> For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt.

# DeterministicModuleIdsPlugin

`DeterministicModuleIdsPlugin` assigns short numeric ids to modules based on their module identifiers. The ids are stable between compilations, which makes the plugin useful for long-term caching.

`optimization.moduleIds: 'deterministic'` uses this plugin internally. Use the plugin directly when you need to customize the deterministic id generation behavior.

## Examples

```js title="rspack.config.mjs"
import rspack from '@rspack/core';

export default {
  optimization: {
    moduleIds: false,
  },
  plugins: [
    new rspack.ids.DeterministicModuleIdsPlugin({
      maxLength: 4,
    }),
  ],
};
```

## Options

### context

- **Type:** `string`
- **Default:** `compiler.context`

The context used to create relative module identifiers before ids are generated.

### test

- **Type:** `(module: Module) => boolean`
- **Default:** `undefined`

Selects which modules should receive deterministic ids from this plugin. When omitted, all modules that need ids are included.

```js title="rspack.config.mjs"
import rspack from '@rspack/core';

export default {
  optimization: {
    moduleIds: false,
  },
  plugins: [
    new rspack.ids.DeterministicModuleIdsPlugin({
      test: (module) => module.type.startsWith('css'),
    }),
  ],
};
```

### maxLength

- **Type:** `number`
- **Default:** `3`

The maximum id length in digits used as the starting id space. The generated numeric id space starts at `10 ** maxLength`.

### salt

- **Type:** `number`
- **Default:** `0`

The hash salt used when generating ids. Change this value to try a different hash starting value in the same id space.

### fixedLength

- **Type:** `boolean`
- **Default:** `false`

When enabled, Rspack does not increase the id length to find a larger id space.

### failOnConflict

- **Type:** `boolean`
- **Default:** `false`

When enabled, Rspack reports an error if deterministic id assignment produces conflicts. By default, Rspack resolves conflicts by retrying with a larger id space.

## Usage with optimization.moduleIds

For the common long-term caching setup, prefer the built-in optimization option:

```js title="rspack.config.mjs"
export default {
  optimization: {
    moduleIds: 'deterministic',
  },
};
```

Use `DeterministicModuleIdsPlugin` directly only when you need the options above.


This page is adapted from [webpack documentation](https://webpack.js.org/configuration/optimization/#optimizationmoduleids) under the [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/), with modifications.