Improve public JavaScript API for code initialisation

CHANGELOG.md

@@ -20,6 +20,40 @@ You can safely delete the old image files, named `govuk-crest.png` and `govuk-cr
 
 We introduced this change in [pull request #5376: Update the Royal Arms graphic in footer (v5.x)](https://github.com/alphagov/govuk-frontend/pull/5376).
 
+#### Components can no longer be initialised twice on the same element
+
+GOV.UK Frontend components now throw an error if they've already been initialised on the DOM Element they're receiving for initialisation.
+This prevents components from being initialised more than once and therefore not working properly.
+
+We introduced this change in [pull request #5272: Prevent multiple initialisations of a single component instance](https://github.com/alphagov/govuk-frontend/pull/5272)
+
+#### Respond to initialisation errors when using `createAll` and `initAll`
+
+We've added a new `onError` option for `createAll` and `initAll` that lets you respond to initialisation errors.
+The functions will continue catching errors and initialising components further down the page if one component fails to initialise,
+but this option will let you react to a component failing to initialise (for example, reporting to an error monitoring service).
+
+We introduced this change in:
+
+- [pull request #5252: Add `onError` to `createAll`](https://github.com/alphagov/govuk-frontend/pull/5252)
+- [pull request #5276: Add `onError` to `initAll`](https://github.com/alphagov/govuk-frontend/pull/5276)
+
+#### Check if GOV.UK Frontend is supported
+
+We've added the `isSupported` function to let you check if GOV.UK Frontend is supported in the browser where your script is running.
+GOV.UK Frontend components will check this automatically, but you may want to use the `isSupported` function to avoid running some code when GOV.UK Frontend is not supported.
+
+We introduced this change in [pull request #5250: Add `isSupported` to `all.mjs`](https://github.com/alphagov/govuk-frontend/pull/5250)
+
+#### Use our base component to build your own components
+
+We've added a `Component` class to help you build your own components. It allows you to focus on your components' specific features by handling these shared behaviours across components:
+
+- Checking that GOV.UK Frontend is supported
+- Checking that the component is not already initialised on its root element
+
+We introduced this change in [pull request #5350: Export a base `Component` class](https://github.com/alphagov/govuk-frontend/pull/5350).
+
 ### Fixes
 
 We've made fixes to GOV.UK Frontend in the following pull requests:

docs/contributing/coding-standards/component-options.md

@@ -10,7 +10,7 @@ First, make sure the component class has a constructor parameter for passing in
 
 ```mjs
 export class Accordion {
-  constructor($module, config = {}) {
+  constructor($root, config = {}) {
     // ...
   }
 }
@@ -36,7 +36,7 @@ There is no guarantee `config` will have any value at all, so we set the default
 import { mergeConfigs } from '../../common/index.mjs'
 
 export class Accordion {
-  constructor($module, config = {}) {
+  constructor($root, config = {}) {
     this.config = mergeConfigs(
       Accordion.defaults,
       config
@@ -101,26 +101,26 @@ You can find `data-*` attributes in JavaScript by looking at an element's `datas
 
 See ['Naming configuration options'](#naming-configuration-options) for exceptions to how names are transformed.
 
-As we expect configuration-related `data-*` attributes to always be on the component's root element (the same element with the `data-module` attribute), we can access them all using `$module.dataset`.
+As we expect configuration-related `data-*` attributes to always be on the component's root element (the same element with the `data-module` attribute), we can access them all using `$root.dataset`.
 
-Using the `mergeConfigs` call discussed earlier in this document, update it to include `$module.dataset` as the highest priority.
+Using the `mergeConfigs` call discussed earlier in this document, update it to include `$root.dataset` as the highest priority.
 
 ```mjs
 import { mergeConfigs } from '../../common/index.mjs'
 import { normaliseDataset } from '../../common/normalise-dataset.mjs'
 
 export class Accordion {
-  constructor($module, config = {}) {
+  constructor($root, config = {}) {
     this.config = mergeConfigs(
       Accordion.defaults,
       config,
-      normaliseDataset(Accordion, $module.dataset)
+      normaliseDataset(Accordion, $root.dataset)
     )
   }
 }
 ```
 
-Here, we pass the value of `$module.dataset` through our `normaliseDataset` function. This is because attribute values in dataset are always interpreted as strings. `normaliseDataset` looks at the component's configuration schema and converts values into numbers or booleans where needed.
+Here, we pass the value of `$root.dataset` through our `normaliseDataset` function. This is because attribute values in dataset are always interpreted as strings. `normaliseDataset` looks at the component's configuration schema and converts values into numbers or booleans where needed.
 
 Now, in our HTML, we could pass configuration options by using the kebab-case version of the option's name.
 
@@ -164,11 +164,11 @@ import { normaliseDataset } from '../../common/normalise-dataset.mjs'
 import { ConfigError } from '../../errors/index.mjs'
 
 export class Accordion {
-  constructor($module, config = {}) {
+  constructor($root, config = {}) {
     this.config = mergeConfigs(
       Accordion.defaults,
       config,
-      normaliseDataset(Accordion, $module.dataset)
+      normaliseDataset(Accordion, $root.dataset)
     )
 
     // Check that the configuration provided is valid
@@ -248,11 +248,11 @@ import { mergeConfigs, extractConfigByNamespace } from '../../common/index.mjs'
 import { normaliseDataset } from '../../common/normalise-dataset.mjs'
 
 export class Accordion {
-  constructor($module, config = {}) {
+  constructor($root, config = {}) {
     this.config = mergeConfigs(
       Accordion.defaults,
       config,
-      normaliseDataset(Accordion, $module.dataset)
+      normaliseDataset(Accordion, $root.dataset)
     )
 
     this.stateInfo = extractConfigByNamespace(Accordion, this.config, 'stateInfo');

docs/contributing/coding-standards/js.md

@@ -13,27 +13,22 @@ component
 ## Skeleton
 
 ```mjs
+import { GOVUKFrontendComponent } from '../../govuk-frontend-component.mjs'
+
 /**
  * Component name
  *
  * @preserve
  */
-export class Example {
+export class Example extends GOVUKFrontendComponent {
   /**
-   * @param {Element | null} $module - HTML element to use for component
+   * @param {Element | null} $root - HTML element to use for component
    */
-  constructor($module) {
-    if (
-      !($module instanceof HTMLElement) ||
-      !document.body.classList.contains('govuk-frontend-supported')
-    ) {
-      return this
-    }
-
-    this.$module = $module
+  constructor($root){
+    super($root)
 
     // Code goes here
-    this.$module.addEventListener('click', () => {
+    this.$root.addEventListener('click', () => {
       // ...
     })
   }

packages/govuk-frontend/rollup.release.config.mjs

@@ -2,16 +2,35 @@ import config from '@govuk-frontend/config'
 import { babel } from '@rollup/plugin-babel'
 import replace from '@rollup/plugin-replace'
 import terser from '@rollup/plugin-terser'
-import * as GOVUKFrontend from 'govuk-frontend/src/govuk/all.mjs'
 import { defineConfig } from 'rollup'
 
+// GOV.UK Frontend uses browser APIs at `import` time
+// because of static properties. These APIs are not available
+// in Node.js.
+// We mock them the time of the `import` so we can read
+// the name of GOV.UK Frontend's exports without errors
+async function getGOVUKFrontendExportsNames() {
+  try {
+    global.HTMLElement = /** @type {any} */ (function () {})
+    global.HTMLAnchorElement = /** @type {any} */ (function () {})
+    return Object.keys(await import('govuk-frontend/src/govuk/all.mjs'))
+  } finally {
+    delete global.HTMLElement
+    delete global.HTMLAnchorElement
+  }
+}
+
 /**
  * Rollup config for GitHub release
  *
  * ECMAScript (ES) module bundles for browser <script type="module">
  * or using `import` for modern browsers and Node.js scripts
+ *
+ * @param {import('rollup').RollupOptions} input
+ * @returns {Promise<import('rollup').RollupOptions|import('rollup').RollupOptions[]>} rollup config
  */
-export default defineConfig(({ i: input }) => ({
+
+export default defineConfig(async ({ i: input }) => ({
   input,
 
   /**
@@ -37,7 +56,7 @@ export default defineConfig(({ i: input }) => ({
           keep_fnames: true,
           // Ensure all top-level exports skip mangling, for example
           // non-function string constants like `export { version }`
-          reserved: Object.keys(GOVUKFrontend)
+          reserved: await getGOVUKFrontendExportsNames()
         },
 
         // Include sources content from source maps to inspect

packages/govuk-frontend/src/govuk/all.mjs

@@ -13,6 +13,8 @@ export { ServiceNavigation } from './components/service-navigation/service-navig
 export { SkipLink } from './components/skip-link/skip-link.mjs'
 export { Tabs } from './components/tabs/tabs.mjs'
 export { initAll, createAll } from './init.mjs'
+export { isSupported } from './common/index.mjs'
+export { GOVUKFrontendComponent as Component } from './govuk-frontend-component.mjs'
 
 /**
  * @typedef {import('./init.mjs').Config} Config

packages/govuk-frontend/src/govuk/all.puppeteer.test.js

@@ -41,10 +41,24 @@ describe('GOV.UK Frontend', () => {
       expect(typeofCreateAll).toBe('function')
     })
 
+    it('exports `isSupported` function', async () => {
+      const typeofIsSupported = await page.evaluate(
+        async (importPath, exportName) => {
+          const namespace = await import(importPath)
+          return typeof namespace[exportName]
+        },
+        scriptsPath.href,
+        'isSupported'
+      )
+
+      expect(typeofIsSupported).toBe('function')
+    })
+
     it('exports Components', async () => {
       const components = exported
         .filter(
-          (method) => !['initAll', 'createAll', 'version'].includes(method)
+          (method) =>
+            !['initAll', 'createAll', 'version', 'isSupported'].includes(method)
         )
         .sort()
 
@@ -54,6 +68,7 @@ describe('GOV.UK Frontend', () => {
         'Button',
         'CharacterCount',
         'Checkboxes',
+        'Component',
         'ErrorSummary',
         'ExitThisPage',
         'Header',

packages/govuk-frontend/src/govuk/common/index.mjs

@@ -188,14 +188,28 @@ export function setFocus($element, options = {}) {
   $element.focus()
 }
 
+/**
+ * Checks if component is already initialised
+ *
+ * @internal
+ * @param {Element} $root - HTML element to be checked
+ * @param {string} moduleName - name of component module
+ * @returns {boolean} Whether component is already initialised
+ */
+export function isInitialised($root, moduleName) {
+  return (
+    $root instanceof HTMLElement &&
+    $root.hasAttribute(`data-${moduleName}-init`)
+  )
+}
+
 /**
  * Checks if GOV.UK Frontend is supported on this page
  *
  * Some browsers will load and run our JavaScript but GOV.UK Frontend
  * won't be supported.
  *
- * @internal
- * @param {HTMLElement | null} [$scope] - HTML element `<body>` checked for browser support
+ * @param {HTMLElement | null} [$scope] - (internal) `<body>` HTML element checked for browser support
  * @returns {boolean} Whether GOV.UK Frontend is supported on this page
  */
 export function isSupported($scope = document.body) {
@@ -266,6 +280,18 @@ function isObject(option) {
   return !!option && typeof option === 'object' && !isArray(option)
 }
 
+/**
+ * Format error message
+ *
+ * @internal
+ * @param {ComponentWithModuleName} Component - Component that threw the error
+ * @param {string} message - Error message
+ * @returns {string} - Formatted error message
+ */
+export function formatErrorMessage(Component, message) {
+  return `${Component.moduleName}: ${message}`
+}
+
 /**
  * Schema for component config
  *
@@ -294,3 +320,16 @@ function isObject(option) {
  * @typedef {keyof ObjectNested} NestedKey
  * @typedef {{ [key: string]: string | boolean | number | ObjectNested | undefined }} ObjectNested
  */
+
+/* eslint-disable jsdoc/valid-types --
+ * `{new(...args: any[] ): object}` is not recognised as valid
+ * https://github.com/gajus/eslint-plugin-jsdoc/issues/145#issuecomment-1308722878
+ * https://github.com/jsdoc-type-pratt-parser/jsdoc-type-pratt-parser/issues/131
+ **/
+
+/**
+ * @typedef ComponentWithModuleName
+ * @property {string} moduleName - Name of the component
+ */
+
+/* eslint-enable jsdoc/valid-types */