Allow ES2015+ supported features

[colinrotherham]

This PR pulls together a few ES2015+ code changes with their supporting ESLint config

These were recommendations from the ESLint stylistic typed linting plugin in #4106

Already supported by ES modules

• ES2015 Array.from() Done in #4322
• ES2015 ''.includes()
• ES2016 [].includes()
• ES2017 Object.entries() Done in #4322

Total bundle size

all.mjs 69.3 KiB 68.66 KiB

Babel transforms without polyfills

• ES2020 ?. optional chaining (+ 0.09 KiB)
• ES2020 ?? nullish coalescing (+ 0.55 KiB)

Total bundle size

all.mjs 68.66 KiB 69.3 KiB

Transform examples

Note the following size increases using != versus !== to check for nullish rather than falsy values

Before

if ($summary?.parentNode) {
  // ...
}

if (config.scope ?? document) {
  // ...
}

After

if ($summary != null && $summary.parentNode) {
  // ...
}

var _config$scope
if ((_config$scope = config.scope) != null ? _config$scope : document)) {
  // ...
}

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-5.0.0-beta.1.min.css	114 KiB
dist/govuk-frontend-5.0.0-beta.1.min.js	37.93 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	77.58 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	72.89 KiB
packages/govuk-frontend/dist/govuk/all.mjs	3.86 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	113.53 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	38.11 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.38 KiB

Modules

File	Size
all.mjs	69.22 KiB
components/accordion/accordion.mjs	21.47 KiB
components/button/button.mjs	4.5 KiB
components/character-count/character-count.mjs	21.05 KiB
components/checkboxes/checkboxes.mjs	5.63 KiB
components/error-summary/error-summary.mjs	5.89 KiB
components/exit-this-page/exit-this-page.mjs	15.89 KiB
components/header/header.mjs	3.71 KiB
components/notification-banner/notification-banner.mjs	4.34 KiB
components/radios/radios.mjs	4.63 KiB
components/skip-link/skip-link.mjs	3.61 KiB
components/tabs/tabs.mjs	9.47 KiB

View stats and visualisations on the review app

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Action run for 47f697d

[romaricpascal]

Nicely handy features to be able to use 😊