Drop support for legacy browsers

[AshGDS]

What / Why

• Adds type=module to our JavaScript to drop support for legacy browsers
• Moves the js-enabled class so it only works in ES6 browsers
• Deletes polyfill files that are no longer needed
• We will combine es6-components.js again as a separate task unless you disagree with this
• Trello card: https://trello.com/c/Hoa45SMD/141-turn-off-javascript-in-legacy-browsers
• type=module shouldn't have an affect on how our JS is executed, because we use DOMContentLoaded, and according to MDN:

The DOMContentLoaded event fires when the HTML document has been completely parsed, and all deferred scripts (<script defer src="…"> and <script type="module">) have downloaded and executed.

• In legacy browsers, this will stop the static layout_for_public JS from executing. The application specific JS will stop running once the other frontend app PRs are merged. This means for a period of time between static being deployed, and the frontend apps being deployed, the legacy browsers may have their application JS being executed, but the JS disabled styles from this gem. I'm not sure the full impact of this, and I'm not sure it will be that big of an issue. I did test this scenario in finder-frontend, and the finder-frontend search had the non-JS styles from this gem, but it still had the client-side realtime search from it's own application.js. The page functionality still worked fine in that example, so I don't think there will be an issue. And I guess we can try and deploy the frontend apps quickly to avoid any issues once this change is deployed.

• I've put this as a breaking change in the changelog, since it stops the JS working in old browsers.

Visual Changes

Browsers that do not support ES6 will have the non js-enabled styles.

[MartinJJones]

Changes look good to me overall, just a couple of things to look at, happy to discuss further 👍

[MartinJJones]

Nice work Ash, just a few more questions and suggestions.

strict mode

This change was originally included in the tech spike as modules will always run in strict mode, but the change is not included here, this may be fine as it appears standardx will capture strict mode JS errors when running js:lint or bundle exec rake locally or as part of the CI, but won't be captured in the jasmine tests, just want to check if this is something we need to explore further now or in the future?

Thinking about the scope of the PR to Drop support for legacy browsers, there could be a couple more changes that are worth including, non-blocking for this PR, just want to highlight them as it seems a good opportunity.

Remove ie.js

This file is served to Internet Explorer 8 and below in layout_for_public

This file bundles together html5shiv-printshiv.js and json2 which can also be removed, json2.js example from the readme 😅

There is no reason to use this file unless fate compels you to support IE8, which is something that no one should ever have to do again.

Remove X-UA-Compatible meta tag
Removing the X-UA-Compatible meta tag is a recommendation in the release notes of v5.0.0 of govuk-frontend, I did not remove this when completing the upgrade work to V5 as we still provided some level of support for legacy browsers.

The change would involve removing the tag and related comment in layout_for_public, interestingly it does not appear to be used in the admin template.

Not essential, and can always come back to this as part of another cleanup PR when looking at removing/updating CSS/JS where we support legacy browsers, IE8 code comments example

[AshGDS]

Thanks @MartinJJones 👍

1. Regarding use strict, because the goal of this is to just drop support for grade X browsers, the extra work of adding use strict is not needed as type=module achieves what we want in this case.

2. I've removed ie.js in the latest commits, good spot, thanks for flagging this.

3. Thanks, I've removed this meta tag in the latest commits.

[MartinJJones]

Great work on this Ash! 👍