(node:61774) ExperimentalWarning: `--experimental-loader` may be removed in the future

You may have encountered a warning when using the --loader Node.js CLI flag, despite the message specifically mentioning --experimental-loader. The warning typically appears as follows:

(node:82007) ExperimentalWarning: `--experimental-loader` may be removed in the future; instead use `register()`:
--import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register("./loader.js", pathToFileURL("./"));'

There are several potential solutions to address this issue, depending on your specific use case.

Table of contents

Case 1: Own loader

This warning may appear when using your own custom loader, such as the one described in this article.

1. Edit your ./loader.js file (or the file containing your custom loader, regardless of its name):

// add `register` import
import { isBuiltin, register } from "node:module";
import { dirname } from "node:path";
import { cwd } from "node:process";
import { fileURLToPath, pathToFileURL } from "node:url";
import { promisify } from "node:util";
import resolveCallback from "resolve/async.js";

// add `register call
register("./loader.js", import.meta.url);

const resolveAsync = promisify(resolveCallback);

const baseURL = pathToFileURL(cwd() + "/").href;

export async function resolve(specifier, context, next) {
  // the rest is the same here
  // ...
}

2. After modifying your ./loader.js file, you’ll need to change the flags used to start your Node.js application.

-`node --loader=./loader.js`
+`node --import=./loader.js`

Note: The fix provided is applicable for ESM (ECMAScript) modules. If you’re using CommonJS (CJS) modules, replace the --import flag with --require and adjust the imports in your loader file accordingly.

Case 2: Library provided loader

For library-provided loaders, the process is similar. Let’s examine an example using the ts-node/esm loader.

1. Create a new file in the root of your project, for example, loader.js.

import { register } from "node:module";

register("ts-node/esm", import.meta.url);

2. Replace all instances of --loader ts-node/esm with --import ./loader.js throughout your project.

Summary

For more detailed information about the node:module API, refer to the official Node.js documentation. This resource provides comprehensive guidance on implementing and using the new loader system.

Thank you for reading this article!