The Node.js ecosystem has seen rapid evolution, with the introduction of ES modules (ECMAScript modules) being one of the most significant changes. While ES modules bring enhanced features and efficiency, they’ve also led to some common errors. One such error is the infamous “error [err_require_esm]: require() of ES module”, which arises when developers attempt to use require() to load ES modules in a project that hasn’t been configured properly to handle them.

In this blog post, we’ll take a deep dive into what causes this error, how to resolve it, and best practices to avoid running into the issue again. This is crucial because, as the JavaScript ecosystem leans more toward ES modules, it’s important to ensure compatibility with the latest standards without disrupting your existing workflows.

How Can You Prevent the [err_require_esm] Error in Future Projects?

To prevent the “[err_require_esm]: require() of ES module” error in future projects, you need to establish best practices when working with ES modules and CommonJS. The Node.js ecosystem is moving toward ECMAScript modules (ESM), and understanding the distinctions and requirements is key to avoiding such issues.

1. Use ES Modules Exclusively

The most straightforward way to avoid the err_require_esm error is to stick with a single module system. If you’re working on a new project, consider using ES modules exclusively. By using the import and export syntax from the beginning, you ensure consistency and avoid conflicts between module types. Adding “type”: “module” in the package.json file signals to Node.js that your project should use ES modules by default. This also avoids the need to rename .js files to .mjs, as Node.js will recognize .js files as ES modules when the “type”: “module” flag is present.

2. Be Careful with Third-Party Dependencies

If your project relies on external dependencies, ensure that they are compatible with ES modules. Many libraries still use CommonJS, so mixing these with your ES module setup can lead to errors. Research and check the module type of each dependency, or use an intermediary like Babel to compile incompatible code. If a library is CommonJS-based, consider loading it dynamically using import() to avoid errors when using require().

3. Avoid Mixing CommonJS and ESM

One of the biggest culprits of the [err_require_esm] error is mixing require() and import within the same project. It’s important to choose one module format per project to avoid inconsistencies. If your project already relies on CommonJS, gradually migrate to ES modules by refactoring one section at a time. Ensure that you don’t use both systems within the same file, as this can lead to errors and make your code harder to manage.

4. Stay Updated with Node.js Versions

Node.js continues to improve its support for ES modules, so it’s essential to use the latest stable version. ES module support has significantly evolved since its introduction in Node.js 12. Staying up-to-date ensures you’re benefiting from the latest bug fixes and features, reducing the likelihood of running into issues like [err_require_esm].

5. Use Babel for Compatibility

If you’re working on a project that requires both CommonJS and ES modules, using Babel can help bridge the gap. Babel can transpile ES module syntax into CommonJS, ensuring compatibility across your project while still allowing you to use modern JavaScript features.

By following these best practices, you can future-proof your project against the [err_require_esm] error, ensuring smooth development workflows and consistent module usage across your Node.js applications.

What Causes “Error [err_require_esm]: require() of ES Module”?

The “error [err_require_esm]: require() of ES module” occurs when there is a conflict between the CommonJS and ES module systems in Node.js. Understanding the root causes of this error requires a basic knowledge of the two different module systems in JavaScript: CommonJS and ES modules (ESM). Node.js has traditionally used CommonJS, which relies on the require() function to load modules. ES modules, on the other hand, are a newer standard and use the import and export syntax.

The error is typically triggered when a developer attempts to use the CommonJS require() function to load an ES module. Since require() is not compatible with ES modules, Node.js throws the [err_require_esm] error to signal that the two systems cannot be mixed in this way.

Here are the primary causes of this error:

• Attempting to Load ES Modules with require(): One of the most common causes is trying to use the require() function, which is designed for CommonJS modules, to load an ES module. ES modules must be imported using the import statement instead of require(). The two systems are not interchangeable, so using require() with an ES module will lead to this error.
• Incorrect Project Configuration: Another frequent cause is the absence of proper configuration in your project’s package.json file. By default, Node.js treats .js files as CommonJS. To instruct Node.js that your project uses ES modules, you must add “type”: “module” to your package.json. Without this flag, Node.js assumes the use of CommonJS, and trying to load ES modules will result in the error.
• File Extensions: In some cases, using the .js extension for ES module files without proper configuration can lead to issues. While Node.js can handle .js files for both module systems if the “type”: “module” flag is set, it’s common to rename ES module files to .mjs for clarity. This ensures that Node.js treats these files as ES modules.
• Outdated Node.js Version: ES modules were introduced in Node.js 12, so running an older version of Node.js can also trigger this error. If you are using a Node.js version below 12, ES module support will not be available, and attempting to load an ES module will result in the [err_require_esm] error.

By ensuring that your project is properly configured for ES modules, using the correct syntax, and running a compatible Node.js version, you can prevent this error from occurring.

Summary

The “error [err_require_esm]: require() of ES module” is a common issue in Node.js, especially as the ecosystem moves towards ES modules. Understanding the differences between CommonJS and ES modules, and knowing how to configure your project correctly, is crucial for avoiding this error. By following the steps outlined in this guide—such as updating your package.json, switching from require() to import, and ensuring your Node.js version is up to date—you can fix this error and prevent it from recurring in future projects.

FAQ

Do I always have to rename my files to .mjs?

No, if you add “type”: “module” in package.json, you can continue using .js extensions for ES modules. However, .mjs is still a recommended practice to avoid ambiguity.

What version of Node.js supports ES Modules?

ES Modules were introduced in Node.js 12. To use them, ensure your Node.js version is 12 or higher.

Can I use both CommonJS and ES Modules in the same project?

While it is possible to mix the two, it’s not recommended as it can lead to errors. Stick to one system for consistency, or use Babel to bridge the gap.