--require doesn't work with import

[daveisfera]

• Version: 12.18.3
• Platform: macOS 10.15.6
• Subsystem:

What steps will reproduce the bug?

Run node with --require and --experimental-specifier-resolution=node

An example can be found here when running npm test

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior?

There's a way to preload a file using import rather than require

What do you see instead?

internal/modules/cjs/loader.js:1154
      throw new ERR_REQUIRE_ESM(filename, parentPath, packageJsonPath);
      ^

Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: /Users/dlj/projects/test_import_preload/preload.js
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1154:13)
    at Module.load (internal/modules/cjs/loader.js:986:32)
    at Function.Module._load (internal/modules/cjs/loader.js:879:14)
    at Module.require (internal/modules/cjs/loader.js:1026:19)
    at Module._preloadModules (internal/modules/cjs/loader.js:1278:12)
    at loadPreloadModules (internal/bootstrap/pre_execution.js:439:5)
    at prepareMainThreadExecution (internal/bootstrap/pre_execution.js:71:3)
    at internal/main/run_main_module.js:7:1 {
  code: 'ERR_REQUIRE_ESM'
}
npm ERR! Test failed.  See above for more details.

Additional information

It would be nice if there was a --import or if --require worked with imports

[Trott]

@nodejs/modules

[devsnek]

this is definitely a duplicate but I'm not able to go diving for the original atm.

[daveisfera]

Is it a duplicate in the sense that there's already a resolution/workaround for this? Or in that this is an existing bug that needs to be addressed?

[jkrems]

If you need a workaround, you can generally use a small wrapper CJS script that only contains import("./actual.mjs"). The big downside is that it won't run synchronously, if that's something you need.

I don't think anybody is working on support for --import=x.mjs right now. So unless somebody who wants that feature steps up, it's unlikely to happen.

[daveisfera]

Ya, we're using --require to load settings for our application before the actual app starts and we're using deasync to make sure it's synchronous to accomplish that right now and it works with esm but we'd like to use the built-in support from node, if possible

[jkrems]

I think adding an --import flag wouldn't be too contentious (I hope). Let me know if you'd like to look into it and want some hints for where to start!

[devsnek]

cc @bmeck ^

[targos]

It's probably a very bad idea, but using node --loader something.mjs script.js actually works as a workaround.

[daveisfera]

@jkrems I've never worked with the code for node, but I'd be willing to give it a try, so we're would I get started?

@targos I gave that a whirl and this was the error I ran into:

(node:20876) ExperimentalWarning: --experimental-loader is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
internal/modules/esm/module_job.js:112
          const namedImports = StringPrototypeMatch(importStatement, /{.*}/)[0];
                                                                            ^

TypeError: Cannot read property '0' of null
    at ModuleJob._instantiate (internal/modules/esm/module_job.js:112:77)
    at async ModuleJob.run (internal/modules/esm/module_job.js:137:5)
    at async Loader.import (internal/modules/esm/loader.js:165:24)
    at async internal/process/esm_loader.js:57:9
    at async Object.loadESM (internal/process/esm_loader.js:67:5)

[devsnek]

There was an effort to add --import to node by bmeck, but iirc there were a lot of issues relating to timing.

[bmeck]

Yep, I had trouble since everything in the bootstrap assumes it is a synchronous startup, adding asynchronous ticks made things get weird. I'd be wary of using --loader for any sort of global mutation outside of the getGlobalPreloadCode hook. There isn't a guarantee that the Loader will continue to use the same global space as user code.

[jkrems]

IIRC --loader already got us to a place where we sometimes have ticks during startup. So maybe --import would be possible now, at least for user code where the user can verify whatever timing effect it has?

I'd be wary of using --loader for any sort of global mutation outside of the getGlobalPreloadCode hook.

Yeah, loading arbitrary code in --loader will do weird stuff™. Among other things: It generates a separate module map.

I've never worked with the code for node, but I'd be willing to give it a try, so we're would I get started?

The general starting point would be this guide: https://github.com/nodejs/node/blob/master/doc/guides/contributing/pull-requests.md. It starts with setting up a local checkout and covers how to test and commit changes.

The --require flag is internally called "preloadModules". For adding a new flag in general, there's a C++ class to register flags.

One possible place to handle --import itself would be around where we also run the --loader-provided global preload code:

node/lib/internal/process/esm_loader.js

Lines 40 to 63 in 05539c1

	async function initializeLoader() {
	const { getOptionValue } = require('internal/options');
	const userLoader = getOptionValue('--experimental-loader');
	if (!userLoader)
	return;
	let cwd;
	try {
	cwd = process.cwd() + '/';
	} catch {
	cwd = 'file:///';
	}
	// If --experimental-loader is specified, create a loader with user hooks.
	// Otherwise create the default loader.
	const { emitExperimentalWarning } = require('internal/util');
	emitExperimentalWarning('--experimental-loader');
	return (async () => {
	const hooks =
	await ESMLoader.import(userLoader, pathToFileURL(cwd).href);
	ESMLoader = new Loader();
	ESMLoader.hook(hooks);
	ESMLoader.runGlobalPreloadCode();
	return exports.ESMLoader = ESMLoader;
	})();
	}

It may require changing the conditions under which we call initializeLoader.

One important trick to make the development go more quickly: ./configure --node-builtin-modules-path=$PWD. Use that instead of the default ./configure and the node binary you build will load the JS code from your nodejs checkout directly. This can save a lot of time on rebuilding. For this change, you can likely stop rebuilding the binary after the flag is registered.

[devsnek]

@jkrems --loader flags are processed after --require flags. But what do you expect --import x --require y --import z to do?