feat: implement remaining ClassList methods

packages/@lwc/engine-server/src/__tests__/fixtures/getter-class-list-modify/config.json

@@ -0,0 +1,7 @@
+{
+    "entry": "x/getter-class-list",
+    "ssrFiles": {
+        "error": "error-ssr.txt",
+        "expected": "expected-ssr.html"
+    }
+}

packages/@lwc/engine-server/src/__tests__/fixtures/getter-class-list-modify/error.txt

@@ -0,0 +1 @@
+classList.forEach is not a function

packages/@lwc/engine-server/src/__tests__/fixtures/getter-class-list-modify/expected-ssr.html

@@ -0,0 +1,4 @@
+<fixture-test class="a b c d-e f g h i" data-lwc-host-mutated="class">
+  <template shadowrootmode="open">
+  </template>
+</fixture-test>

packages/@lwc/engine-server/src/__tests__/fixtures/getter-class-list-modify/modules/x/getter-class-list/getter-class-list.js

@@ -0,0 +1,25 @@
+import { expect } from 'vitest';
+import { LightningElement } from 'lwc';
+
+export default class GetterClassList extends LightningElement {
+    connectedCallback() {
+        const { classList } = this;
+
+        classList.add('a', 'b', 'c', 'd-e', 'f', 'g', 'h', 'i');
+        classList.forEach((value) => {
+            classList.remove(value);
+        });
+        expect(this.getAttribute('class')).toBe('');
+        expect(this.classList.length).toBe(0);
+
+        classList.add('a', 'b', 'c', 'd-e', 'f', 'g', 'h', 'i');
+        while (classList.length > 0) {
+            classList.remove(classList.item(0));
+        }
+        expect(this.getAttribute('class')).toBe('');
+        expect(this.classList.length).toBe(0);
+
+        classList.add('a', 'b', 'c', 'd-e', 'f', 'g', 'h', 'i');
+        expect(classList.item(3)).toBe('d-e');
+    }
+}

packages/@lwc/ssr-runtime/src/class-list.ts

@@ -7,8 +7,6 @@
 
 import type { LightningElement } from './lightning-element';
 
-const MULTI_SPACE = /\s+/g;
-
 // Copied from lib.dom
 interface DOMTokenList {
     readonly length: number;
@@ -28,6 +26,15 @@ interface DOMTokenList {
     [index: number]: string;
 }
 
+const MULTI_SPACE = /\s+/g;
+
+function parseClassName(className: string | null): string[] {
+    return (className ?? '')
+        .split(MULTI_SPACE)
+        .map((item) => item.trim())
+        .filter(Boolean);
+}
+
 export class ClassList implements DOMTokenList {
     el: LightningElement;
 
@@ -36,22 +43,19 @@ export class ClassList implements DOMTokenList {
     }
 
     add(...newClassNames: string[]) {
-        const className = this.el.className;
-        const set = new Set(className.split(MULTI_SPACE).filter(Boolean));
+        const set = new Set(parseClassName(this.el.className));
         for (const newClassName of newClassNames) {
             set.add(newClassName);
         }
         this.el.className = Array.from(set).join(' ');
     }
 
     contains(className: string) {
-        const currentClassNameStr = this.el.className;
-        return currentClassNameStr.split(MULTI_SPACE).includes(className);
+        return parseClassName(this.el.className).includes(className);
     }
 
     remove(...classNamesToRemove: string[]) {
-        const className = this.el.className;
-        const set = new Set(className.split(MULTI_SPACE).filter(Boolean));
+        const set = new Set(parseClassName(this.el.className));
         for (const newClassName of classNamesToRemove) {
             set.delete(newClassName);
         }
@@ -60,8 +64,7 @@ export class ClassList implements DOMTokenList {
 
     replace(oldClassName: string, newClassName: string) {
         let classWasReplaced = false;
-        const className = this.el.className;
-        const listOfClasses = className.split(MULTI_SPACE).filter(Boolean) as string[];
+        const listOfClasses = parseClassName(this.el.className);
         listOfClasses.forEach((value, idx) => {
             if (value === oldClassName) {
                 classWasReplaced = true;
@@ -73,8 +76,7 @@ export class ClassList implements DOMTokenList {
     }
 
     toggle(classNameToToggle: string, force?: boolean) {
-        const classNameStr = this.el.className;
-        const set = new Set(classNameStr.split(MULTI_SPACE).filter(Boolean));
+        const set = new Set(parseClassName(this.el.className));
         if (!set.has(classNameToToggle) && force !== false) {
             set.add(classNameToToggle);
         } else if (set.has(classNameToToggle) && force !== true) {
@@ -93,22 +95,28 @@ export class ClassList implements DOMTokenList {
     }
 
     get length(): number {
-        const currentClassNameStr = this.el.className ?? '';
-        return currentClassNameStr.split(MULTI_SPACE).length;
+        return parseClassName(this.el.className).length;
     }
 
     // Stubs to satisfy DOMTokenList interface
     [index: number]: never; // Can't implement arbitrary index getters without a proxy
-    item(_index: number): string | null {
-        throw new Error('Method "item" not implemented.');
-    }
-    supports(_token: string): boolean {
-        throw new Error('Method "supports" not implemented.');
+
+    item(index: number): string | null {
+        return parseClassName(this.el.className ?? '')[index] ?? null;
     }
+
     forEach(
-        _callbackfn: (value: string, key: number, parent: DOMTokenList) => void,
-        _thisArg?: any
+        callbackFn: (value: string, key: number, parent: DOMTokenList) => void,
+        thisArg?: any
     ): void {
-        throw new Error('Method "forEach" not implemented.');
+        parseClassName(this.el.className).forEach((value, index) =>
+            callbackFn.call(thisArg, value, index, this)
+        );
+    }
+
+    // This method is present on DOMTokenList but throws an error in the browser when used
+    // in connection with Element#classList.
+    supports(_token: string): boolean {
+        throw new TypeError('DOMTokenList has no supported tokens.');
     }
 }