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# lib.autoExtension

- **Type:** `boolean`
- **Default:** `true`
- **CLI:** `--auto-extension` / `--no-auto-extension`
- **Top-level config:** Supported

Whether to automatically set the file extension based on the [`format`](/config/lib/format.md) option in the JavaScript output files.

## Default extension

By default that when `autoExtension` is set to `true`, the file extension will be:

- `.js` with `esm` format and `.cjs` with `cjs` format when `type: module` in `package.json`.

- `.js` with `cjs` format and `.mjs` with `esm` format when `type: commonjs` or no `type` field in `package.json`.

When `autoExtension` is set to `false`, the file extension will be default to `.js`.

## Customize extension

You can set `autoExtension` to `false` and use [output.filename](https://rsbuild.rs/config/output/filename) to customize the JavaScript output files.

```ts title="rslib.config.ts"
export default defineConfig({
  lib: [
    {
      format: 'cjs',
      autoExtension: false,
      output: {
        filename: {
          js: '[name].cjs',
        },
      },
    },
    {
      format: 'esm',
      output: {
        filename: {
          js: '[name].mjs',
        },
      },
    },
  ],
});
```