ESM & CommonJS Interop Support

Bare uses the same module resolution between CommonJS and ESM modules allowing an ESM module to be require()'d.

Snippet

index.js

const { foo } = require('./esm')
foo().then(() => console.log('done'))

esm.mjs

await new Promise((resolve) => setTimeout(resolve, 1 * 1000))
console.log('After top-level await')

export async function foo () {
  await new Promise((resolve) => setTimeout(resolve, 2 * 1000))
  console.log('foo')
}

Running & Explanation

Bare

bare index.js

Will output:

After top-level await
foo
done

Node

node index.js

Should error like so:

node:internal/modules/cjs/loader:1252
  throw err;
  ^

Error: Cannot find module './esm'
Require stack:
- ./bare-snippets/esm-cjs-interop/index.js
    at Function._resolveFilename (node:internal/modules/cjs/loader:1249:15)
    at Function._load (node:internal/modules/cjs/loader:1075:27)
    at TracingChannel.traceSync (node:diagnostics_channel:315:14)
    at wrapModuleLoad (node:internal/modules/cjs/loader:218:24)
    at Module.require (node:internal/modules/cjs/loader:1340:12)
    at require (node:internal/modules/helpers:141:16)
    at Object.<anonymous> (./bare-snippets/esm-cjs-interop/index.js:1:17)
    at Module._compile (node:internal/modules/cjs/loader:1546:14)
    at Object..js (node:internal/modules/cjs/loader:1689:10)
    at Module.load (node:internal/modules/cjs/loader:1318:32) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    './bare-snippets/esm-cjs-interop/index.js'
  ]
}

Nodejs cannot resolve ./esm identifier because it doesn't automatically add the .mjs to identifiers like it does for .js. Even if the ./esm is changed to ./esm.mjs, it outputs the following error:

node:internal/modules/esm/module_job:392
      throw new ERR_REQUIRE_ASYNC_MODULE();
      ^

Error [ERR_REQUIRE_ASYNC_MODULE]: require() cannot be used on an ESM graph with top-level await. Use import() instead. To see where the top-level await comes from, use --experimental-print-required-tla.
    at ModuleJobSync.runSync (node:internal/modules/esm/module_job:392:13)
    at ModuleLoader.importSyncForRequire (node:internal/modules/esm/loader:329:47)
    at Object.loadESMFromCJS [as .mjs] (node:internal/modules/cjs/loader:1411:24)
    at Module.load (node:internal/modules/cjs/loader:1313:32)
    at Function._load (node:internal/modules/cjs/loader:1123:12)
    at TracingChannel.traceSync (node:diagnostics_channel:322:14)
    at wrapModuleLoad (node:internal/modules/cjs/loader:217:24)
    at Module.require (node:internal/modules/cjs/loader:1335:12)
    at require (node:internal/modules/helpers:136:16)
    at Object.<anonymous> (./bare-snippets/esm-cjs-interop/index.js:1:17) {
  code: 'ERR_REQUIRE_ASYNC_MODULE'
}

This is because it doesn't allow require()'ing an ESM module with top-level awaits. Having a top-level await in a module requires the module to be evaluated asynchronously, but require() is synchronous. Bare runs this async module for you synchronously. This is why the first line output by Bare is After top-level await.

See bare-module-resolve's "Algorithm" section for more details.