Reduce JavaScript bundle size

[paulrobertlloyd]

We currently use Stimulus to find components in markup and initiate their respective JavaScript modules from NHS.UK frontend.

However, we’re using Stimulus to do a job NHS.UK frontend is already set up to do; it already finds components and initiates any required modules.

Where we may require Stimulus is for our own custom components for which, once you remove those we no longer use, we only have one: the accessible autocomplete component.

Turns out, NHS.UK frontend has an extendable API that lets us register additional components, so why not use that?

Removing unused controllers, and removing Stimulus, we can reduce the generated application bundle by ~20%.

We can also create a separate bundle for parent-facing pages (that doesn’t include Turbo, which doesn’t appear to be used on public pages) that is ~67% smaller!

	Before (bytes)	Remove unused controllers	Remove Stimulus
application.js	239,731	228,826	177,587
public.js	-	-	66,979
application.js.gz	68,196	65,226	53,465
public.js.gz	-	-	23,343

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This PR also does a few other bits of frontend tidy-up:

• Removes test for service worker removed in Precompile fewer assets, remove service worker #4397
• Adds a unit test for the autocomplete component (to check that it gets initiated)
• Removes jest.setup.js and its unused mutationobserver-shim dependency
• Imports styles for the action link component from nhsuk-frontend, and ensures we are using the updated markup introduced for this component in v10

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud