perf(engine-core): reduce fragment cache objects

packages/@lwc/engine-core/src/framework/fragment-cache.ts

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2024, Salesforce, Inc.
+ * All rights reserved.
+ * SPDX-License-Identifier: MIT
+ * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
+ */
+import { ArrayFrom } from '@lwc/shared';
+
+export const enum FragmentCacheKey {
+    HAS_SCOPED_STYLE = 1,
+    SHADOW_MODE_SYNTHETIC = 2,
+}
+
+// HAS_SCOPED_STYLE | SHADOW_MODE_SYNTHETIC = 3
+const MAX_CACHE_KEY = 3;
+
+// Mapping of cacheKeys to `string[]` (assumed to come from a tagged template literal) to an Element.
+// Note that every unique tagged template literal will have a unique `string[]`. So by using `string[]`
+// as the WeakMap key, we effectively associate each Element with a unique tagged template literal.
+// See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals#tagged_templates
+// Also note that this array only needs to be large enough to account for the maximum possible cache key
+const fragmentCache: WeakMap<string[], Element>[] = ArrayFrom(
+    { length: MAX_CACHE_KEY + 1 },
+    () => new WeakMap()
+);
+
+// Only used in LWC's Karma tests
+if (process.env.NODE_ENV === 'test-karma-lwc') {
+    (window as any).__lwcResetFragmentCache = () => {
+        for (let i = 0; i < fragmentCache.length; i++) {
+            fragmentCache[i] = new WeakMap();
+        }
+    };
+}
+
+function checkIsBrowser() {
+    // The fragment cache only serves prevent calling innerHTML multiple times which doesn't happen on the server.
+    /* istanbul ignore next */
+    if (!process.env.IS_BROWSER) {
+        throw new Error(
+            'The fragment cache is intended to only be used in @lwc/engine-dom, not @lwc/engine-server'
+        );
+    }
+}
+
+export function getFromFragmentCache(cacheKey: number, strings: string[]) {
+    checkIsBrowser();
+    return fragmentCache[cacheKey].get(strings);
+}
+
+export function setInFragmentCache(cacheKey: number, strings: string[], element: Element) {
+    checkIsBrowser();
+    fragmentCache[cacheKey].set(strings, element);
+}

packages/@lwc/engine-core/src/framework/template.ts

@@ -16,7 +16,6 @@ import {
     isUndefined,
     KEY__SCOPED_CSS,
     keys,
-    noop,
     StringCharAt,
     STATIC_PART_TOKEN_ID,
     toString,
@@ -47,6 +46,7 @@ import { getTemplateOrSwappedTemplate, setActiveVM } from './hot-swaps';
 import { MutableVNodes, VNodes, VStaticPart, VStaticPartElement, VStaticPartText } from './vnodes';
 import { RendererAPI } from './renderer';
 import { getMapFromClassName } from './modules/computed-class-attr';
+import { FragmentCacheKey, getFromFragmentCache, setInFragmentCache } from './fragment-cache';
 
 export interface Template {
     (api: RenderAPI, cmp: object, slotSet: SlotSet, cache: TemplateCache): VNodes;
@@ -238,40 +238,11 @@ function serializeClassAttribute(part: VStaticPartElement, classToken: string) {
     return computedClassName.length ? ` class="${htmlEscape(computedClassName, true)}"` : '';
 }
 
-const enum FragmentCache {
-    HAS_SCOPED_STYLE = 1,
-    SHADOW_MODE_SYNTHETIC = 2,
-}
-
-// This should be a no-op outside of LWC's Karma tests, where it's not needed
-let registerFragmentCache: (fragmentCache: any) => void = noop;
-
-// Only used in LWC's Karma tests
-if (process.env.NODE_ENV === 'test-karma-lwc') {
-    // Keep track of fragmentCaches, so we can clear them in LWC's Karma tests
-    const fragmentCaches: any[] = [];
-    registerFragmentCache = (fragmentCache: any) => {
-        fragmentCaches.push(fragmentCache);
-    };
-
-    (window as any).__lwcResetFragmentCaches = () => {
-        for (const fragmentCache of fragmentCaches) {
-            for (const key of keys(fragmentCache)) {
-                delete fragmentCache[key];
-            }
-        }
-    };
-}
-
 function buildParseFragmentFn(
     createFragmentFn: (html: string, renderer: RendererAPI) => Element
 ): (strings: string[], ...keys: (string | number)[]) => () => Element {
-    return (strings: string[], ...keys: (string | number)[]) => {
-        const cache = create(null);
-
-        registerFragmentCache(cache);
-
-        return function (parts?: VStaticPart[]): Element {
+    return function parseFragment(strings: string[], ...keys: (string | number)[]) {
+        return function applyFragmentParts(parts?: VStaticPart[]): Element {
             const {
                 context: { hasScopedStyles, stylesheetToken, legacyStylesheetToken },
                 shadowMode,
@@ -284,16 +255,16 @@ function buildParseFragmentFn(
 
             let cacheKey = 0;
             if (hasStyleToken && hasScopedStyles) {
-                cacheKey |= FragmentCache.HAS_SCOPED_STYLE;
+                cacheKey |= FragmentCacheKey.HAS_SCOPED_STYLE;
             }
             if (hasStyleToken && isSyntheticShadow) {
-                cacheKey |= FragmentCache.SHADOW_MODE_SYNTHETIC;
+                cacheKey |= FragmentCacheKey.SHADOW_MODE_SYNTHETIC;
             }
 
             // Cache is only here to prevent calling innerHTML multiple times which doesn't happen on the server.
             if (process.env.IS_BROWSER) {
                 // Disable this on the server to prevent cache poisoning when expressions are used.
-                const cached = cache[cacheKey];
+                const cached = getFromFragmentCache(cacheKey, strings);
                 if (!isUndefined(cached)) {
                     return cached;
                 }
@@ -343,9 +314,14 @@ function buildParseFragmentFn(
 
             htmlFragment += strings[strings.length - 1];
 
-            cache[cacheKey] = createFragmentFn(htmlFragment, renderer);
+            const element = createFragmentFn(htmlFragment, renderer);
+
+            // Cache is only here to prevent calling innerHTML multiple times which doesn't happen on the server.
+            if (process.env.IS_BROWSER) {
+                setInFragmentCache(cacheKey, strings, element);
+            }
 
-            return cache[cacheKey];
+            return element;
         };
     };
 }

packages/@lwc/integration-karma/test/rendering/fragment-cache/index.spec.js

@@ -0,0 +1,80 @@
+import { createElement } from 'lwc';
+import { LOWERCASE_SCOPE_TOKENS } from 'test-utils';
+
+import NativeScopedStyles from 'x/nativeScopedStyles';
+import NativeStyles from 'x/nativeStyles';
+import NoStyles from 'x/noStyles';
+import ScopedStyles from 'x/scopedStyles';
+import Styles from 'x/styles';
+
+const scenarios = [
+    {
+        name: 'no styles',
+        Ctor: NoStyles,
+        tagName: 'x-no-styles',
+        expectedColor: 'rgb(0, 0, 0)',
+        expectClass: false,
+        expectAttribute: false,
+    },
+    {
+        name: 'styles',
+        Ctor: Styles,
+        tagName: 'x-styles',
+        expectedColor: 'rgb(255, 0, 0)',
+        expectClass: false,
+        expectAttribute: !process.env.NATIVE_SHADOW,
+    },
+    {
+        name: 'scoped styles',
+        Ctor: ScopedStyles,
+        tagName: 'x-scoped-styles',
+        expectedColor: 'rgb(0, 128, 0)',
+        expectClass: true,
+        expectAttribute: !process.env.NATIVE_SHADOW,
+    },
+    {
+        name: 'native styles',
+        Ctor: NativeStyles,
+        tagName: 'x-native-styles',
+        expectedColor: 'rgb(255, 0, 0)',
+        expectClass: false,
+        expectAttribute: false,
+    },
+    {
+        name: 'native scoped styles',
+        Ctor: NativeScopedStyles,
+        tagName: 'x-native-scoped-styles',
+        expectedColor: 'rgb(0, 128, 0)',
+        expectClass: true,
+        expectAttribute: false,
+    },
+];
+
+// These tests confirm that the fragment cache (from `fragment-cache.ts`) is working correctly. Fragments should be
+// unique based on 1) synthetic vs native shadow, and 2) presence or absence of scoped styles. If the fragment cache is
+// not working correctly, then we may end up rendering the wrong styles or the wrong attribute/class scope token due to
+// the cache being poisoned, e.g. an HTML string for scoped styles being rendered for non-scoped styles.
+// To test this, we re-use the same `template.html` but change the `static stylesheets` in each component.
+scenarios.forEach(({ name, Ctor, tagName, expectedColor, expectClass, expectAttribute }) => {
+    describe(name, () => {
+        let h1;
+
+        beforeEach(async () => {
+            const elm = createElement(tagName, { is: Ctor });
+            document.body.appendChild(elm);
+            await Promise.resolve();
+            h1 = elm.shadowRoot.querySelector('h1');
+        });
+
+        it('renders the correct styles', () => {
+            expect(getComputedStyle(h1).color).toBe(expectedColor);
+        });
+
+        it('renders the correct attributes/classes', () => {
+            const scopeToken = LOWERCASE_SCOPE_TOKENS ? 'lwc-2it5vhebv0i' : 'x-template_template';
+
+            expect(h1.getAttribute('class')).toBe(expectClass ? scopeToken : null);
+            expect(h1.hasAttribute(scopeToken)).toBe(expectAttribute);
+        });
+    });
+});

packages/@lwc/integration-karma/test/rendering/fragment-cache/x/nativeScopedStyles/nativeScopedStyles.js

@@ -0,0 +1,12 @@
+import { LightningElement } from 'lwc';
+import template from '../template/template.html';
+import styles from '../stylesheets/scopedStyles.scoped.css';
+
+export default class extends LightningElement {
+    static shadowSupportMode = 'native';
+    static stylesheets = [styles];
+
+    render() {
+        return template;
+    }
+}

packages/@lwc/integration-karma/test/rendering/fragment-cache/x/nativeStyles/nativeStyles.js

@@ -0,0 +1,12 @@
+import { LightningElement } from 'lwc';
+import template from '../template/template.html';
+import styles from '../stylesheets/styles.css';
+
+export default class extends LightningElement {
+    static shadowSupportMode = 'native';
+    static stylesheets = [styles];
+
+    render() {
+        return template;
+    }
+}

packages/@lwc/integration-karma/test/rendering/fragment-cache/x/noStyles/noStyles.js

@@ -0,0 +1,8 @@
+import { LightningElement } from 'lwc';
+import template from '../template/template.html';
+
+export default class extends LightningElement {
+    render() {
+        return template;
+    }
+}

packages/@lwc/integration-karma/test/rendering/fragment-cache/x/scopedStyles/scopedStyles.js

@@ -0,0 +1,11 @@
+import { LightningElement } from 'lwc';
+import template from '../template/template.html';
+import styles from '../stylesheets/scopedStyles.scoped.css';
+
+export default class extends LightningElement {
+    static stylesheets = [styles];
+
+    render() {
+        return template;
+    }
+}

packages/@lwc/integration-karma/test/rendering/fragment-cache/x/styles/styles.js

@@ -0,0 +1,11 @@
+import { LightningElement } from 'lwc';
+import template from '../template/template.html';
+import styles from '../stylesheets/styles.css';
+
+export default class extends LightningElement {
+    static stylesheets = [styles];
+
+    render() {
+        return template;
+    }
+}

packages/@lwc/integration-karma/test/rendering/fragment-cache/x/stylesheets/scopedStyles.scoped.css

@@ -0,0 +1,3 @@
+h1 {
+    color: green;
+}

packages/@lwc/integration-karma/test/rendering/fragment-cache/x/stylesheets/styles.css

@@ -0,0 +1,3 @@
+h1 {
+    color: red;
+}

packages/@lwc/integration-karma/test/rendering/fragment-cache/x/template/template.html

@@ -0,0 +1,3 @@
+<template>
+    <h1>hello</h1>
+</template>

packages/@lwc/integration-karma/test/rendering/legacy-scope-tokens/index.spec.js

@@ -14,7 +14,7 @@ describe('legacy scope tokens', () => {
                 setFeatureFlagForTest('ENABLE_LEGACY_SCOPE_TOKENS', false);
                 // We keep a cache of parsed static fragments; these need to be reset
                 // since they can vary based on whether we use the legacy scope token or not.
-                window.__lwcResetFragmentCaches();
+                window.__lwcResetFragmentCache();
             });
 
             function getAttributes(elm) {

packages/@lwc/shared/src/language.ts

@@ -38,8 +38,12 @@ const {
     setPrototypeOf,
 } = Object;
 
-/** Detached {@linkcode Array.isArray}; see {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray MDN Reference}. */
-const { isArray } = Array;
+const {
+    /** Detached {@linkcode Array.isArray}; see {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray MDN Reference}. */
+    isArray,
+    /** Detached {@linkcode Array.from}; see {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from MDN Reference}. */
+    from: ArrayFrom,
+} = Array;
 
 /** The most extensible array type. */
 type BaseArray = readonly unknown[];
@@ -152,6 +156,8 @@ export {
      */
     /** Detached {@linkcode Array.isArray}; see {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray MDN Reference}. */
     isArray,
+    /** Detached {@linkcode Array.from}; see {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from MDN Reference}. */
+    ArrayFrom,
     /*
      * Array prototype
      */