Run Jest Tests with Bun Using Babel: A Complete Guide (2026)

As the JavaScript ecosystem evolves, developers often look for new tools to optimize their workflows. Bun is a fast, all-in-one JavaScript runtime that promises speed improvements over traditional Node.js setups. However, integrating Bun with existing setups, especially those that rely on Babel for code transformation, can be challenging. In this guide, we will explore how to make Jest tests, which are traditionally run in Node.js, compatible with Bun when using Babel for code transformation.

Key Takeaways

• Understand the role of Babel in transforming JavaScript code for different environments.
• Learn how to configure Jest and Babel to work seamlessly with Bun.
• Explore the necessary changes to the Babel configuration to ensure compatibility.
• Identify common errors when integrating Bun and how to troubleshoot them.

This tutorial will guide you through configuring your Jest tests with Bun while using Babel to ensure your tests run smoothly across different environments. By the end, you'll understand how to adjust your setup for maximum compatibility and performance.

Prerequisites

• Basic knowledge of JavaScript and Node.js.
• Familiarity with Jest and Babel.
• Bun installed on your system (version 0.5.0 or later).
• A working Node.js environment (version 16 or later).

Step 1: Understand Your Babel Configuration

Your Babel configuration is crucial for transforming your JavaScript code to run in different environments. The provided babel.config.js file is already set up to target the current Node.js version:

module.exports = { presets: [['@babel/preset-env', {targets: {node: 'current'}}]], plugins: ['babel-plugin-rewire', 'export-toplevel'], };

This configuration ensures that your code is transpiled to be compatible with the current Node.js environment. However, Bun might not respect this configuration out-of-the-box, leading to compatibility issues.

Step 2: Install Necessary Bun and Babel Support

To make Babel work with Bun, you need to ensure that all necessary dependencies are installed. Run the following command to install Bun-specific Babel plugins:

bun add @babel/preset-env babel-plugin-rewire export-toplevel

This command ensures that Babel can properly transpile your code when running tests with Bun.

Step 3: Configure Bun to Use Babel

Bun does not automatically use Babel for transforming JavaScript, unlike Node.js. You need to explicitly configure Bun to use Babel by setting up a script in your package.json:

"scripts": { "bun-test": "BUN_BABEL_CONFIG=./babel.config.js bun test" }

This script specifies that Bun should use the provided Babel configuration when running tests.

Step 4: Run Your Jest Tests with Bun

Now that Bun is configured to use Babel, you can run your Jest tests with the following command:

bun run bun-test

This command will execute your tests using Bun, leveraging the Babel transformations specified in your configuration file.

Step 5: Verify Test Compatibility

After running your tests, check the output to ensure they pass without errors. If you encounter issues, verify that your Babel configuration is correctly targeting all necessary features and syntax used in your code.

If your tests fail, consider checking for syntax or feature usage that may require additional Babel plugins or configuration adjustments.

Common Errors/Troubleshooting

When integrating Bun with Babel, you might encounter the following common issues:

• Syntax Errors: Ensure that your Babel configuration includes all necessary plugins for your code syntax.
• Module Resolution Errors: Check that all dependencies are correctly installed and accessible by Bun.
• Configuration Ignored: Ensure that the BUN_BABEL_CONFIG environment variable is correctly set to your Babel configuration file.

By carefully following the steps and troubleshooting tips provided, you can successfully run Jest tests with Bun while utilizing Babel for code transformation.

Frequently Asked Questions

Why isn't my Babel configuration working with Bun?

Bun requires explicit configuration to use Babel. Ensure you set the BUN_BABEL_CONFIG environment variable to point to your Babel config file.

How do I install Bun and Babel plugins?

Use bun add to install necessary plugins like @babel/preset-env and others listed in your Babel configuration.

Can I use the same Jest configuration for both Node.js and Bun?

Yes, but you may need to adjust Babel configurations to ensure compatibility with both runtimes.