fix some pass-by-reference issues

lib/page-data.js

@@ -37,35 +37,39 @@ async function loadSiteTree (unversionedTree) {
   const rawTree = Object.assign({}, (unversionedTree || await loadUnversionedTree()))
   const siteTree = {}
 
+  // For every language...
   await Promise.all(Object.keys(languages).map(async (langCode) => {
     const treePerVersion = {}
-
+    // in every version...
     await Promise.all(versions.map(async (version) => {
-      versionPages(rawTree[langCode])
+      // "version" the pages.
+      treePerVersion[version] = versionPages(Object.assign({}, rawTree[langCode]), version)
+    }))
 
-      // This step can't be asynchronous because the order of child pages matters.
-      function versionPages (item) {
-        // Add a versioned href as a convenience for use in layouts.
-        item.href = item.page.permalinks
-          .find(pl => pl.pageVersion === version || (pl.pageVersion === 'homepage' && version === nonEnterpriseDefaultVersion))
-          .href
+    siteTree[langCode] = treePerVersion
+  }))
 
-        if (!item.childPages) return item
+  return siteTree
+}
 
-        // Drop child pages that do not apply to the current version.
-        item.childPages = item.childPages.filter(childPage => childPage.page.applicableVersions.includes(version))
+// This step can't be asynchronous because the order of child pages matters.
+function versionPages (obj, version) {
+  // Add a versioned href as a convenience for use in layouts.
+  obj.href = obj.page.permalinks
+    .find(pl => pl.pageVersion === version || (pl.pageVersion === 'homepage' && version === nonEnterpriseDefaultVersion))
+    .href
 
-        // Run on the child pages recursively.
-        item.childPages.forEach(childPage => versionPages(childPage))
-      }
+  if (!obj.childPages) return obj
 
-      treePerVersion[version] = rawTree[langCode]
-    }))
+  const versionedChildPages = obj.childPages
+    // Drop child pages that do not apply to the current version.
+    .filter(childPage => childPage.page.applicableVersions.includes(version))
+    // Version the child pages recursively.
+    .map(childPage => versionPages(Object.assign({}, childPage), version))
 
-    siteTree[langCode] = treePerVersion
-  }))
+  obj.childPages = [...versionedChildPages]
 
-  return siteTree
+  return obj
 }
 
 // Derive a flat array of Page objects in all languages.