"Differential Loading" - A New Feature of Angular CLI v8

TL;DR

• Angular CLI understands browsers support range from browserslist configuration.
• If the application needs to support ES5 browsers and TypeScript target is higher than es5, the CLI automatically makes additional bundles for compatibility.
• browserslist is the single source of truth, so es5BrowserSupport will be deprecated.

Angular CLI v8 (now in beta.15) ships new feature called "Differential Loading". It allows us to get free from considering browser compatibility of your application.

The CLI can understand browsers which the app needs to support and can make different bundles for both ES5 browsers and not.

How to use

To enable differential loading, the app must have browserslist configuration. It can be placed in package.json or browserslist file. This configuration has already been used by autoprefixer process of postcss. Apps created recently by CLI should contain it and you can find it in the project.

browserslist/browserslist

Even if you don't have it now, you can create easily with online demo. Angular CLI can look it up if browserslist file is placed at the same directory to package.json .

Preparation is over! If your tsconfig's target is out of browser range determined by browserslist , Angular CLI will separate bundles; one is for original target, and another is for ES5 browsers.

For example, let's support the latest 2 versions of Chrome and IE 11. browserslist is the following;

last 2 Chrome versions, IE 11

And tsconfig.json is like below.

{
  "compilerOptions": {
    "target": "es2015",
        ...
  }
}

As you may know, IE11 is an ES5 browser. So without differential loading, this application will throw errors on IE11 because of missing es2015 features like arrow functions, class or etc...

With differential loading, Angular CLI understand this problem in advance. The CLI judges whether the app has to support ES5 browsers, and check the current tsconfig's target can support them.
If they are mismatched, all bundles are separated as like main-es5.bundle.js and main-es2015.bundle.js.

Then, <script> tags for ES5 bundles are placed with nomodule attribute. It avoids loading ES5 bundles on non-ES5 browsers. As a result, on modern browsers, users will load smaller bundles just that the browser needs. It can improve loading performance.

How about es5BrowserSupport option?

Yes, Angular CLI v7.3 added a feature like differential loading but it is only for polyfills. It uses es5BrowserSupport option in angular.json .

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After Angular CLI v8, it will be deprecated because it is not simple to manage supporting browsers in both of browserslist for CSS and es5BrowserSupport for JavaScript. So the CLI team adopt browserslist as the single source of truth to judge whether the application needs to support ES5 browsers.

Conclusion

• Differential loading has been landed in Angular CLI v8 beta.
• CLI uses browserslist to judge the application needs to support ES5 browsers.
• If tsconfig doesn't match that, CLI adds different bundles loaded only by ES5 browsers.

To try the feature, let's create an application with the following command;

$ npx @angular/cli@next new example-app
$ cd example-app
$ npm run build

Thanks for reading!