[v10] Add character count countFunction option

eslint.config.mjs

@@ -199,6 +199,9 @@ export default defineConfig([
       // https://browsersl.ist/#q=supports+es6-module+and+not+supports+array-includes
       'es-x/no-array-prototype-includes': 'off',
 
+      // Babel transpiles ES2021 `??=` logical assignment
+      'es-x/no-logical-assignment-operators': 'off',
+
       // Babel transpiles ES2020 `??` nullish coalescing
       'es-x/no-nullish-coalescing-operators': 'off',
 

packages/nhsuk-frontend/src/nhsuk/components/character-count/character-count.jsdom.test.mjs

@@ -428,6 +428,40 @@ describe('Character count', () => {
 
         expect(component.getCountMessage()).toBe('You have 97 words remaining')
       })
+
+      it('uses custom `countFunction` for `maxlength` limit when set', async () => {
+        const component = new CharacterCount($root, {
+          maxlength: 100,
+          countFunction: jest.fn().mockReturnValue(10)
+        })
+
+        $textarea.focus()
+        await user.keyboard('Newly updated value')
+
+        expect(component.config.countFunction).toHaveBeenLastCalledWith(
+          'Newly updated value'
+        )
+
+        expect(component.getCountMessage()).toBe(
+          'You have 90 characters remaining'
+        )
+      })
+
+      it('uses custom `countFunction` for `maxwords` limit when set', async () => {
+        const component = new CharacterCount($root, {
+          maxwords: 100,
+          countFunction: jest.fn().mockReturnValue(10)
+        })
+
+        $textarea.focus()
+        await user.keyboard('Newly updated value')
+
+        expect(component.config.countFunction).toHaveBeenLastCalledWith(
+          'Newly updated value'
+        )
+
+        expect(component.getCountMessage()).toBe('You have 90 words remaining')
+      })
     })
 
     describe('with HTML lang attribute', () => {

packages/nhsuk-frontend/src/nhsuk/components/character-count/character-count.mjs

@@ -61,6 +61,7 @@ export class CharacterCount extends ConfigurableComponent {
     const {
       i18n,
       maxlength,
+      countFunction,
       countType,
       screenReaderCountMessageClass,
       textareaDescriptionClass,
@@ -72,7 +73,7 @@ export class CharacterCount extends ConfigurableComponent {
       locale: closestAttributeValue(this.$root, 'lang')
     })
 
-    if (countType === 'characters') {
+    if (countType === 'characters' || !!countFunction) {
       if (!('Segmenter' in Intl)) {
         throw new SupportError(
           formatErrorMessage(
@@ -83,7 +84,7 @@ export class CharacterCount extends ConfigurableComponent {
       }
 
       this.segmenter = new Intl.Segmenter(this.i18n.locale, {
-        granularity: 'grapheme'
+        granularity: countType === 'words' ? 'word' : 'grapheme'
       })
     }
 
@@ -220,31 +221,12 @@ export class CharacterCount extends ConfigurableComponent {
    * @param {string} [text] - Deprecated
    */
   updateCount(text) {
-    const { $textarea } = this
-    const { countType } = this.config
+    const { $textarea, countFunctions } = this
+    let { countType, countFunction } = this.config
 
-    text = text ?? $textarea.value
-
-    switch (countType) {
-      case 'length':
-        // Count code points (string length)
-        this.length = text.length
-        break
-
-      case 'characters': {
-        // Count grapheme clusters (user-perceived characters)
-        this.length = this.segmenter
-          ? Array.from(this.segmenter.segment(text)).length
-          : 0
-
-        break
-      }
-
-      case 'words':
-        // Count consecutive non-whitespace results
-        this.length = text.match(/\S+/g)?.length ?? 0
-        break
-    }
+    text ??= $textarea.value
+    countFunction ??= countFunctions[countType]
+    this.length = countFunction.call(this, text)
   }
 
   /**
@@ -467,6 +449,46 @@ export class CharacterCount extends ConfigurableComponent {
     }
   }
 
+  /**
+   * Character count functions
+   *
+   * @constant
+   * @satisfies {Record<string, CharacterCountConfig['countFunction']>}
+   */
+  countFunctions = Object.freeze({
+    /**
+     * Count code points (string length)
+     *
+     * @param {string} text - Textarea value
+     * @returns {number} Count
+     */
+    length(text) {
+      return text.length
+    },
+
+    /**
+     * Count grapheme clusters (user-perceived characters)
+     *
+     * @param {string} text - Textarea value
+     * @returns {number} Count
+     */
+    characters(text) {
+      return this.segmenter
+        ? Array.from(this.segmenter.segment(text)).length
+        : 0
+    },
+
+    /**
+     * Count consecutive non-whitespace results
+     *
+     * @param {string} text - Textarea value
+     * @returns {number} Count
+     */
+    words(text) {
+      return text.match(/\S+/g)?.length ?? 0
+    }
+  })
+
   /**
    * Name for the component used when initialising using data-module attributes
    */
@@ -524,6 +546,7 @@ export class CharacterCount extends ConfigurableComponent {
       maxlength: { type: 'number' },
       threshold: { type: 'number' },
       countType: { type: 'string' },
+      countFunction: { type: 'function' },
       textareaDescriptionClass: { type: 'string' },
       visibleCountMessageClass: { type: 'string' },
       screenReaderCountMessageClass: { type: 'string' },
@@ -574,12 +597,20 @@ export function initCharacterCounts(options) {
  *   count message will be hidden by default.
  * @property {'characters' | 'length' | 'words'} countType - The count type
  *   (`"characters"`, `"length"` or `"words"`) used to count the text.
+ * @property {CharacterCountFunction} [countFunction] - Custom character or
+ *   word counting function.
  * @property {string} textareaDescriptionClass - Textarea description class
  * @property {string} visibleCountMessageClass - Visible count message class
  * @property {string} screenReaderCountMessageClass - Screen reader count message class
  * @property {CharacterCountTranslations} [i18n=CharacterCount.defaults.i18n] - Character count translations
  */
 
+/**
+ * Character count function
+ *
+ * @typedef {(this: CharacterCount, text: string) => number} CharacterCountFunction
+ */
+
 /**
  * Character count translations
  *