refactor(core): prefetch/preload refactor (#7282)

packages/docusaurus-module-type-aliases/src/index.d.ts

@@ -346,3 +346,10 @@ declare module '*.css' {
   const src: string;
   export default src;
 }
+
+interface Window {
+  docusaurus: {
+    prefetch: (url: string) => false | Promise<void[]>;
+    preload: (url: string) => false | Promise<void[]>;
+  };
+}

packages/docusaurus/src/client/PendingNavigation.tsx

@@ -62,7 +62,9 @@ class PendingNavigation extends React.Component<Props, State> {
       location: nextLocation,
     })!;
 
-    // Load data while the old screen remains.
+    // Load data while the old screen remains. Force preload instead of using
+    // `window.docusaurus`, because we want to avoid loading screen even when
+    // user is on saveData
     preload(nextLocation.pathname)
       .then(() => {
         this.routeUpdateCleanupCb?.();

packages/docusaurus/src/client/docusaurus.ts

@@ -12,8 +12,8 @@ import prefetchHelper from './prefetch';
 import preloadHelper from './preload';
 import flat from './flat';
 
-const fetched: {[key: string]: boolean} = {};
-const loaded: {[key: string]: boolean} = {};
+const fetched = new Set<string>();
+const loaded = new Set<string>();
 
 declare global {
   // eslint-disable-next-line camelcase, no-underscore-dangle
@@ -25,14 +25,14 @@ declare global {
 
 // If user is on slow or constrained connection.
 const isSlowConnection = () =>
-  navigator.connection?.effectiveType.includes('2g') &&
+  navigator.connection?.effectiveType.includes('2g') ||
   navigator.connection?.saveData;
 
 const canPrefetch = (routePath: string) =>
-  !isSlowConnection() && !loaded[routePath] && !fetched[routePath];
+  !isSlowConnection() && !loaded.has(routePath) && !fetched.has(routePath);
 
 const canPreload = (routePath: string) =>
-  !isSlowConnection() && !loaded[routePath];
+  !isSlowConnection() && !loaded.has(routePath);
 
 const getChunkNamesToLoad = (path: string): string[] =>
   Object.entries(routesChunkNames)
@@ -46,12 +46,11 @@ const getChunkNamesToLoad = (path: string): string[] =>
     .flatMap(([, routeChunks]) => Object.values(flat(routeChunks)));
 
 const docusaurus = {
-  prefetch: (routePath: string): boolean => {
+  prefetch(routePath: string): false | Promise<void[]> {
     if (!canPrefetch(routePath)) {
       return false;
     }
-    // Prevent future duplicate prefetch of routePath.
-    fetched[routePath] = true;
+    fetched.add(routePath);
 
     // Find all webpack chunk names needed.
     const matches = matchRoutes(routes, routePath);
@@ -61,32 +60,30 @@ const docusaurus = {
     );
 
     // Prefetch all webpack chunk assets file needed.
-    chunkNamesNeeded.forEach((chunkName) => {
-      // "__webpack_require__.gca" is a custom function provided by
-      // ChunkAssetPlugin. Pass it the chunkName or chunkId you want to load and
-      // it will return the URL for that chunk.
-      // eslint-disable-next-line camelcase
-      const chunkAsset = __webpack_require__.gca(chunkName);
+    return Promise.all(
+      chunkNamesNeeded.map((chunkName) => {
+        // "__webpack_require__.gca" is injected by ChunkAssetPlugin. Pass it
+        // the name of the chunk you want to load and it will return its URL.
+        // eslint-disable-next-line camelcase
+        const chunkAsset = __webpack_require__.gca(chunkName);
 
-      // In some cases, webpack might decide to optimize further & hence the
-      // chunk assets are merged to another chunk/previous chunk.
-      // Hence, we can safely filter it out/don't need to load it.
-      if (chunkAsset && !/undefined/.test(chunkAsset)) {
-        prefetchHelper(chunkAsset);
-      }
-    });
-
-    return true;
+        // In some cases, webpack might decide to optimize further, leading to
+        // the chunk assets being merged to another chunk. In this case, we can
+        // safely filter it out and don't need to load it.
+        if (chunkAsset && !/undefined/.test(chunkAsset)) {
+          return prefetchHelper(chunkAsset);
+        }
+        return Promise.resolve();
+      }),
+    );
   },
 
-  preload: (routePath: string): boolean => {
+  preload(routePath: string): false | Promise<void[]> {
     if (!canPreload(routePath)) {
       return false;
     }
-
-    loaded[routePath] = true;
-    preloadHelper(routePath);
-    return true;
+    loaded.add(routePath);
+    return preloadHelper(routePath);
   },
 };
 

packages/docusaurus/src/client/exports/Link.tsx

@@ -21,13 +21,6 @@ import {useBaseUrlUtils} from './useBaseUrl';
 import {applyTrailingSlash} from '@docusaurus/utils-common';
 
 import type {Props} from '@docusaurus/Link';
-import type docusaurus from '../docusaurus';
-
-declare global {
-  interface Window {
-    docusaurus: typeof docusaurus;
-  }
-}
 
 // TODO all this wouldn't be necessary if we used ReactRouter basename feature
 // We don't automatically add base urls to all links,

packages/docusaurus/src/client/flat.ts

@@ -25,18 +25,18 @@ export default function flat(target: ChunkNames): {[keyPath: string]: string} {
   const delimiter = '.';
   const output: {[keyPath: string]: string} = {};
 
-  function step(object: Tree, prefix?: string | number) {
+  function dfs(object: Tree, prefix?: string | number) {
     Object.entries(object).forEach(([key, value]) => {
       const newKey = prefix ? `${prefix}${delimiter}${key}` : key;
 
       if (isTree(value)) {
-        step(value, newKey);
+        dfs(value, newKey);
       } else {
         output[newKey] = value;
       }
     });
   }
 
-  step(target);
+  dfs(target);
   return output;
 }

packages/docusaurus/src/client/prefetch.ts

@@ -14,7 +14,7 @@ function supports(feature: string) {
   }
 }
 
-function linkPrefetchStrategy(url: string) {
+function linkPrefetchStrategy(url: string): Promise<void> {
   return new Promise((resolve, reject) => {
     if (typeof document === 'undefined') {
       reject();
@@ -25,8 +25,8 @@ function linkPrefetchStrategy(url: string) {
     link.setAttribute('rel', 'prefetch');
     link.setAttribute('href', url);
 
-    link.onload = resolve;
-    link.onerror = reject;
+    link.onload = () => resolve();
+    link.onerror = () => reject();
 
     const parentElement =
       document.getElementsByTagName('head')[0] ??
@@ -57,20 +57,6 @@ const supportedPrefetchStrategy = supports('prefetch')
   ? linkPrefetchStrategy
   : xhrPrefetchStrategy;
 
-const preFetched: {[url: string]: boolean} = {};
-
 export default function prefetch(url: string): Promise<void> {
-  return new Promise((resolve) => {
-    if (preFetched[url]) {
-      resolve();
-      return;
-    }
-
-    supportedPrefetchStrategy(url)
-      .then(() => {
-        resolve();
-        preFetched[url] = true;
-      })
-      .catch(() => {}); // 404s are logged to the console anyway.
-  });
+  return supportedPrefetchStrategy(url).catch(() => {}); // 404s are logged to the console anyway.
 }