Upgrading an Existing Angular Application to CLI 7.3 for Conditional Polyfills

Jared Youtsey |ng-conf | May 2019

Angular has allowed us to polyfill features back to browsers like IE 9, 10, and 11 for a while using the polyfills.ts file. With the introduction of Angular CLI 7.3 we get conditional polyfills, where we only ship the older polyfills to pre-ES2105 browsers. This results in a savings of about 56Kb. This won’t take you from average to superhero load times, but this is an easy optimization.

If you’re starting a new project, just make sure you have at least @angular/cli@7.3.0 installed globally via npm and ng new away!

If you have an older project there are a few more steps. Here is the outline:

Upgrade global CLI to 7.3

Upgrade project to CLI 7.3

Delete IE polyfills from polyfills.ts

Add flag to angular.json

Validate it’s working

First, upgrade your global installation of the CLI. I recommend uninstalling the old one first for good measure.

Next, upgrade your project to the latest Angular version with:

This will have made a few changes to your package.json, but you may need to make a few more.

typescript version should be > 3.2.0 and < 3.3.0.

@angular-devkit/build-angular version should be at least 0.13.6

Next, delete the polyfills for IE 9, 10, and 11 from the polyfills.ts file:

Now add one flag to angular.json under your project in architect/build/options:

Now, let’s verify that it works.

In your project’s dist folder under your project name you should find a new es2015-polyfills.XXX.js file. This is the file that will be conditionally loaded if the browser requires those polyfills.

How do the browsers know whether or not to load the es2015-polyfills? Open the index.html from your project’s dist folder and look down at the bottom. The following script tags were added by the builder to your index.html during compilation (I’ve formatted these for the purpose of this article):

Notice the nomodule attribute on the es2105-polyfills line. The HTML spec for nomodule says:

The nomodule attribute is a boolean attribute that prevents a script from being executed in user agents that support module scripts.

Basically, if you put nomodule on a script tag, any evergreen browser which supports modules is going to ignore that script. So, your older browsers will download this, but the newer ones will not. This will save you about 56Kb on startup!

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