The externals configuration option provides a way of excluding dependencies from the output bundles. Instead, the created bundle relies on that dependency to be present in the consumer's environment. This feature is typically most useful to library developers, however there are a variety of applications for it.

consumer here is any end-user application.

externals

string object function regex

Prevent bundling of certain imported packages and instead retrieve these external dependencies at runtime.

For example, to include jQuery from a CDN instead of bundling it:

index.html

<script
  src="https://code.jquery.com/jquery-3.1.0.js"
  integrity="sha256-slogkvB1K3VOkzAI8QITxV3VzpOnkeNVsKvtkYLMjfk="
  crossorigin="anonymous">
</script>

webpack.config.js

module.exports = {
  //...
  externals: {
    jquery: 'jQuery'
  }
};

This leaves any dependent modules unchanged, i.e. the code shown below will still work:

import $ from 'jquery';

$('.my-element').animate(/* ... */);

The bundle with external dependencies can be used in various module contexts, such as CommonJS, AMD, global and ES2015 modules. The external library may be available in any of these forms:

• root: The library should be available as a global variable (e.g. via a script tag).
• commonjs: The library should be available as a CommonJS module.
• commonjs2: Similar to the above but where the export is module.exports.default.
• amd: Similar to commonjs but using AMD module system.

The following syntaxes are accepted...

string

See the example above. The property name jquery indicates that the module jquery in import $ from 'jquery' should be excluded. In order to replace this module, the value jQuery will be used to retrieve a global jQuery variable. In other words, when a string is provided it will be treated as root (defined above and below).

array

module.exports = {
  //...
  externals: {
    subtract: ['./math', 'subtract']
  }
};

subtract: ['./math', 'subtract'] allows you select part of a commonjs module, where ./math is the module and your bundle only requires the subset under the subtract variable. This example would translate to require('./math').subtract;

object

An object with { root, amd, commonjs, ... } is only allowed for libraryTarget: 'umd'. It's not allowed for other library targets.

module.exports = {
  //...
  externals : {
    react: 'react'
  },

  // or

  externals : {
    lodash : {
      commonjs: 'lodash',
      amd: 'lodash',
      root: '_' // indicates global variable
    }
  },

  // or

  externals : {
    subtract : {
      root: ['math', 'subtract']
    }
  }
};

This syntax is used to describe all the possible ways that an external library can be available. lodash here is available as lodash under AMD and CommonJS module systems but available as _ in a global variable form. subtract here is available via the property subtract under the global math object (e.g. window['math']['subtract']).

function

It might be useful to define your own function to control the behavior of what you want to externalize from webpack. webpack-node-externals, for example, excludes all modules from the node_modules directory and provides some options too, for example, whitelist packages.

It basically comes down to this:

module.exports = {
  //...
  externals: [
    function(context, request, callback) {
      if (/^yourregex$/.test(request)){
        return callback(null, 'commonjs ' + request);
      }
      callback();
    }
  ]
};

The 'commonjs ' + request defines the type of module that needs to be externalized.

regex

Every dependency that matches the given regular expression will be excluded from the output bundles.

module.exports = {
  //...
  externals: /^(jquery|\$)$/i
};

In this case, any dependency named jQuery, capitalized or not, or $ would be externalized.

Combining syntaxes

Sometimes you may want to use a combination of the above syntaxes. This can be done in the following manner:

module.exports = {
  //...
  externals: [
    {
      // String
      react: 'react',
      // Object
      lodash : {
        commonjs: 'lodash',
        amd: 'lodash',
        root: '_' // indicates global variable
      },
      // Array
      subtract: ['./math', 'subtract']
    },
    // Function
    function(context, request, callback) {
      if (/^yourregex$/.test(request)){
        return callback(null, 'commonjs ' + request);
      }
      callback();
    },
    // Regex
    /^(jquery|\$)$/i
  ]
};

For more information on how to use this configuration, please refer to the article on how to author a library.