bundle = await rollup(inputOptions);
        await bundle.write(outputOptions);
        return;
    });

    // returns promise that resolves when all rollups complete
    return Promise.all(pending);
}

export default buildEntries;

Where entries would look something like:

[[{
    input: 'component.js',
    plugins: []
},
{
    dir: '/dist',
    name: 'Component',
    format: 'umd'
}]]