Serve modern code to modern browsers for faster page loads

In this codelab, improve the performance of this simple application that allows users to rate random cats. Learn how to optimize the JavaScript bundle by minizming how much code is transpiled.

In the sample app, you can select a word or emoji to convey how much you like each cat. When you click a button, the app displays the button's value underneath the current cat image.

Measure

It's always a good idea to begin by inspecting a website before adding any optimizations:

1. To preview the site, press View App. Then press Fullscreen .
2. Press `Control+Shift+J` (or `Command+Option+J` on Mac) to open DevTools.
3. Click the Network tab.
4. Select the Disable cache checkbox.
5. Reload the app.

Over 80 KB is used for this application! Time to find out if parts of the bundle aren't being used:

1. Press Control+Shift+P (or Command+Shift+P on Mac) to open the Command menu.

2. Enter Show Coverage and hit Enter to display the Coverage tab.

3. In the Coverage tab, click Reload to reload the application while capturing coverage.

   

4. Take a look at how much code was used versus how much was loaded for the main bundle:

   

Over half the bundle (44 KB) is not even utilized. This is because a lot of the code within consists of polyfills to ensure that the application works in older browsers.

Use @babel/preset-env

The syntax of the JavaScript language conforms to a standard known as ECMAScript, or ECMA-262. Newer versions of the specification are released every year and include new features that have passed the proposal process. Each major browser is always at a different stage of supporting these features.

The following ES2015 features are used in the application:

• Arrow functions
• Template literals
• For…of loop
• Destructuring assignment

The following ES2017 feature is used as well:

• Async functions

Feel free to dive into the source code in src/index.js to see how all of this is used.

All of these features are supported in the latest version of Chrome, but what about other browsers that don't support them? Babel, which is included in the application, is the most popular library used to compile code that contains newer syntax into code that older browsers and environments can understand. It does this in two ways:

• Polyfills are included to emulate newer ES2015+ functions so that their APIs can be used even if it is not supported by the browser. Here is an example of a polyfill of the Array.includes method.
• Plugins are used to transform ES2015 code (or later) into older ES5 syntax. Since these are syntax related changes (such as arrow functions), they cannot be emulated with polyfills.

Look at package.json to see which Babel libraries are included:

"dependencies": {
  "@babel/polyfill": "^7.0.0"
},
"devDependencies": {
  //...
  "babel-loader": "^8.0.2",
  "@babel/core": "^7.1.0",
  "@babel/preset-env": "^7.1.0",
  //...
}

• @babel/core is the core Babel compiler. With this, all the Babel configurations are defined in a .babelrc at the root of the project.
• babel-loader includes Babel in the webpack build process.

Now look at webpack.config.js to see how babel-loader is included as a rule:

module: {
  rules: [
    //...
    {
      test: /\.js$/,
      exclude: /node_modules/,
      loader: "babel-loader"
    }
  ]
},

• @babel/polyfill provides all the necessary polyfills for any newer ECMAScript features so that they can work in environments that do not support them. It is already imported at the very top of src/index.js.

import "./style.css";
import "@babel/polyfill";

• @babel/preset-env identifies which transforms and polyfills are necessary for any browsers or environments chosen as targets.

Take a look at the Babel configurations file, .babelrc, to see how it's included:

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "targets": "last 2 versions"
      }
    ]
  ]
}

This is a Babel and webpack setup. Learn how to include Babel in your application if you happen to use a different module bundler than webpack.

The targets attribute in .babelrc identifies which browsers are being targeted. @babel/preset-env integrates with browserslist, which means you can find a full list of compatible queries that can be used in this field in the browserlist documentation.

The "last 2 versions" value transpiles the code in the application for the last two versions of every browser.

Debugging

To get a complete look at all the browser's Babel targets as well as all the transforms and polyfills that are included, add a debug field to .babelrc:

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "targets": "last 2 versions",
        "debug": true
      }
    ]
  ]
}

• Click Tools.
• Click Logs.

Reload the application and take a look at the Glitch status logs at the bottom of the editor.

Targeted browsers

Babel logs a number of details to the console about the compilation process, including all the target environments that the code has been compiled for.

Notice how discontinued browsers, such as Internet Explorer, are included in this list. This is a problem because unsupported browsers won't have newer features added, and Babel continues to transpile specific syntax for them. This unnecessarily increases the size of your bundle if users are not using this browser to access your site.

Babel also logs a list of transform plugins used:

That's a pretty long list! These are all the plugins that Babel needs to use to transform any ES2015+ syntax to older syntax for all the targeted browsers.

However, Babel doesn't show any specific polyfills that are used:

This is because the entire @babel/polyfill is being imported directly.

Load polyfills individually

By default, Babel includes every polyfill needed for a complete ES2015+ environment when @babel/polyfill is imported into a file. To import specific polyfills needed for the target browsers, add a useBuiltIns: 'entry' to the configuration.

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "targets": "last 2 versions",
        "debug": true
        "useBuiltIns": "entry"
      }
    ]
  ]
}

Reload the application. You can now see all the specific polyfills included:

Although only needed polyfills for "last 2 versions" is now included, it is still a super long list! This is because polyfills needed for the target browsers for every newer feature is still included. Change the value of the attribute to usage to only include those needed for features that are being used in the code.

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "targets": "last 2 versions",
        "debug": true,
        "useBuiltIns": "entry"
        "useBuiltIns": "usage"
      }
    ]
  ]
}

With this, polyfills are automatically included where needed. This means you can remove the @babel/polyfill import in src/index.js.

import "./style.css";
import "@babel/polyfill";

Now only the required polyfills needed for the application are included.

The application bundle size is reduced significantly.

Narrowing the list of supported browsers

The number of browser targets included is still quite large, and not many users use discontinued browsers such as Internet Explorer. Update the configurations to the following:

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "targets": "last 2 versions",
        "targets": [">0.25%", "not ie 11"],
        "debug": true,
        "useBuiltIns": "usage",
      }
    ]
  ]
}

Take a look at the details for the fetched bundle.

Since the application is so small, there really isn't much of a difference with these changes. However, using a browser market share percentage (such as ">0.25%") along with excluding specific browsers that you are confident your users are not using is the recommended approach. Take a look at the "Last 2 versions" considered harmful article by James Kyle to learn more about this.

Use <script type="module">

There is still more room for improvement. Although a number of unused polyfills have been removed, there are many that are being shipped that are not needed for some browsers. By using modules, newer syntax can be written and shipped to browsers directly without the use of any unnecessary polyfills.

JavaScript modules are a relatively new feature supported in all major browsers. Modules can be created using a type="module" attribute to define scripts that import and export from other modules. For example:

// math.mjs
export const add = (x, y) => x + y;

<!-- index.html -->
<script type="module">
  import { add } from './math.mjs';

  add(5, 2); // 7
</script>

Many newer ECMAScript features are already supported in environments that support JavaScript modules (instead of needing Babel.) This means that the Babel config can be modified to send two different versions of your application to the browser:

• A version that would work in newer browsers that support modules and that includes a module that is largely untranspiled but has a smaller file size
• A version that includes a larger, transpiled script that would work in any legacy browser

Using ES Modules with Babel

To have separate @babel/preset-env settings for the two versions of the application, remove the .babelrc file. Babel settings can be added to the webpack configuration by specifying two different compilation formats for each version of the application.

Begin by adding a configuration for the legacy script to webpack.config.js:

const legacyConfig = {
  entry,
  output: {
    path: path.resolve(__dirname, "public"),
    filename: "[name].bundle.js"
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "babel-loader",
        options: {
          presets: [
            ["@babel/preset-env", {
              useBuiltIns: "usage",
              targets: {
                esmodules: false
              }
            }]
          ]
        }
      },
      cssRule
    ]
  },
  plugins
}

Notice that instead of using the targets value for "@babel/preset-env", esmodules with a value of false is used instead. This means that Babel includes all the necessary transforms and polyfills to target every browser that does not yet support ES modules.

Add entry, cssRule, and corePlugins objects to the beginning of the webpack.config.js file. These are all shared between both the module and legacy scripts served to the browser.

const entry = {
  main: "./src"
};

const cssRule = {
  test: /\.css$/,
  use: ExtractTextPlugin.extract({
    fallback: "style-loader",
    use: "css-loader"
  })
};

const plugins = [
  new ExtractTextPlugin({filename: "[name].css", allChunks: true}),
  new HtmlWebpackPlugin({template: "./src/index.html"})
];

Now similarly, create a config object for the module script below where legacyConfig is defined:

const moduleConfig = {
  entry,
  output: {
    path: path.resolve(__dirname, "public"),
    filename: "[name].mjs"
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: "babel-loader",
        options: {
          presets: [
            ["@babel/preset-env", {
              useBuiltIns: "usage",
              targets: {
                esmodules: true
              }
            }]
          ]
        }
      },
      cssRule
    ]
  },
  plugins
}

The main difference here is that a .mjs file extension is used for the output filename. The esmodules value is set to true here which means that the code that is outputted into this module is a smaller, less compiled script that does not go through any transformation in this example since all the features used are already supported in browsers that support modules.

At the very end of the file, export both configurations in a single array.

module.exports = [
  legacyConfig, moduleConfig
];

Now this builds both a smaller module for browsers that support it and a larger transpiled script for older browsers.

Browsers that support modules ignore scripts with a nomodule attribute. Conversely, browsers that do not support modules ignore script elements with type="module". This means you can include a module as well as a compiled fallback. Ideally, the two versions of the application should be in index.html like this:

<script type="module" src="main.mjs"></script>
<script nomodule src="main.bundle.js"></script>

Browsers that support modules fetch and execute main.mjs and ignore main.bundle.js. The browsers that do not support modules do the opposite.

It is important to note that unlike regular scripts, module scripts are always deferred by default. If you would like the equivalent nomodule script to also be deferred and only executed after parsing, then you'll need to add the defer attribute:

<script type="module" src="main.mjs"></script>
<script nomodule src="main.bundle.js" defer></script>

The last thing that needs to be done here is to add the module and nomodule attributes to the module and legacy script respectively, Import the ScriptExtHtmlWebpackPlugin at the very top of webpack.config.js:

const path = require("path");

const webpack = require("webpack");
const HtmlWebpackPlugin = require("html-webpack-plugin");
const ScriptExtHtmlWebpackPlugin = require("script-ext-html-webpack-plugin");

Now update the plugins array in the configurations to include this plugin:

const plugins = [
  new ExtractTextPlugin({filename: "[name].css", allChunks: true}),
  new HtmlWebpackPlugin({template: "./src/index.html"}),
  new ScriptExtHtmlWebpackPlugin({
    module: /\.mjs$/,
    custom: [
      {
        test: /\.js$/,
        attribute: 'nomodule',
        value: ''
    },
    ]
  })
];

These plugin settings add a type="module" attribute for all .mjs script elements as well as a nomodule attribute for all .js script modules.

Serving modules in the HTML document

The last thing that needs to be done is to output both the legacy and modern script elements to the HTML file. Unfortunately, the plugin that creates the final HTML file, HTMLWebpackPlugin, does not currently support the output of both the module and nomodule scripts. Although there are workarounds and separate plugins created to solve this problem, such as BabelMultiTargetPlugin and HTMLWebpackMultiBuildPlugin, a simpler approach of adding the module script element manually is used for the purpose of this tutorial.

Add the following to src/index.js at the end of the file:

    ...
    </form>
    <script type="module" src="main.mjs"></script>
  </body>
</html>

Now load the application in a browser that supports modules, such as the latest version of Chrome.

Only the module is fetched, with a much smaller bundle size due to it being largely untranspiled! The other script element is completely ignored by the browser.

If you load the application on an older browser, only the larger, transpiled script with all the needed polyfills and transforms are be fetched. Here is a screenshot for all the requests made on an older version of Chrome (version 38).

Conclusion

You now understand how to use @babel/preset-env to provide only the necessary polyfills required for targeted browsers. You also know how JavaScript modules can improve performance further by shipping two different transpiled versions of an application. With a decent understanding of how both these techniques can cut your bundle size down significantly, go forth and optimize!