Extract function for core logic of "staying on the same page when switching versions"

We will add unit tests for this functions in the next commit.

The function gets its own file because I was unable to get the test runner ("mocha") to work otherwise. See the child commit's description for more details.

Author: ilyagr

src/templates/assets/javascripts/integrations/version/correspondingPage.ts

@@ -0,0 +1,37 @@
+// This is a separate file so that `mocha` can load it without needing
+// DOM shims.
+
+import { Sitemap } from "../sitemap"
+
+type CorrespondingURLParams = {
+  selectedVersionSitemap: Sitemap
+  selectedVersionBaseURL: URL
+  currentLocation: URL
+  currentBaseURL: string
+}
+
+/**
+ * Choose a URL to navigate to when the user chooses a version in the version
+ * selector.
+ *
+ * @param selectedVersionSitemap - as obtained by fetchSitemap from `${selectedVersionBaseURL}/sitemap.xml`
+ * @param selectedVersionBaseURL - usually `${currentBaseURL}/../selectedVersion`
+ * @param currentLocation - current web browser location
+ * @param currentBaseURL - as obtained from `config.base`
+ * @returns the URL to navigate to or null if we can't be sure that the
+ * corresponding page to the current page exists in the selected version
+ */
+export function selectedVersionCorrespondingURL(
+  {selectedVersionSitemap,
+    selectedVersionBaseURL,
+    currentLocation,
+    currentBaseURL}: CorrespondingURLParams
+): URL | undefined {
+  const result = currentLocation.href.replace(
+    currentBaseURL,
+    selectedVersionBaseURL.href,
+  )
+  return selectedVersionSitemap.has(result.split("#")[0])
+    ? new URL(result)
+    : undefined
+}

src/templates/assets/javascripts/integrations/version/index.ts

@@ -48,6 +48,8 @@ import {
 
 import { fetchSitemap } from "../sitemap"
 
+import { selectedVersionCorrespondingURL } from "./correspondingPage"
+
 /* ----------------------------------------------------------------------------
  * Helper types
  * ------------------------------------------------------------------------- */
@@ -122,22 +124,23 @@ export function setupVersionSelector(
                     return EMPTY
                 }
                 ev.preventDefault()
-                return of(url)
+                return of(new URL(url))
               }
             }
             return EMPTY
           }),
-          switchMap(url => {
-            return fetchSitemap(new URL(url))
-              .pipe(
-                map(sitemap => {
-                  const location = getLocation()
-                  const path = location.href.replace(config.base, url)
-                  return sitemap.has(path.split("#")[0])
-                    ? new URL(path)
-                    : new URL(url)
-                })
-              )
+          switchMap(selectedVersionBaseURL => {
+            return fetchSitemap(selectedVersionBaseURL).pipe(
+              map(
+                sitemap =>
+                  selectedVersionCorrespondingURL({
+                    selectedVersionSitemap: sitemap,
+                    selectedVersionBaseURL,
+                    currentLocation: getLocation(),
+                    currentBaseURL: config.base
+                  }) ?? selectedVersionBaseURL,
+              ),
+            )
           })
         )
       )