Does @babel/preset-env preserve ES modules when modules is auto?

When modules is set to "auto" in @babel/preset-env, Babel does not make a fixed decision on whether to preserve ES modules; instead, it defers the transformation to the caller (the tool invoking Babel).

Caller-Based Resolution

The final output depends entirely on the integration used to run the compilation. Because "auto" is a signal to check the caller's preferences, the same configuration file can produce different results in different environments:

• ESM-aware Bundlers: Tools like Webpack (via babel-loader) or Rollup typically signal Babel to preserve ES modules. This allows the bundler to perform tree-shaking by analyzing the static import and export statements.
• Babel CLI: When running Babel as a standalone command-line tool, it generally resolves "auto" to "commonjs", transforming ES modules into require and module.exports.

Stability and Configuration

A shared configuration using modules: "auto" is not stable across different workflows. If your pipeline uses both the Babel CLI (e.g., for quick tests or legacy scripts) and a bundler (e.g., for production builds), you will get inconsistent module syntax.

Ensuring Consistent ESM Output

To guarantee that ES modules are preserved regardless of the caller, you must explicitly disable module transformation:

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "modules": false
      }
    ]
  ]
}

Verification Steps

To verify how your current environment resolves the "auto" setting, you can run these two tests:

1. CLI Test: Run npx babel src --out-dir dist. If the output files contain require() instead of import, the CLI is resolving "auto" to CommonJS.
2. Bundler Test: Run your Webpack or Rollup build and inspect the intermediate transformed code. If import statements remain, the bundler is successfully requesting ESM preservation.

Diagnostic Note: To provide a more specific recommendation, please specify if you are using a custom babel-loader configuration or a specific version of the Babel CLI.