Shimming

The webpack compiler can understand modules written as ES2015 modules, CommonJS or AMD. However, some third party libraries may expect global dependencies (e.g. $ for jQuery). The libraries might also create globals which need to be exported. These "broken modules" are one instance where shimming comes into play.

Another instance where shimming can be useful is when you want to polyfill browser functionality to support more users. In this case, you may only want to deliver those polyfills to the browsers that need patching (i.e. load them on demand).

The following article will walk through both of these use cases.

Shimming Globals

Let's start with the first use case of shimming global variables. Before we do anything let's take another look at our project:

project

webpack-demo
|- package.json
|- webpack.config.js
|- /dist
  |- index.html
|- /src
  |- index.js
|- /node_modules

Remember that lodash package we were using? For demonstration purposes, let's say we wanted to instead provide this as a global throughout our application. To do this, we can use ProvidePlugin.

The ProvidePlugin makes a package available as a variable in every module compiled through webpack. If webpack sees that variable used, it will include the given package in the final bundle. Let's go ahead by removing the import statement for lodash and instead provide it via the plugin:

src/index.js

-import _ from 'lodash';
-
 function component() {
   const element = document.createElement('div');
 
-  // Lodash, now imported by this script
   element.innerHTML = _.join(['Hello', 'webpack'], ' ');
 
   return element;
 }
 
 document.body.appendChild(component());

webpack.config.js

 const path = require('path');
+const webpack = require('webpack');
 
 module.exports = {
   entry: './src/index.js',
   output: {
     filename: 'main.js',
     path: path.resolve(__dirname, 'dist'),
   },
+  plugins: [
+    new webpack.ProvidePlugin({
+      _: 'lodash',
+    }),
+  ],
 };

What we've essentially done here is tell webpack...

If you encounter at least one instance of the variable _, include the lodash package and provide it to the modules that need it.

If we run a build, we should still see the same output:

$ npm run build

..

[webpack-cli] Compilation finished
asset main.js 69.1 KiB [emitted] [minimized] (name: main) 1 related asset
runtime modules 344 bytes 2 modules
cacheable modules 530 KiB
  ./src/index.js 191 bytes [built] [code generated]
  ./node_modules/lodash/lodash.js 530 KiB [built] [code generated]
webpack 5.4.0 compiled successfully in 2910 ms

We can also use the ProvidePlugin to expose a single export of a module by configuring it with an "array path" (e.g. [module, child, ...children?]). So let's imagine we only wanted to provide the join method from lodash wherever it's invoked:

src/index.js

 function component() {
   const element = document.createElement('div');
 
-  element.innerHTML = _.join(['Hello', 'webpack'], ' ');
+  element.innerHTML = join(['Hello', 'webpack'], ' ');
 
   return element;
 }
 
 document.body.appendChild(component());

webpack.config.js

 const path = require('path');
 const webpack = require('webpack');
 
 module.exports = {
   entry: './src/index.js',
   output: {
     filename: 'main.js',
     path: path.resolve(__dirname, 'dist'),
   },
   plugins: [
     new webpack.ProvidePlugin({
-      _: 'lodash',
+      join: ['lodash', 'join'],
     }),
   ],
 };

This would go nicely with Tree Shaking as the rest of the lodash library should get dropped.

Granular Shimming

Some legacy modules rely on this being the window object. Let's update our index.js so this is the case:

 function component() {
   const element = document.createElement('div');
 
   element.innerHTML = join(['Hello', 'webpack'], ' ');
 
+  // Assume we are in the context of `window`
+  this.alert("Hmmm, this probably isn't a great idea...");
+
   return element;
 }
 
 document.body.appendChild(component());

This becomes a problem when the module is executed in a CommonJS context where this is equal to module.exports. In this case you can override this using the imports-loader:

webpack.config.js

 const path = require('path');
 const webpack = require('webpack');
 
 module.exports = {
   entry: './src/index.js',
   output: {
     filename: 'main.js',
     path: path.resolve(__dirname, 'dist'),
   },
+  module: {
+    rules: [
+      {
+        test: require.resolve('./src/index.js'),
+        use: 'imports-loader?wrapper=window',
+      },
+    ],
+  },
   plugins: [
     new webpack.ProvidePlugin({
       join: ['lodash', 'join'],
     }),
   ],
 };

Global Exports

Let's say a library creates a global variable that it expects its consumers to use. We can add a small module to our setup to demonstrate this:

project

  webpack-demo
  |- package.json
  |- webpack.config.js
  |- /dist
  |- /src
    |- index.js
+   |- globals.js
  |- /node_modules

src/globals.js

const file = 'blah.txt';
const helpers = {
  test: function () {
    console.log('test something');
  },
  parse: function () {
    console.log('parse something');
  },
};

Now, while you'd likely never do this in your own source code, you may encounter a dated library you'd like to use that contains similar code to what's shown above. In this case, we can use exports-loader, to export that global variable as a normal module export. For instance, in order to export file as file and helpers.parse as parse:

webpack.config.js

 const path = require('path');
 const webpack = require('webpack');
 
 module.exports = {
   entry: './src/index.js',
   output: {
     filename: 'main.js',
     path: path.resolve(__dirname, 'dist'),
   },
   module: {
     rules: [
       {
         test: require.resolve('./src/index.js'),
         use: 'imports-loader?wrapper=window',
       },
+      {
+        test: require.resolve('./src/globals.js'),
+        use:
+          'exports-loader?type=commonjs&exports[]=file&exports[]=multiple|helpers.parse|parse',
+      },
     ],
   },
   plugins: [
     new webpack.ProvidePlugin({
       join: ['lodash', 'join'],
     }),
   ],
 };

Now from within our entry script (i.e. src/index.js), we could use const { file, parse } = require('./globals.js'); and all should work smoothly.

Loading Polyfills

Almost everything we've discussed thus far has been in relation to handling legacy packages. Let's move on to our second topic: polyfills.

There's a lot of ways to load polyfills. For example, to include the babel-polyfill we might simply:

npm install --save babel-polyfill

and import it so as to include it in our main bundle:

src/index.js

+import 'babel-polyfill';
+
 function component() {
   const element = document.createElement('div');
 
   element.innerHTML = join(['Hello', 'webpack'], ' ');
 
   // Assume we are in the context of `window`
   this.alert("Hmmm, this probably isn't a great idea...");
 
   return element;
 }
 
 document.body.appendChild(component());

Note that this approach prioritizes correctness over bundle size. To be safe and robust, polyfills/shims must run before all other code, and thus either need to load synchronously, or, all app code needs to load after all polyfills/shims load. There are many misconceptions in the community, as well, that modern browsers "don't need" polyfills, or that polyfills/shims merely serve to add missing features - in fact, they often repair broken implementations, even in the most modern of browsers. The best practice thus remains to unconditionally and synchronously load all polyfills/shims, despite the bundle size cost this incurs.

If you feel that you have mitigated these concerns and wish to incur the risk of brokenness, here's one way you might do it: Let's move our import to a new file and add the whatwg-fetch polyfill:

npm install --save whatwg-fetch

src/index.js

-import 'babel-polyfill';
-
 function component() {
   const element = document.createElement('div');
 
   element.innerHTML = join(['Hello', 'webpack'], ' ');
 
   // Assume we are in the context of `window`
   this.alert("Hmmm, this probably isn't a great idea...");
 
   return element;
 }
 
 document.body.appendChild(component());

project

  webpack-demo
  |- package.json
  |- webpack.config.js
  |- /dist
  |- /src
    |- index.js
    |- globals.js
+   |- polyfills.js
  |- /node_modules

src/polyfills.js

import 'babel-polyfill';
import 'whatwg-fetch';

webpack.config.js

 const path = require('path');
 const webpack = require('webpack');
 
 module.exports = {
-  entry: './src/index.js',
+  entry: {
+    polyfills: './src/polyfills',
+    index: './src/index.js',
+  },
   output: {
-    filename: 'main.js',
+    filename: '[name].bundle.js',
     path: path.resolve(__dirname, 'dist'),
   },
   module: {
     rules: [
       {
         test: require.resolve('./src/index.js'),
         use: 'imports-loader?wrapper=window',
       },
       {
         test: require.resolve('./src/globals.js'),
         use:
           'exports-loader?type=commonjs&exports[]=file&exports[]=multiple|helpers.parse|parse',
       },
     ],
   },
   plugins: [
     new webpack.ProvidePlugin({
       join: ['lodash', 'join'],
     }),
   ],
 };

With that in place, we can add the logic to conditionally load our new polyfills.bundle.js file. How you make this decision depends on the technologies and browsers you need to support. We'll just do some simple testing to determine whether our polyfills are needed:

dist/index.html

 <!DOCTYPE html>
 <html>
   <head>
     <meta charset="utf-8" />
     <title>Getting Started</title>
+    <script>
+      const modernBrowser = 'fetch' in window && 'assign' in Object;
+
+      if (!modernBrowser) {
+        const scriptElement = document.createElement('script');
+
+        scriptElement.async = false;
+        scriptElement.src = '/polyfills.bundle.js';
+        document.head.appendChild(scriptElement);
+      }
+    </script>
   </head>
   <body>
-    <script src="main.js"></script>
+    <script src="index.bundle.js"></script>
   </body>
 </html>

Now we can fetch some data within our entry script:

src/index.js

 function component() {
   const element = document.createElement('div');
 
   element.innerHTML = join(['Hello', 'webpack'], ' ');
 
   // Assume we are in the context of `window`
   this.alert("Hmmm, this probably isn't a great idea...");
 
   return element;
 }
 
 document.body.appendChild(component());
+
+fetch('https://jsonplaceholder.typicode.com/users')
+  .then((response) => response.json())
+  .then((json) => {
+    console.log(
+      "We retrieved some data! AND we're confident it will work on a variety of browser distributions."
+    );
+    console.log(json);
+  })
+  .catch((error) =>
+    console.error('Something went wrong when fetching this data: ', error)
+  );

If we run our build, another polyfills.bundle.js file will be emitted and everything should still run smoothly in the browser. Note that this set up could likely be improved upon but it should give you a good idea of how you can provide polyfills only to the users that actually need them.

Further Optimizations

The babel-preset-env package uses browserslist to transpile only what is not supported in your browsers matrix. This preset comes with the useBuiltIns option, false by default, which converts your global babel-polyfill import to a more granular feature by feature import pattern:

import 'core-js/modules/es7.string.pad-start';
import 'core-js/modules/es7.string.pad-end';
import 'core-js/modules/web.timers';
import 'core-js/modules/web.immediate';
import 'core-js/modules/web.dom.iterable';

See the babel-preset-env documentation for more information.

Node Built-Ins

Node built-ins, like process, can be polyfilled right directly from your configuration file without the use of any special loaders or plugins. See the node configuration page for more information and examples.

Other Utilities

There are a few other tools that can help when dealing with legacy modules.

When there is no AMD/CommonJS version of the module and you want to include the dist, you can flag this module in noParse. This will cause webpack to include the module without parsing it or resolving require() and import statements. This practice is also used to improve the build performance.

Lastly, there are some modules that support multiple module styles; e.g. a combination of AMD, CommonJS, and legacy. In most of these cases, they first check for define and then use some quirky code to export properties. In these cases, it could help to force the CommonJS path by setting additionalCode=var%define%20=%20false; via the imports-loader.