Enable ESLint stylistic typed linting in source code

[colinrotherham]

This PR enables ESLint stylistic typed linting

It highlights opportunities to replace older JavaScript with new, but also uses TypeScript { "strict": true } to add warnings when our GOV.UK Frontend source code is too loose or unnecessarily defensive

As part of #4323 we've already fixed:

• Class fields with incorrect null default
• Selector null return values not handled
• Timeout and interval IDs cleared when null
• Confg allows null as typeof value === 'object'

But this PR includes a few more:

1. Potentially null attribute NamedNodeMap values
2. Potentially null element .parentNode values
3. Potentially null element .textContent values

Whilst quite pedantic for internal code, it's a huge safety net as we clarify our public API and catch issues like:

• Improve SupportError when document.body is not set #4394

Stylistic typed linting

This change lets ESLint use types to reduce unnecessary code with suggestions to use modern syntax

Strict mode checks (existing)

Here's a list of the compiler options we already enabled in the past:

• strictBindCallApply – Function bind, call and apply must be checked (added in 73a706a)
• strictFunctionTypes – Function parameters must be checked (added in 73a706a)
• noImplicitThis – Context for this must be checked (added in 8b3d6f3)

Strict mode checks (new)

Here's a list of the compiler options enabled in this PR:

• alwaysStrict – Assume code has "use strict"
• strictNullChecks – Possible null and undefined must be checked
• strictPropertyInitialization – Class field values must be checked
• noImplicitAny – All types must be added (e.g. in JSDoc)
• noUncheckedIndexedAccess – Array and object key/values must be checked
• useUnknownInCatchVariables – Assume errors are unknown until error instanceof

[romaricpascal]

Cheers! Not sure it's one we'll merge straight away, but it's good to have the steps to enable it. I'm also wondering if us throwing (rather than returning early) during initialisation will not remove a couple of these "watch out it may be null" warnings, which would be nice and may make the list of things to fix less daunting 😊

[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	77.67 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	72.97 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	38.14 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.38 KiB

Modules

File	Size
all.mjs	69.3 KiB
components/accordion/accordion.mjs	21.55 KiB
components/button/button.mjs	4.58 KiB
components/character-count/character-count.mjs	21.13 KiB
components/checkboxes/checkboxes.mjs	5.71 KiB
components/error-summary/error-summary.mjs	5.97 KiB
components/exit-this-page/exit-this-page.mjs	15.97 KiB
components/header/header.mjs	3.79 KiB
components/notification-banner/notification-banner.mjs	4.42 KiB
components/radios/radios.mjs	4.71 KiB
components/skip-link/skip-link.mjs	3.69 KiB
components/tabs/tabs.mjs	9.55 KiB

View stats and visualisations on the review app

---

Action run for 331d7f0

[domoscargin]

Just rebasing to keep this branch up to date

[colinrotherham]

Just rebasing to keep this branch up to date

Thanks @domoscargin

Shame GitHub limits the number of annotations on the Summary (scroll down)

But you'll see them all in the logs

[github-actions]

JavaScript changes to dist

diff --git a/dist/govuk-frontend-5.0.0-beta.0.min.js b/dist/govuk-frontend-5.0.0-beta.1.min.js
index 45cebefc1..5a255dbcb 100644
--- a/dist/govuk-frontend-5.0.0-beta.0.min.js
+++ b/dist/govuk-frontend-5.0.0-beta.1.min.js
@@ -1,4 +1,4 @@
-const version = "5.0.0-beta.0";
+const version = "5.0.0-beta.1";
 
 function mergeConfigs(...t) {
     function flattenObject(t) {
@@ -35,7 +35,7 @@ function getFragmentFromUrl(t) {
 }
 
 function isSupported(t = document.body) {
-    return t.classList.contains("govuk-frontend-supported")
+    return !!t && t.classList.contains("govuk-frontend-supported")
 }
 
 function normaliseString(t) {
@@ -55,8 +55,8 @@ class GOVUKFrontendError extends Error {
     }
 }
 class SupportError extends GOVUKFrontendError {
-    constructor() {
-        super("GOV.UK Frontend is not supported in this browser"), this.name = "SupportError"
+    constructor(t = document.body) {
+        super(t ? "GOV.UK Frontend is not supported in this browser" : 'GOV.UK Frontend initialised without `<script type="module">`'), this.name = "SupportError"
     }
 }
 class ConfigError extends GOVUKFrontendError {
@@ -971,4 +971,4 @@ export {
     Tabs,
     initAll,
     version
-}; //# sourceMappingURL=govuk-frontend-5.0.0-beta.0.min.js.map
\ No newline at end of file
+}; //# sourceMappingURL=govuk-frontend-5.0.0-beta.1.min.js.map
\ No newline at end of file

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Action run for 331d7f0

[github-actions]

Other changes to dist

diff --git a/dist/VERSION.txt b/dist/VERSION.txt
index 5183a25c3..9e25fc0ae 100644
--- a/dist/VERSION.txt
+++ b/dist/VERSION.txt
@@ -1 +1 @@
-5.0.0-beta.0
+5.0.0-beta.1

---

Action run for 331d7f0

[github-actions]

Stylesheets changes to dist

diff --git a/dist/govuk-frontend-5.0.0-beta.0.min.css b/dist/govuk-frontend-5.0.0-beta.1.min.css
index a25f51b2e..cd71490d1 100644
--- a/dist/govuk-frontend-5.0.0-beta.0.min.css
+++ b/dist/govuk-frontend-5.0.0-beta.1.min.css
@@ -1,7 +1,7 @@
 @charset "UTF-8";
 
 :root {
-    --govuk-frontend-version: "5.0.0-beta.0"
+    --govuk-frontend-version: "5.0.0-beta.1"
 }
 
 .govuk-link {
@@ -7692,4 +7692,4 @@ screen and (-ms-high-contrast:active) {
     }
 }
 
-/*# sourceMappingURL=govuk-frontend-5.0.0-beta.0.min.css.map */
\ No newline at end of file
+/*# sourceMappingURL=govuk-frontend-5.0.0-beta.1.min.css.map */
\ No newline at end of file

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Action run for 331d7f0

[colinrotherham]

Following dev catch-up this week we agreed to get this PR merged to test things out

I've assumed we're undecided on ES2015+ syntax suggestions from #4286 so they're turned off in 331d7f0

@domoscargin @romaricpascal Mind giving this the green tick please?

Thanks 🙌

[domoscargin]

Makes sense, let's give it a go.