SplitChunksPlugin

Originally, chunks (and modules imported inside them) were connected by a parent-child relationship in the internal webpack graph. The CommonsChunkPlugin was used to avoid duplicated dependencies across them, but further optimizations were not possible

Since webpack v4, the CommonsChunkPlugin was removed in favor of optimization.splitChunks.

Defaults

Out of the box SplitChunksPlugin should work great for most users.

By default it only affects on-demand chunks because changing initial chunks would affect the script tags the HTML file should include to run the project.

webpack will automatically split chunks based on these conditions:

• New chunk can be shared OR modules are from the node_modules folder
• New chunk would be bigger than 30kb (before min+gz)
• Maximum number of parallel requests when loading chunks on demand would be lower or equal to 5
• Maximum number of parallel requests at initial page load would be lower or equal to 3

When trying to fulfill the last two conditions, bigger chunks are preferred.

Configuration

For developers that want to have more control over this functionality, webpack provides a set of options to better fit your needs. If you're changing the configuration, it's a good idea to measure the impact of your changes to ensure there's a real benefit.

Default configuration was chosen to fit web performance best practices but the optimum strategy for your project might defer depending on the nature of it.

optimization.splitChunks

This configuration object represents the default behavior of the SplitChunksPlugin.

module.exports = {
  //...
  optimization: {
    splitChunks: {
      chunks: 'async',
      minSize: 30000,
      maxSize: 0,
      minChunks: 1,
      maxAsyncRequests: 5,
      maxInitialRequests: 3,
      automaticNameDelimiter: '~',
      name: true,
      cacheGroups: {
        vendors: {
          test: /[\\/]node_modules[\\/]/,
          priority: -10
        },
        default: {
          minChunks: 2,
          priority: -20,
          reuseExistingChunk: true
        }
      }
    }
  }
};

splitChunks.automaticNameDelimiter

string

By default webpack will generate names using origin and name of the chunk (e.g. vendors~main.js). This option lets you specify the delimiter to use for the generated names.

splitChunks.chunks

function string

This indicates which chunks will be selected for optimization. If a string is provided, possible values are all, async, and initial. Providing all can be particularly powerful because it means that chunks can be shared even between async and non-async chunks.

module.exports = {
  //...
  optimization: {
    splitChunks: {
      // include all types of chunks
      chunks: 'all'
    }
  }
};

Alternatively, you can provide a function for more control. The return value will indicate whether to include each chunk.

module.exports = {
  //...
  optimization: {
    splitChunks: {
      chunks (chunk) {
        // exclude `my-excluded-chunk`
        return chunk.name !== 'my-excluded-chunk';
      }
    }
  }
};

You can combine this configuration with the HtmlWebpackPlugin. It will inject all the generated vendor chunks for you.

splitChunks.maxAsyncRequests

number

Maximum number of parallel requests when on-demand loading.

splitChunks.maxInitialRequests

number

Maximum number of parallel requests at an entry point.

splitChunks.minChunks

number

Minimum number of chunks that must share a module before splitting.

splitChunks.minSize

number

Minimum size, in bytes, for a chunk to be generated.

splitChunks.maxSize

number

Using maxSize (either globally optimization.splitChunks.maxSize per cache group optimization.splitChunks.cacheGroups[x].maxSize or for the fallback cache group optimization.splitChunks.fallbackCacheGroup.maxSize) tells webpack to try to split chunks bigger than maxSize into smaller parts. Parts will be at least minSize (next to maxSize) in size. The algorithm is deterministic and changes to the modules will only have local impact. So that it is usable when using long term caching and doesn't require records. maxSize is only a hint and could be violated when modules are bigger than maxSize or splitting would violate minSize.

When the chunk has a name already, each part will get a new name derived from that name. Depending on the value of optimization.splitChunks.hidePathInfo it will add a key derived from the first module name or a hash of it.

maxSize options is intended to be used with HTTP/2 and long term caching. It increase the request count for better caching. It could also be used to decrease the file size for faster rebuilding.

maxSize takes higher priority than maxInitialRequest/maxAsyncRequests. Actual priority is maxInitialRequest/maxAsyncRequests < maxSize < minSize.

splitChunks.name

boolean: true function string

The name of the split chunk. Providing true will automatically generate a name based on chunks and cache group key. Providing a string or function will allow you to use a custom name. If the name matches an entry point name, the entry point will be removed.

module.exports = {
  //...
  optimization: {
    splitChunks: {
      name (module) {
        // generate a chunk name...
        return; //...
      }
    }
  }
};

When assigning equal names to different split chunks, all vendor modules are placed into a single shared chunk, though it's not recommend since it can result in more code downloaded.

splitChunks.cacheGroups

Cache groups can inherit and/or override any options from splitChunks.*; but test, priority and reuseExistingChunk can only be configured on cache group level. To disable any of the default cache groups, set them to false.

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        default: false
      }
    }
  }
};

splitChunks.cacheGroups.priority

number

A module can belong to multiple cache groups. The optimization will prefer the cache group with a higher priority. The default groups have a negative priority to allow custom groups to take higher priority (default value is 0 for custom groups).

splitChunks.cacheGroups.reuseExistingChunk

boolean

If the current chunk contains modules already split out from the main bundle, it will be reused instead of a new one being generated. This can impact the resulting file name of the chunk.

splitChunks.cacheGroups.test

function RegExp string

Controls which modules are selected by this cache group. Omitting it selects all modules. It can match the absolute module resource path or chunk names. When a chunk name is matched, all modules in the chunk are selected.

module.exports = {
  //...
  optimization: {
    splitChunks: {
      test (chunks) {
        //...
        return true;
      }
    }
  }
};

Examples

Defaults: Example 1

// index.js

import('./a'); // dynamic import

// a.js
import 'react';

//...

Result: A separate chunk would be created containing react. At the import call this chunk is loaded in parallel to the original chunk containing ./a.

Why:

• Condition 1: The chunk contains modules from node_modules
• Condition 2: react is bigger than 30kb
• Condition 3: Number of parallel requests at the import call is 2
• Condition 4: Doesn't affect request at initial page load

What's the reasoning behind this? react probably won't change as often as your application code. By moving it into a separate chunk this chunk can be cached separately from your app code (assuming you are using chunkhash, records, Cache-Control or other long term cache approach).

Defaults: Example 2

// entry.js

// dynamic imports
import('./a');
import('./b');

// a.js
import './helpers'; // helpers is 40kb in size

//...

// b.js
import './helpers';
import './more-helpers'; // more-helpers is also 40kb in size

//...

Result: A separate chunk would be created containing ./helpers and all dependencies of it. At the import calls this chunk is loaded in parallel to the original chunks.

Why:

• Condition 1: The chunk is shared between both import calls
• Condition 2: helpers is bigger than 30kb
• Condition 3: Number of parallel requests at the import calls is 2
• Condition 4: Doesn't affect request at initial page load

Putting the content of helpers into each chunk will result into its code being downloaded twice. By using a separate chunk this will only happen once. We pay the cost of an additional request, which could be considered a tradeoff. That's why there is a minimum size of 30kb.

Split Chunks: Example 1

Create a commons chunk, which includes all code shared between entry points.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        commons: {
          name: 'commons',
          chunks: 'initial',
          minChunks: 2
        }
      }
    }
  }
};

This configuration can enlarge your initial bundles, it is recommended to use dynamic imports when a module is not immediately needed.

Split Chunks: Example 2

Create a vendors chunk, which includes all code from node_modules in the whole application.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        commons: {
          test: /[\\/]node_modules[\\/]/,
          name: 'vendors',
          chunks: 'all'
        }
      }
    }
  }
};

This might result in a large chunk containing all external packages. It is recommended to only include your core frameworks and utilities and dynamically load the rest of the dependencies.

Further Reading

• webpack's automatic deduplication algorithm example
• webpack 4: Code Splitting, chunk graph and the splitChunks optimization