Experimental modules warning does not show when using dynamic import

[guybedford]

As reported by @coreyfarrell here -

Interesting note, the following code executed by CJS does not produce any warning or error in node.js 13.2.0:

import('./nyc.config.mjs')
	.then(console.log)
	.catch(e => console.error(e.message));

When using dynamic import the experimental modules implementation is effectively used "lazily". Even when loaded at this point the warning should still be displayed.

Currently the warning is implemented at https://github.com/nodejs/node/blob/master/lib/internal/process/esm_loader.js#L45.

When calling the first dynamic import through https://github.com/nodejs/node/blob/master/lib/internal/process/esm_loader.js#L26 we should also be able to ensure the warning is provided.

[coreyfarrell]

From looking at this code I suspect we cannot prevent the warning if import('./nyc.config.js') loads a CJS file? This makes me lean towards just not supporting nyc.config.js when type: 'module' is specified.

[juanarbol]

How can I recreate this?

I've change esm_loader.js, hard coded warning:

exports.importModuleDynamicallyCallback = async function(wrap, specifier) {
  assert(calledInitialize === true || !userLoader);
  process.emitWarning(
    'The ESM module loader is experimental.',
    'ExperimentalWarning', undefined);
  const { callbackMap } = internalBinding('module_wrap');
  if (callbackMap.has(wrap)) {
    const { importModuleDynamically } = callbackMap.get(wrap);
    if (importModuleDynamically !== undefined) {
      return importModuleDynamically(
        specifier, getModuleFromWrap(wrap) || wrap);
    }
  }
  throw new ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING();
};

$ ./node mod/file.js
node:6329) ExperimentalWarning: The ESM module loader is experimental.
(node:6329) ExperimentalWarning: The ESM module loader is experimental.
[Module] { default: [Function: default] }

The warning is already emitted on another place, so it appears two times.

[coreyfarrell]

@juamedgod create two files:

// test.cjs
import('./test.mjs')
  .then(console.log)
  .catch(e => console.error(e.message));

/// test.mjs
export default 'test.mjs';

Then run ./node ./test.cjs. Output on current master branch:

[Module] { default: 'test.mjs' }