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 load the whole `index.ts` file; that would seem to require writing some DOM shims and be a general headache.

For the record, using `tsx` and `jsdom-global` instead of `ts-node` to run `mocha` seemed like the most promising approach.

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
+ * @param selectedVersionBaseURL
+ * @param currentLocation
+ * @param currentBaseURL
+ * @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
  * ------------------------------------------------------------------------- */
@@ -120,24 +122,25 @@ export function setupVersionSelector(
                   const version = urls.get(url)!
                   if (version === current)
                     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,
+              ),
+            )
           })
         )
       )