#Optimization

Rspack will select appropriate optimization configuration based on the mode. You can also customize the configuration via optimization.

#optimization.moduleIds

• Type: 'natural' | 'named' | 'deterministic'
• Default:production mode is'deterministic', development mode is'named'

Tells Rspack which algorithm to use when generating module ids.

The following string values are supported:

export default {
  optimization: {
    moduleIds: 'deterministic',
  },
};

The deterministic option is useful for long term caching, and results in smaller bundles compared to hashed. Length of the numeric value is chosen to fill a maximum of 80% of the id space. By default a minimum length of 3 digits is used when optimization.moduleIds is set to deterministic.

#optimization.chunkIds

• Type: 'natural' | 'named' | 'deterministic' | 'size' | 'total-size'
• Default:development mode is'named', production mode is'deterministic'

Tells Rspack which algorithm to use when generating chunk ids.

The following string values are supported:

export default {
  optimization: {
    chunkIds: 'deterministic',
  },
};

#optimization.mergeDuplicateChunks

• Type: boolean
• Default:true

Whether to merge chunks which contain the same modules. Setting optimization.mergeDuplicateChunks to false will disable this optimization.

export default {
  optimization: {
    mergeDuplicateChunks: false,
  },
};

#optimization.minimize

• Type: boolean
• Default:production mode istrue, development mode isfalse

Whether to use the minimizer declared in optimization.minimizer to minimize the bundle.

export default {
  optimization: {
    minimize: true,
  },
};

#optimization.minimizer

• Type: Array<Plugin>
• Default:[new SwcJsMinimizerRspackPlugin(), new LightningCssMinimizerRspackPlugin()]

Customize the minimizer. By default, rspack.SwcJsMinimizerRspackPlugin and rspack.LightningCssMinimizerRspackPlugin are used.

When optimization.minimizer is specified, the default minimizers will be disabled.

import TerserPlugin from 'terser-webpack-plugin';

export default {
  optimization: {
    minimizer: [new TerserPlugin()],
  },
};

Use Rspack's built-in minimizer with custom options:

import { rspack } from '@rspack/core';

export default {
  optimization: {
    // when `optimization.minimizer` is specified, the default minimizers are disabled by default
    // but you can use '...', it represents the default minimizers
    minimizer: [
      new rspack.SwcJsMinimizerRspackPlugin({
        minimizerOptions: {
          format: {
            comments: false,
          },
        },
      }),
      new rspack.LightningCssMinimizerRspackPlugin({
        minimizerOptions: {
          errorRecovery: false,
        },
      }),
    ],
  },
};

#optimization.removeAvailableModules

• Type: boolean
• Default:true

Whether to detect and remove modules from chunks when these modules are already included in parent chunks. This optimization helps to reduce duplicated modules in the bundle.

This optimization is enabled by default and setting optimization.removeAvailableModules to false will disable this optimization:

export default {
  optimization: {
    removeAvailableModules: false,
  },
};

#optimization.removeEmptyChunks

• Type: boolean
• Default:true

Detect and remove empty chunks generated in the compilation. Setting optimization.removeEmptyChunks to false will disable this optimization.

export default {
  optimization: {
    removeEmptyChunks: false,
  },
};

#optimization.runtimeChunk

• Type: boolean | string | { name: string } | { name: (entrypoint: { name: string }) => string }
• Default:false

Used to control how the Rspack's runtime chunk is generated.

Defaults to false, which means the runtime code is inlined into the entry chunks.

Setting it to true or 'multiple' will add an additional chunk containing only the runtime for each entry point. This setting is an alias for:

export default {
  optimization: {
    runtimeChunk: {
      name: entrypoint => `runtime~${entrypoint.name}`,
    },
  },
};

Setting it to 'single' will extract the runtime code of all entry points into a single separate chunk. This setting is an alias for:

export default {
  optimization: {
    runtimeChunk: 'runtime',
  },
};

By setting optimization.runtimeChunk to an object it can provide the name property which stands for the name for the runtime chunks.

export default {
  optimization: {
    runtimeChunk: {
      // this will generate a chunk named `my-name.js`
      name: 'my-name',
    },
  },
};

#optimization.realContentHash

• Type: boolean | 'flag'
• Default:production mode istrue, development mode isfalse

Adds an additional hash compilation pass after the assets have been processed to get the correct asset content hashes. This feature will enable by default in production mode.

If realContentHash is set to false, internal data is used to calculate the hash and it can change when assets are identical in some cases.

export default {
  //...
  optimization: {
    realContentHash: true,
  },
};

#optimization.splitChunks

• Type: false | object

Rspack supports splitting chunks with the optimization.splitChunks configuration item.

It is enabled by default for dynamically imported modules.

To turn it off, set it to false.

See available options for configuring this behavior in the SplitChunksPlugin page.

#optimization.sideEffects

• Type: boolean | 'flag'
• Default:production mode istrue, development mode is'flag'

If you only want Rspack use the manual sideEffects flag via (package.json and module.rule.sideEffects) and don't analyse source code:

export default {
  //...
  optimization: {
    sideEffects: 'flag',
  },
};

flag tells Rspack to recognise the sideEffects flag in package.json or module.rule.sideEffects to skip over modules which are flagged to contain no side effects when exports are not used.

true tells Rspack not only recognise the sideEffects flag, but also analyse modules which are not flagged explicitly, and determine if they have side effects or not.

export default {
  //...
  optimization: {
    sideEffects: true,
  },
};

#optimization.providedExports

• Type: boolean
• Default:true

After enabling, Rspack will analyze which exports the module provides, including re-exported modules. A warning or error will be issued when importing members that reference non-existent exports. By default, optimization.providedExports is enabled. This analysis will increase build time. You may consider disabling this configuration in development mode. Disabling it may lead to errors related to runtime circular dependencies as mentioned in the SideEffects section.

export default {
  //...
  optimization: {
    providedExports: false,
  },
};

#optimization.usedExports

• Type: boolean | 'global'
• Default:production mode istrue, development mode isfalse

Tells Rspack to determine used exports for each module. This depends on optimization.providedExports. Information collected by optimization.usedExports is used by other optimizations or code generation i.e. Exports are not generated for unused exports, export names are mangled to single char identifiers when all usages are compatible. Dead code elimination in minimizers will benefit from this and can remove unused exports.

export default {
  //...
  optimization: {
    usedExports: false,
  },
};

To opt-out from used exports analysis per runtime:

export default {
  //...
  optimization: {
    usedExports: 'global',
  },
};

#optimization.mangleExports

• Type: boolean | 'deterministic' | 'size'
• Default:production mode isdeterministic, development mode isfalse

optimization.mangleExports allows to control export mangling.

The following values are supported:

#optimization.innerGraph

• Type: boolean
• Default:production mode istrue, development mode isfalse

optimization.innerGraph tells Rspack whether to perform a more detailed analysis of variable assignments. This helps Rspack to identify unused module exports, thereby reducing the size of the bundled output.

For example:

import { value } from 'lib';

const value2 = value;

function f1() {
  console.log(value);
}

function f2() {
  console.log(value2);
}

Here we assign the value to value2. Both value2 and value are accessed within the functions f2 and f1 respectively, but the functions are not called, hence value2 and value are not actually used, thus the import of value can be removed.

#optimization.concatenateModules

• Type: boolean
• Default:production mode istrue, development mode isfalse

Tells Rspack to find segments of the module graph which can be safely concatenated into a single module. Depends on optimization.providedExports and optimization.usedExports. By default optimization.concatenateModules is enabled in production mode and disabled elsewise.

export default {
  optimization: {
    concatenateModules: false,
  },
};

#optimization.nodeEnv

• Type: boolean | string
• Default:false

Tells Rspack to set process.env.NODE_ENV to a given string value. optimization.nodeEnv uses DefinePlugin unless set to false. optimization.nodeEnv defaults to mode if set, else falls back to 'production'.

Possible values:

• any string: the value to set process.env.NODE_ENV to.
• false: do not modify/set the value of process.env.NODE_ENV.

export default {
  //...
  optimization: {
    nodeEnv: 'production',
  },
};

#optimization.emitOnErrors

• Type: boolean
• Default:production mode isfalse, development mode istrue

Use the optimization.emitOnErrors to emit assets whenever there are errors while compiling. This ensures that erroring assets are emitted. The errors are emitted into the generated code and will cause errors at runtime.

export default {
  optimization: {
    emitOnErrors: true,
  },
};

#optimization.avoidEntryIife

• Type: boolean
• Default:false

Use optimization.avoidEntryIife to avoid wrapping the entry module in an IIFE when it is required (search for "This entry needs to be wrapped in an IIFE because" in rspack_plugin_javascript). This approach helps optimize performance for JavaScript engines and helps tree shaking when building ESM libraries.

Currently, optimization.avoidEntryIife can only optimize a single entry module along with other modules.

export default {
  //...
  optimization: {
    avoidEntryIife: true,
  },
};