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

This commit is a no-op refactor to make reviewing the following commit
easier.

Author: ilyagr

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

@@ -0,0 +1,34 @@
+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,
+              ),
+            )
           })
         )
       )