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 well 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 20kb (before min+gz)
• Maximum number of parallel requests when loading chunks on demand would be lower or equal to 30
• Maximum number of parallel requests at initial page load would be lower or equal to 30

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

Configuration

webpack provides a set of options for developers that want more control over this functionality.

The default configuration was chosen to fit web performance best practices, but the optimal strategy for your project might differ. If you're changing the configuration, you should measure the impact of your changes to ensure there's a real benefit.

optimization.splitChunks

This configuration object represents the default behavior of the SplitChunksPlugin.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      chunks: 'async',
      minSize: 20000,
      minRemainingSize: 0,
      maxSize: 0,
      minChunks: 1,
      maxAsyncRequests: 30,
      maxInitialRequests: 30,
      automaticNameDelimiter: '~',
      enforceSizeThreshold: 50000,
      cacheGroups: {
        defaultVendors: {
          test: /[\\/]node_modules[\\/]/,
          priority: -10,
          reuseExistingChunk: true
        },
        default: {
          minChunks: 2,
          priority: -20,
          reuseExistingChunk: true
        }
      }
    }
  }
};

When files paths are processed by webpack, they always contain / on Unix systems and \ on Windows. That's why using [\\/] in {cacheGroup}.test fields is necessary to represent a path separator. / or \ in {cacheGroup}.test will cause issues when used cross-platform.

Since webpack 5, passing an entry name to {cacheGroup}.test and using a name of an existing chunk for {cacheGroup}.name is no longer allowed.

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

string = 'async' function (chunk)

This indicates which chunks will be selected for optimization. When a string is provided, valid 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.

webpack.config.js

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

Alternatively, you may 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 = 30

Maximum number of parallel requests when on-demand loading.

splitChunks.maxInitialRequests

number = 30

Maximum number of parallel requests at an entry point.

splitChunks.minChunks

number = 1

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

splitChunks.minSize

number = 20000

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

splitChunks.enforceSizeThreshold

splitChunks.cacheGroups.{cacheGroup}.enforceSizeThreshold

number = 50000

Size threshold at which splitting is enforced and other restrictions (minRemainingSize, maxAsyncRequests, maxInitialRequests) are ignored.

splitChunks.minRemainingSize

splitChunks.cacheGroups.{cacheGroup}.minRemainingSize

number = 0

splitChunks.minRemainingSize option was introduced in webpack 5 to avoid zero sized modules by ensuring that the minimum size of the chunk which remains after splitting is above a limit. Defaults to 0 in 'development' mode. For other cases splitChunks.minRemainingSize defaults to the value of splitChunks.minSize so it doesn't need to be specified manually except for the rare cases where deep control is required.

splitChunks.minRemainingSize only takes effect when a single chunk is remaining.

splitChunks.maxSize

number = 0

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 bytes 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 option is intended to be used with HTTP/2 and long term caching. It increases 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.

Setting the value for maxSize sets the value for both maxAsyncSize and maxInitialSize.

splitChunks.maxAsyncSize

number

Like maxSize, maxAsyncSize can be applied globally (splitChunks.maxAsyncSize), to cacheGroups (splitChunks.cacheGroups.{cacheGroup}.maxAsyncSize), or to the fallback cache group (splitChunks.fallbackCacheGroup.maxAsyncSize).

The difference between maxAsyncSize and maxSize is that maxAsyncSize will only affect on-demand loading chunks.

splitChunks.maxInitialSize

number

Like maxSize, maxInitialSize can be applied globally (splitChunks.maxInitialSize), to cacheGroups (splitChunks.cacheGroups.{cacheGroup}.maxInitialSize), or to the fallback cache group (splitChunks.fallbackCacheGroup.maxInitialSize).

The difference between maxInitialSize and maxSize is that maxInitialSize will only affect initial load chunks.

splitChunks.name

boolean = false function (module, chunks, cacheGroupKey) => string string

Also available for each cacheGroup: splitChunks.cacheGroups.{cacheGroup}.name.

The name of the split chunk. Providing false will keep the same name of the chunks so it doesn't change names unnecessarily. It is the recommended value for production builds.

Providing a string or a function allows you to use a custom name. Specifying either a string or a function that always returns the same string will merge all common modules and vendors into a single chunk. This might lead to bigger initial downloads and slow down page loads.

If you choose to specify a function, you may find the chunk.name and chunk.hash properties (where chunk is an element of the chunks array) particularly useful in choosing a name for your chunk.

If the splitChunks.name matches an entry point name, the entry point will be removed.

main.js

import _ from 'lodash';

console.log(_.join(['Hello', 'webpack'], ' '));

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        commons: {
          test: /[\\/]node_modules[\\/]/,
          // cacheGroupKey here is `commons` as the key of the cacheGroup
          name(module, chunks, cacheGroupKey) {
            const moduleFileName = module.identifier().split('/').reduceRight(item => item);
            const allChunksNames = chunks.map((item) => item.name).join('~');
            return `${cacheGroupKey}-${allChunksNames}-${moduleFileName}`;
          },
          chunks: 'all'
        }
      }
    }
  }
};

Running webpack with following splitChunks configuration would also output a chunk of the group common with next name: commons-main-lodash.js.e7519d2bb8777058fa27.js (hash given as an example of real world output).

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.automaticNamePrefix

string = ''

Sets the name prefix for created chunks.

module.exports = {
  //...
  optimization: {
    splitChunks: {
      automaticNamePrefix: 'general-prefix',
      cacheGroups: {
        react: {
          // ...
          automaticNamePrefix: 'react-chunks-prefix'
        }
      }
    }
  }
};

splitChunks.usedExports

splitChunks.cacheGroups{cacheGroup}.usedExports

boolean = true

Figure out which exports are used by modules to mangle export names, omit unused exports and generate more efficient code. When it is true: analyse used exports for each runtime, when it is "global": analyse exports globally for all runtimes combined).

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.

webpack.config.js

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

splitChunks.cacheGroups.{cacheGroup}.priority

number = -20

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.{cacheGroup}.reuseExistingChunk

boolean = true

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.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        defaultVendors: {
          reuseExistingChunk: true
        }
      }
    }
  }
};

splitChunks.cacheGroups.{cacheGroup}.type

function RegExp string

Allows to assign modules to a cache group by module type.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        json: {
          type: 'json'
        }
      }
    }
  }
};

splitChunks.cacheGroups.test

splitChunks.cacheGroups.{cacheGroup}.test

function (module, chunk) => boolean 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.

Providing a function to{cacheGroup}.test:

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        svgGroup: {
          test(module, chunks) {
            // `module.resource` contains the absolute path of the file on disk.
            // Note the usage of `path.sep` instead of / or \, for cross-platform compatibility.
            const path = require('path');
            return module.resource &&
                 module.resource.endsWith('.svg') &&
                 module.resource.includes(`${path.sep}cacheable_svgs${path.sep}`);
          }
        },
        byModuleTypeGroup: {
          test(module, chunks) {
            return module.type === 'javascript/auto';
          }
        }
      }
    }
  }
};

In order to see what information is available in module and chunks objects, you can put debugger; statement in the callback. Then run your webpack build in debug mode to inspect the parameters in Chromium DevTools.

Providing a RegExp to {cacheGroup}.test:

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        defaultVendors: {
          // Note the usage of `[\\/]` as a path separator for cross-platform compatibility.
          test: /[\\/]node_modules[\\/]|vendor[\\/]analytics_provider|vendor[\\/]other_lib/
        }
      }
    }
  }
};

splitChunks.cacheGroups.{cacheGroup}.filename

string function (pathData, assetInfo) => string

Allows to override the filename when and only when it's an initial chunk. All placeholders available in output.filename are also available here.

This option can also be set globally in splitChunks.filename, but this isn't recommended and will likely lead to an error if splitChunks.chunks is not set to 'initial'. Avoid setting it globally.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        defaultVendors: {
          filename: '[name].bundle.js'
        }
      }
    }
  }
};

And as a function:

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        defaultVendors: {
          filename: (pathData) => {
            // Use pathData object for generating filename string based on your requirements
            return `${pathData.chunk.name}-bundle.js`;
          }
        }
      }
    }
  }
};

It is possible to create a folder structure by providing path prefixing the filename: 'js/vendor/bundle.js'.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        defaultVendors: {
          filename: 'js/[name]/bundle.js'
        }
      }
    }
  }
};

splitChunks.cacheGroups.{cacheGroup}.enforce

boolean = false

Tells webpack to ignore splitChunks.minSize, splitChunks.minChunks, splitChunks.maxAsyncRequests and splitChunks.maxInitialRequests options and always create chunks for this cache group.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        defaultVendors: {
          enforce: true
        }
      }
    }
  }
};

splitChunks.cacheGroups.{cacheGroup}.idHint

string

Sets the hint for chunk id. It will be added to chunk's filename.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        defaultVendors: {
          idHint: 'vendors'
        }
      }
    }
  }
};

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.

Split Chunks: Example 3

Create a custom vendor chunk, which contains certain node_modules packages matched by RegExp.

webpack.config.js

module.exports = {
  //...
  optimization: {
    splitChunks: {
      cacheGroups: {
        vendor: {
          test: /[\\/]node_modules[\\/](react|react-dom)[\\/]/,
          name: 'vendor',
          chunks: 'all',
        }
      }
    }
  }
};

This will result in splitting react and react-dom into a separate chunk. If you're not sure what packages have been included in a chunk you may refer to Bundle Analysis section for details.