diff --git a/biome.json b/biome.json
index 20ba54c3..b4487259 100644
--- a/biome.json
+++ b/biome.json
@@ -1,6 +1,6 @@
 {
     "files": {
-        "includes": ["static/**/*.js", "static/**/*.css", "static/**/*.json"]
+        "includes": ["static/**/*.js", "static/**/*.css", "static/**/*.json", "!static/js/vendors/"]
     },
     "formatter": {
         "enabled": true,
@@ -15,7 +15,15 @@
             "recommended": true,
             "correctness": {
                 "noUnusedVariables": "warn",
-                "noUndeclaredVariables": "error"
+                "noUndeclaredVariables": "error",
+                "noUnreachable": "warn",
+                "noUnsafeFinally": "error"
+            },
+            "nursery": {
+                "useExplicitType": "error"
+            },
+            "security": {
+                "noGlobalEval": "error"
             },
             "style": {
                 "noDescendingSpecificity": "off"
diff --git a/jsconfig.json b/jsconfig.json
index f09567bd..ce15aaf8 100644
--- a/jsconfig.json
+++ b/jsconfig.json
@@ -2,17 +2,22 @@
   "compilerOptions": {
     // Enable type checking on all JS files (equivalent to @ts-check globally)
     "checkJs": true,
+    "allowJs": true,
     "strict": true,
-    "noImplicitAny": true,
+    "noEmit": true,
+    "noImplicitAny": false,
     "noImplicitReturns": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "exactOptionalPropertyTypes": true,
-    "target": "ES2022",
     "lib": ["ES2022", "DOM"],
     // Treat all JS files as modules
-    "moduleDetection": "force"
+    "moduleDetection": "force",
+    "strictNullChecks": false, // too much pedantic...
+    "moduleResolution": "bundler",
+    "skipLibCheck": true,
+    "target": "ESNext"
   },
   "include": ["static/js/**/*.js"],
-  "exclude": []
+  "exclude": ["static/js/vendors/**", "static/js/vendors/**/*.mjs"]
 }
diff --git a/static/js/app/filesView.js b/static/js/app/filesView.js
index a9650ee4..41ff4771 100644
--- a/static/js/app/filesView.js
+++ b/static/js/app/filesView.js
@@ -9,23 +9,9 @@ import { app } from './state.js';
 import { ui } from './ui.js';
 import { uiNotifications } from './uiNotifications.js';
 
-let isLoadingFiles = false;
+/** @import {FileInfo, FolderInfo} from '../core/types.js' */
 
-// TODO move to features/files/fileOperations.js ?
-/**
- * @typedef {Object} FolderInfo
- * @property {string} category
- * @property {number} created_at - timestamp
- * @property {string} icon_class
- * @property {string} icon_special_class
- * @property {string} id the uniq id of the folder
- * @property {boolean} is_root
- * @property {number} modified_at
- * @property {string} name
- * @property {string} owner_id
- * @property {string|null} parent_id the folder parent (null if is_root)
- * @property {string} path the full path
- */
+let isLoadingFiles = false;
 
 /**
  * getFolder information
@@ -99,7 +85,7 @@ async function rebuildBreadCrumb() {
             uiNotifications.show('error: folder not found or permission denied', 'the given folder is not available or you do not have sufficient rights');
             app.breadcrumbPath = [];
             id = app.userHomeFolderId;
-            app.currentPath = id;
+            if (id) app.currentPath = id;
         }
     }
 
@@ -114,6 +100,7 @@ async function rebuildBreadCrumb() {
  * @param {Object} options
  * @param {boolean} [options.insertHistory] add browser history (default true)
  * @param {boolean} [options.forceRefresh] force refresh of content
+ *
  */
 async function loadFiles(options = { insertHistory: true }) {
     try {
@@ -185,7 +172,7 @@ async function loadFiles(options = { insertHistory: true }) {
 
         if (forceRefresh) {
             url += `&force_refresh=true`;
-            requestOptions.headers['X-Force-Refresh'] = 'true';
+            if (requestOptions.headers) requestOptions.headers['X-Force-Refresh'] = 'true';
             console.log('Forcing complete refresh ignoring cache');
         }
 
@@ -215,7 +202,10 @@ async function loadFiles(options = { insertHistory: true }) {
             multiSelect.init(); // this will wire buttons & select-all-checkbox
         }
 
+        /** @type {FolderInfo[]} */
         const folderList = Array.isArray(listing.folders) ? listing.folders : [];
+
+        /** @type {FileInfo[]} */
         const fileList = Array.isArray(listing.files) ? listing.files : [];
 
         if (folderList.length === 0 && fileList.length === 0) {
diff --git a/static/js/app/main.js b/static/js/app/main.js
index fa07b9d8..10621899 100644
--- a/static/js/app/main.js
+++ b/static/js/app/main.js
@@ -31,7 +31,7 @@ import { ui } from './ui.js';
 import { setupUserMenu } from './userMenu.js';
 
 // Upload dropdown listener state (prevents accumulated listeners)
-/** @type { function | null } */
+/** @type {((e: MouseEvent) => void) | null} */
 let uploadDropdownDocumentClickHandler = null;
 
 /** @type { AbortController | null } */
@@ -178,7 +178,7 @@ function setupActionsBarDelegation() {
     actionsBarDelegationBound = true;
 
     elements.actionsBar.addEventListener('click', async (e) => {
-        const btn = e.target.closest('button');
+        const btn = /** @type {HTMLElement} */ (e.target)?.closest('button');
         if (!btn) return;
 
         switch (btn.id) {
@@ -321,7 +321,7 @@ function switchSectionTo(section) {
         // no change ...
         return;
 
-    if ((!section) in SECTIONS_MAPPER) {
+    if (!(section in SECTIONS_MAPPER)) {
         console.warn(`context view ${section} unkonwn fallback to files section`);
         section = 'files';
     }
@@ -420,7 +420,7 @@ function initApp() {
 function cacheElements() {
     elements.uploadBtn = document.getElementById('upload-btn');
     elements.dropzone = document.getElementById('dropzone');
-    elements.fileInput = document.getElementById('file-input');
+    elements.fileInput = /** @type {HTMLInputElement} */ (document.getElementById('file-input'));
     elements.filesList = document.getElementById('files-list');
     elements.newFolderBtn = document.getElementById('new-folder-btn');
     elements.gridViewBtn = document.getElementById('grid-view-btn');
@@ -472,7 +472,7 @@ function setupUploadDropdown() {
         document.removeEventListener('click', uploadDropdownDocumentClickHandler);
     }
     uploadDropdownDocumentClickHandler = (e) => {
-        if (e.target.closest('#upload-dropdown')) return;
+        if (/** @type {HTMLElement} */ (e.target)?.closest('#upload-dropdown')) return;
         document.querySelectorAll('.upload-dropdown-menu').forEach((m) => {
             m.classList.add('hidden');
         });
@@ -511,11 +511,11 @@ function setupEventListeners() {
     });
 
     // Search input — Enter key
-    elements.searchInput.addEventListener('keydown', (e) => {
+    elements.searchInput?.addEventListener('keydown', (e) => {
         if (e.key === 'Enter') {
             // Cancel any pending debounce
             if (searchDebounceTimer) clearTimeout(searchDebounceTimer);
-            const query = elements.searchInput.value.trim();
+            const query = elements.searchInput?.value.trim();
 
             // In shared section, filter locally
             if (app.currentSection === 'shared' && sharedView) {
@@ -529,16 +529,17 @@ function setupEventListeners() {
                 // If search is empty and we're in search mode, return to normal view
                 app.isSearchMode = false;
                 app.currentPath = '';
-                ui.updateBreadcrumb('');
+                ui.updateBreadcrumb();
                 loadFiles();
             }
         }
     });
 
     // Search input — Live search (debounced, after 3+ chars)
-    elements.searchInput.addEventListener('input', () => {
+    elements.searchInput?.addEventListener('input', () => {
         if (searchDebounceTimer) clearTimeout(searchDebounceTimer);
-        const query = elements.searchInput.value.trim();
+        const query = elements.searchInput?.value.trim();
+        if (!query) return;
 
         if (query.length >= SEARCH_MIN_CHARS) {
             searchDebounceTimer = setTimeout(() => {
@@ -549,16 +550,16 @@ function setupEventListeners() {
             searchDebounceTimer = setTimeout(() => {
                 app.isSearchMode = false;
                 app.currentPath = '';
-                ui.updateBreadcrumb('');
+                ui.updateBreadcrumb();
                 loadFiles();
             }, SEARCH_DEBOUNCE_MS);
         }
     });
 
     // Search button
-    document.getElementById('search-button').addEventListener('click', () => {
+    document.getElementById('search-button')?.addEventListener('click', () => {
         if (searchDebounceTimer) clearTimeout(searchDebounceTimer);
-        const query = elements.searchInput.value.trim();
+        const query = elements.searchInput?.value.trim();
         if (query) {
             performSearch(query);
         }
@@ -572,10 +573,13 @@ function setupEventListeners() {
     }
 
     // File input
-    elements.fileInput.addEventListener('change', (e) => {
-        if (e.target.files.length > 0) {
-            fileOps.uploadFiles(e.target.files);
-            e.target.value = ''; // reset so same file can be re-uploaded
+    elements.fileInput?.addEventListener('change', (e) => {
+        const target = /** @type {HTMLInputElement} */ (e.target);
+        if (!target) return;
+        if (!target.files) return;
+        if (target.files.length > 0) {
+            fileOps.uploadFiles(target.files);
+            target.value = ''; // reset so same file can be re-uploaded
         }
     });
 
@@ -583,18 +587,21 @@ function setupEventListeners() {
     const folderInput = document.getElementById('folder-input');
     if (folderInput) {
         folderInput.addEventListener('change', (e) => {
-            if (e.target.files.length > 0) {
-                fileOps.uploadFolderFiles(e.target.files);
-                e.target.value = '';
+            const target = /** @type {HTMLInputElement} */ (e.target);
+            if (!target) return;
+            if (!target.files) return;
+            if (target.files.length > 0) {
+                fileOps.uploadFolderFiles(target.files);
+                target.value = '';
             }
         });
     }
 
     // Sidebar navigation
-    elements.navItems.forEach((item) => {
+    elements.navItems?.forEach((item) => {
         item.addEventListener('click', () => {
             // Remove active class from all nav items
-            elements.navItems.forEach((navItem) => {
+            elements.navItems?.forEach((navItem) => {
                 navItem.classList.remove('active');
             });
 
@@ -602,7 +609,7 @@ function setupEventListeners() {
             item.classList.add('active');
             let _updateHistory = true;
 
-            const itemI18nKey = item.querySelector('span').getAttribute('data-i18n');
+            const itemI18nKey = item.querySelector('span')?.getAttribute('data-i18n');
 
             switch (itemI18nKey) {
                 case 'nav.shared':
@@ -661,12 +668,13 @@ function setupEventListeners() {
     // Global events to close context menus and deselect cards
     document.addEventListener('click', (e) => {
         const folderMenu = document.getElementById('folder-context-menu');
-        if (folderMenu && !folderMenu.classList.contains('hidden') && !folderMenu.contains(e.target)) {
+        const target = /** @type {HTMLElement} */ (e.target);
+        if (folderMenu && !folderMenu.classList.contains('hidden') && !folderMenu.contains(target)) {
             ui.closeContextMenu();
         }
 
         const fileMenu = document.getElementById('file-context-menu');
-        if (fileMenu && !fileMenu.classList.contains('hidden') && !fileMenu.contains(e.target)) {
+        if (fileMenu && !fileMenu.classList.contains('hidden') && !fileMenu.contains(target)) {
             ui.closeFileContextMenu();
         }
     });
@@ -714,8 +722,8 @@ function updateStorageUsageDisplay(userData) {
     const quotaFormatted = formatQuotaSize(quotaBytes);
 
     // Update the storage display elements
-    const storageFill = document.querySelector('.storage-fill');
-    const storageInfo = document.querySelector('.storage-info');
+    const storageFill = /** @type {HTMLDivElement} */ (document.querySelector('.storage-fill'));
+    const storageInfo = /** @type {HTMLDivElement} */ (document.querySelector('.storage-info'));
 
     if (storageFill) {
         storageFill.style.width = `${usagePercentage}%`;
diff --git a/static/js/app/navigation.js b/static/js/app/navigation.js
index 3e2c61b5..3bea519a 100644
--- a/static/js/app/navigation.js
+++ b/static/js/app/navigation.js
@@ -27,14 +27,14 @@ function syncViewContainers() {
 
     const isGrid = app.currentView === 'grid';
     if (isGrid) {
-        filesList.classList.remove('files-list-view');
-        filesList.classList.add('files-grid-view');
+        filesList?.classList.remove('files-list-view');
+        filesList?.classList.add('files-grid-view');
 
         gridViewBtn?.classList.add('active');
         listViewBtn?.classList.remove('active');
     } else {
-        filesList.classList.add('files-list-view');
-        filesList.classList.remove('files-grid-view');
+        filesList?.classList.add('files-list-view');
+        filesList?.classList.remove('files-grid-view');
 
         gridViewBtn?.classList.remove('active');
         listViewBtn?.classList.add('active');
@@ -47,7 +47,7 @@ function syncViewContainers() {
  */
 function toggleFileContainer(show) {
     const filesList = document.getElementById('files-list');
-    filesList.classList.toggle('hidden', !show);
+    filesList?.classList.toggle('hidden', !show);
 }
 
 /**
@@ -61,19 +61,19 @@ function initSidebarToggle() {
     if (!sidebarToggle || !sidebar || !sidebarOverlay) return;
 
     function openSidebar() {
-        sidebar.classList.add('open');
-        sidebarOverlay.classList.add('active');
+        sidebar?.classList.add('open');
+        sidebarOverlay?.classList.add('active');
         document.body.style.overflow = 'hidden';
     }
 
     function closeSidebar() {
-        sidebar.classList.remove('open');
-        sidebarOverlay.classList.remove('active');
+        sidebar?.classList.remove('open');
+        sidebarOverlay?.classList.remove('active');
         document.body.style.overflow = '';
     }
 
     function toggleSidebar() {
-        if (sidebar.classList.contains('open')) {
+        if (sidebar?.classList.contains('open')) {
             closeSidebar();
         } else {
             openSidebar();
@@ -118,6 +118,7 @@ function getSectionFromNavItem(navItem) {
 }
 
 // Mapping section name to associated switch functions
+/** @type {Record<String, Function>} */
 export const SECTIONS_MAPPER = {
     files: switchToFilesSection,
     shared: switchToSharedSection,
@@ -144,7 +145,7 @@ function setCurrentSection(section) {
     app.currentSection = section;
 
     // Update nav item active classes by finding matching item from DOM
-    appElements.navItems.forEach((item) => {
+    appElements.navItems?.forEach((item) => {
         const itemSection = getSectionFromNavItem(item);
         item.classList.toggle('active', itemSection === section);
     });
@@ -152,8 +153,10 @@ function setCurrentSection(section) {
     // Update page title
     const titleKey = `nav.${section}`;
     // TODO check why no more used: const defaultTitle = section.charAt(0).toUpperCase() + section.slice(1);
-    appElements.pageTitle.textContent = i18n.t(titleKey);
-    appElements.pageTitle.setAttribute('data-i18n', titleKey);
+    if (appElements.pageTitle) {
+        appElements.pageTitle.textContent = i18n.t(titleKey);
+        appElements.pageTitle.setAttribute('data-i18n', titleKey);
+    }
 
     // Hide sharedView when switching to any other section
     if (section !== 'shared' && sharedView) {
diff --git a/static/js/app/searchView.js b/static/js/app/searchView.js
index 4c7ef06b..1ae99d57 100644
--- a/static/js/app/searchView.js
+++ b/static/js/app/searchView.js
@@ -16,7 +16,7 @@ async function performSearch(query, sortBy) {
 
     try {
         app.isSearchMode = true;
-        ui.updateBreadcrumb(`Search: "${query}"`);
+        ui.updateBreadcrumb();
 
         ui.showError(`<h3><i class="fas fa-spinner fa-spin search-spinner"></i> Searching for "${query}"...</h3>`);
 
@@ -48,9 +48,10 @@ async function performSearch(query, sortBy) {
 }
 
 document.addEventListener('search-resort', (e) => {
-    const searchInput = document.querySelector('.search-container input');
+    const event = /** @type {CustomEvent<{sort_by: string}>} */ (e);
+    const searchInput = /** @type {HTMLInputElement} */ (document.querySelector('.search-container input'));
     if (searchInput?.value.trim()) {
-        performSearch(searchInput.value.trim(), e.detail.sort_by);
+        performSearch(searchInput.value.trim(), event.detail.sort_by);
     }
 });
 
diff --git a/static/js/app/state.js b/static/js/app/state.js
index 3911e264..06bc3da6 100644
--- a/static/js/app/state.js
+++ b/static/js/app/state.js
@@ -3,15 +3,27 @@
  * Centralized mutable state for app and cached DOM references.
  */
 
+/** @import {FolderInfo} from '../core/types.js' */
+
 export const app = {
     currentView: 'grid',
+
+    /** @type {string | null} */
     currentPath: '',
     currentFolder: null,
+
+    /** @type {FolderInfo | null} */
     currentFolderInfo: null,
+
+    /** @type {Object | null} */
     contextMenuTargetFolder: null,
+
+    /** @type {Object | null} */
     contextMenuTargetFile: null,
     selectedTargetFolderId: '',
     moveDialogMode: 'file',
+
+    /** @type {String | null} */
     currentSection: null, // will be defined on first call
     isSearchMode: false,
     shareDialogItem: null,
@@ -19,8 +31,36 @@ export const app = {
     notificationShareUrl: null,
     userHomeFolderId: null,
     userHomeFolderName: null,
+    /** @type {Object[]} */
     breadcrumbPath: [], // Array of {id, name} tracking folder navigation hierarchy
+
+    /** @type {String | null} */
     viewFile: null // current file in inline view
 };
 
-export const appElements = {};
+export const appElements = {
+    /** @type {HTMLElement | null} */
+    uploadBtn: null,
+    /** @type {HTMLElement | null}  */
+    dropzone: null,
+    /** @type {HTMLInputElement | null}  */
+    fileInput: null,
+    /** @type {HTMLElement | null}  */
+    filesList: null,
+    /** @type {HTMLElement | null}  */
+    newFolderBtn: null,
+    /** @type {HTMLElement | null}  */
+    gridViewBtn: null,
+    /** @type {HTMLElement | null}  */
+    listViewBtn: null,
+    /** @type {HTMLElement | null}  */
+    breadcrumb: null,
+    /** @type {HTMLElement | null}  */
+    pageTitle: null,
+    /** @type {HTMLElement | null}  */
+    actionsBar: null,
+    /** @type {NodeListOf<HTMLElement> | null}  */
+    navItems: null,
+    /** @type {HTMLInputElement | null}  */
+    searchInput: null
+};
diff --git a/static/js/app/ui.js b/static/js/app/ui.js
index 0bdd375d..109cd2ff 100644
--- a/static/js/app/ui.js
+++ b/static/js/app/ui.js
@@ -24,8 +24,6 @@ import { app } from './state.js';
 import { uiFileTypes } from './uiFileTypes.js';
 import { uiNotifications } from './uiNotifications.js';
 
-let __rubberBandJustFinished = false;
-
 // UI Module
 const ui = {
     /** @type {HTMLDListElement | null} */
@@ -235,22 +233,22 @@ const ui = {
             document.body.appendChild(shareDialog);
 
             // Add event listeners for share dialog
-            document.getElementById('share-close-btn').addEventListener('click', () => {
+            document.getElementById('share-close-btn')?.addEventListener('click', () => {
                 contextMenus.closeShareDialog();
             });
 
-            document.getElementById('share-confirm-btn').addEventListener('click', async () => {
+            document.getElementById('share-confirm-btn')?.addEventListener('click', async () => {
                 await contextMenus.createSharedLink();
             });
 
-            document.getElementById('copy-share-btn').addEventListener('click', async () => {
-                const shareUrl = document.getElementById('generated-share-url').value;
-                await fileSharing.copyLinkToClipboard(shareUrl);
+            document.getElementById('copy-share-btn')?.addEventListener('click', async () => {
+                const shareUrl = /** @type {HTMLInputElement | null} */ (document.getElementById('generated-share-url'))?.value;
+                if (shareUrl) await fileSharing.copyLinkToClipboard(shareUrl);
             });
 
-            document.getElementById('notify-share-btn').addEventListener('click', () => {
-                const shareUrl = document.getElementById('generated-share-url').value;
-                contextMenus.showEmailNotificationDialog(shareUrl);
+            document.getElementById('notify-share-btn')?.addEventListener('click', () => {
+                const shareUrl = /** @type {HTMLInputElement | null} */ (document.getElementById('generated-share-url'))?.value;
+                if (shareUrl) contextMenus.showEmailNotificationDialog(shareUrl);
             });
 
             // FIXME make generic function (close all dialog / etc)
@@ -301,11 +299,11 @@ const ui = {
             document.body.appendChild(notificationDialog);
 
             // Add event listeners for notification dialog
-            document.getElementById('notification-cancel-btn').addEventListener('click', () => {
+            document.getElementById('notification-cancel-btn')?.addEventListener('click', () => {
                 contextMenus.closeNotificationDialog();
             });
 
-            document.getElementById('notification-send-btn').addEventListener('click', () => {
+            document.getElementById('notification-send-btn')?.addEventListener('click', () => {
                 contextMenus.sendShareNotification();
             });
         }
@@ -332,7 +330,7 @@ const ui = {
             `;
             document.body.appendChild(playlistDialog);
 
-            document.getElementById('playlist-cancel-btn').addEventListener('click', () => {
+            document.getElementById('playlist-cancel-btn')?.addEventListener('click', () => {
                 if (contextMenus) contextMenus.closePlaylistDialog();
             });
         }
@@ -373,9 +371,9 @@ const ui = {
                         entry.file(
                             (file) => {
                                 out.push({ file, relativePath: `${prefix}${file.name}` });
-                                resolve();
+                                resolve(undefined);
                             },
-                            () => resolve()
+                            () => resolve(undefined)
                         );
                     });
                     return;
@@ -407,20 +405,26 @@ const ui = {
         };
 
         // Dropzone events
-        dropzone.addEventListener('dragover', (e) => {
+        dropzone?.addEventListener('dragover', (e) => {
             e.preventDefault();
             dropzone.classList.add('active');
         });
 
-        dropzone.addEventListener('dragleave', () => {
+        dropzone?.addEventListener('dragleave', () => {
             dropzone.classList.remove('active');
         });
 
-        dropzone.addEventListener('drop', async (e) => {
+        // remove previous hack e._oxiHandled
+        // WeakSet will automatically garbage collect entry
+        const handledDropEvents = new WeakSet();
+
+        dropzone?.addEventListener('drop', async (e) => {
             e.preventDefault();
             e.stopPropagation(); // Prevent bubbling to document's drop handler (avoids double upload)
-            e._oxiHandled = true; // Mark as handled for document-level fallback
+            handledDropEvents.add(e); // Mark as handled for document-level fallback
             dropzone.classList.remove('active');
+            if (!e.dataTransfer) return;
+
             if (e.dataTransfer.files.length > 0) {
                 // First try directory-aware extraction (Finder folder drag & drop)
                 const droppedEntries = await collectDroppedEntries(e.dataTransfer);
@@ -453,6 +457,7 @@ const ui = {
         // Document-wide drag and drop
         document.addEventListener('dragover', (e) => {
             e.preventDefault();
+            if (!e.dataTransfer) return;
             if (e.dataTransfer.types.includes('Files')) {
                 dropzone?.classList.remove('hidden');
                 dropzone?.classList.add('active');
@@ -461,9 +466,9 @@ const ui = {
 
         document.addEventListener('dragleave', (e) => {
             if (e.clientX <= 0 || e.clientY <= 0 || e.clientX >= window.innerWidth || e.clientY >= window.innerHeight) {
-                dropzone.classList.remove('active');
+                dropzone?.classList.remove('active');
                 setTimeout(() => {
-                    if (!dropzone.classList.contains('active')) {
+                    if (!dropzone?.classList.contains('active')) {
                         dropzone?.classList.add('hidden');
                     }
                 }, 100);
@@ -472,11 +477,12 @@ const ui = {
 
         document.addEventListener('drop', async (e) => {
             e.preventDefault();
-            dropzone.classList.remove('active');
+            dropzone?.classList.remove('active');
 
             // Skip if already handled by the dropzone handler (defensive against bubble leaks)
-            if (e._oxiHandled) return;
+            if (handledDropEvents.has(e)) return;
 
+            if (!e.dataTransfer) return;
             if (e.dataTransfer.files.length > 0) {
                 // First try directory-aware extraction (Finder folder drag & drop)
                 const droppedEntries = await collectDroppedEntries(e.dataTransfer);
@@ -539,7 +545,9 @@ const ui = {
      */
     updateBreadcrumb() {
         const breadcrumb = document.querySelector('.breadcrumb');
-        breadcrumb.innerHTML = '';
+        if (breadcrumb) {
+            breadcrumb.innerHTML = '';
+        }
         const path = app.breadcrumbPath; // [{id, name}, ...]
 
         // -- Home icon (always present, clickable to go to root) --
@@ -558,7 +566,7 @@ const ui = {
                 loadFiles();
             });
         }
-        breadcrumb.appendChild(homeIcon);
+        breadcrumb?.appendChild(homeIcon);
 
         // -- Root/Home folder name (if available) is always the first element of the breadcrumb --
         // TODO clarify the difference between homeIcon & this first element
@@ -579,7 +587,7 @@ const ui = {
             const separator = document.createElement('span');
             separator.className = 'breadcrumb-separator';
             separator.textContent = '>';
-            breadcrumb.appendChild(separator);
+            breadcrumb?.appendChild(separator);
 
             // Segment item
             const item = document.createElement('span');
@@ -600,7 +608,7 @@ const ui = {
                 // can drag files on this folder
                 // dragover – only folders are valid drop targets
                 item.addEventListener('dragover', (e) => {
-                    const card = e.target.closest('span');
+                    const card = /** @type {HTMLElement} */ (e.target).closest('span');
                     if (!card?.dataset.folderId) return;
                     e.preventDefault();
                     card.classList.add('drop-target');
@@ -609,14 +617,14 @@ const ui = {
                 // dragleave
                 item.addEventListener('dragleave', (e) => {
                     console.log('dragleave ', e);
-                    const card = e.target.closest('span');
+                    const card = /** @type {HTMLElement} */ (e.target).closest('span');
                     if (!card?.dataset.folderId) return;
                     card.classList.remove('drop-target');
                 });
 
                 // drop – only folders accept drops
                 item.addEventListener('drop', async (e) => {
-                    const card = e.target.closest('span');
+                    const card = /** @type {HTMLElement} */ (e.target).closest('span');
                     if (!card) return;
                     const targetFolderId = card.dataset.folderId;
                     if (!targetFolderId) return;
@@ -625,13 +633,15 @@ const ui = {
                     card.classList.remove('drop-target');
 
                     const action = e.dataTransfer?.dropEffect;
-                    await this._dropToFolder(action, targetFolderId, e.dataTransfer);
+                    if (action) {
+                        await this._dropToFolder(action, targetFolderId, e.dataTransfer);
+                    }
                 });
             } else {
                 // Last segment: current location, not clickable
                 item.classList.add('breadcrumb-current');
             }
-            breadcrumb.appendChild(item);
+            breadcrumb?.appendChild(item);
         });
     },
 
@@ -915,7 +925,7 @@ const ui = {
                     parent_id: card.dataset.parentId || ''
                 };
             } else {
-                const fileData = info.data || self._items.get(info.id);
+                const fileData = info.data || this._items.get(info.id);
                 app.contextMenuTargetFile = {
                     id: info.id,
                     name: card.dataset.fileName,
@@ -927,27 +937,27 @@ const ui = {
 
         // ──  click (open / navigate; select only via checkbox) ──
         filesList.addEventListener('click', (e) => {
-            const card = e.target.closest('.file-item');
+            const card = /** @type {HTMLElement} */ (e.target).closest('.file-item');
             if (!card) return;
 
-            if (e.target.closest('.file-actions')) {
+            if (/** @type {HTMLElement} */ (e.target).closest('.file-actions')) {
                 e.stopPropagation();
                 e.preventDefault();
                 const info = itemInfo(card);
                 if (!info) return;
                 setContextTarget(card, info);
                 const menuId = info.type === 'folder' ? 'folder-context-menu' : 'file-context-menu';
-                showContextMenuAtElement(e.target.closest('.file-actions'), menuId);
+                showContextMenuAtElement(/** @type {HTMLElement} */ (e.target).closest('.file-actions'), menuId);
                 return;
             }
 
-            if (e.target.closest('.checkbox-cell')) {
+            if (/** @type {HTMLElement} */ (e.target).closest('.checkbox-cell')) {
                 toggleCardSelection(card, e);
                 return;
             }
 
             // Favorite star – handled by direct onclick on the button
-            if (e.target.closest('.favorite-star')) return;
+            if (/** @type {HTMLElement} */ (e.target).closest('.favorite-star')) return;
 
             // Single-click opens/navigates (selection is only via checkbox)
             const info = itemInfo(card);
@@ -984,7 +994,7 @@ const ui = {
         // ── shared events ──────────────────────
 
         filesList.addEventListener('contextmenu', (e) => {
-            const card = e.target.closest('.file-item');
+            const card = /** @type {HTMLElement} */ (e.target).closest('.file-item');
             if (!card) return;
             e.preventDefault();
             const info = itemInfo(card);
@@ -1001,14 +1011,16 @@ const ui = {
             if (contextMenus && typeof contextMenus.syncAddToPlaylistOption === 'function') {
                 contextMenus.syncAddToPlaylistOption();
             }
-            menu.style.left = `${e.pageX}px`;
-            menu.style.top = `${e.pageY}px`;
-            menu?.classList.remove('hidden');
+            if (menu) {
+                menu.style.left = `${e.pageX}px`;
+                menu.style.top = `${e.pageY}px`;
+                menu.classList.remove('hidden');
+            }
         });
 
         // dragstart
         filesList.addEventListener('dragstart', (e) => {
-            const card = e.target.closest('.file-item');
+            const card = /** @type {HTMLElement} */ (e.target).closest('.file-item');
             if (!card) {
                 e.preventDefault();
                 return;
@@ -1020,6 +1032,8 @@ const ui = {
                 return;
             }
 
+            if (!e.dataTransfer) return;
+
             e.dataTransfer.setData('text/plain', info.id);
             if (info.type === 'folder') {
                 e.dataTransfer.setData('application/oxicloud-folder', 'true');
@@ -1076,7 +1090,7 @@ const ui = {
 
             // if more than maxElements display the fading
             if (selectedCardFromList.length > maxElements) {
-                lastItemDiv.classList.add('fading');
+                lastItemDiv?.classList.add('fading');
             }
 
             this.dragPreview.appendChild(this.draggedItems);
@@ -1093,7 +1107,7 @@ const ui = {
 
         // dragover – only folders are valid drop targets
         filesList.addEventListener('dragover', (e) => {
-            const card = e.target.closest('.file-item');
+            const card = /** @type {HTMLElement} */ (/** @type {HTMLElement} */ (e.target).closest('.file-item'));
             if (!card || card.dataset.fileId) return;
             if (!card.dataset.folderId) return;
             e.preventDefault();
@@ -1102,14 +1116,14 @@ const ui = {
 
         // dragleave
         filesList.addEventListener('dragleave', (e) => {
-            const card = e.target.closest('.file-item');
+            const card = /** @type {HTMLElement} */ (/** @type {HTMLElement} */ (e.target).closest('.file-item'));
             if (!card || card.dataset.fileId) return;
             card.classList.remove('drop-target');
         });
 
         // drop – only folders accept drops
         filesList.addEventListener('drop', async (e) => {
-            const card = e.target.closest('.file-item');
+            const card = /** @type {HTMLElement} */ (/** @type {HTMLElement} */ (e.target).closest('.file-item'));
             if (!card || card.dataset.fileId) return;
             const targetFolderId = card.dataset.folderId;
             if (!targetFolderId) return;
@@ -1117,6 +1131,7 @@ const ui = {
             e.preventDefault();
             card.classList.remove('drop-target');
 
+            if (!e.dataTransfer) return;
             const action = e.dataTransfer.dropEffect;
             await this._dropToFolder(action, targetFolderId, e.dataTransfer);
         });
@@ -1203,7 +1218,7 @@ const ui = {
             const targetPath = isFavorite ? filledPath : outlinePath;
             if (svg && targetPath) {
                 const p = svg.querySelector('path');
-                if (p) p.setAttribute('d', targetPath[1]);
+                if (p) p.setAttribute('d', String(targetPath[1]));
                 svg.setAttribute('viewBox', `0 0 ${targetPath[0]} 512`);
             }
 
@@ -1379,6 +1394,8 @@ const ui = {
     /**
      * Render an array of folders into both grid and list views
      * using DocumentFragment for minimal reflows.
+     *
+     * @param {FolderInfo[]} folders
      */
     renderFolders(folders) {
         if (!this._delegationReady) this.initDelegation();
@@ -1503,6 +1520,8 @@ function showContextMenuAtElement(triggerElement, menuId) {
     menu.classList.remove('hidden');
 }
 
+let __rubberBandJustFinished = false;
+
 /**
  * Rubber band (lasso) selection — click + drag on empty grid area
  * to draw a rectangle and select all cards it touches.
@@ -1635,7 +1654,7 @@ if (document.readyState === 'loading') {
  * @param {boolean} [options.danger=false] - Use danger styling (red)
  * @returns {Promise<boolean>} true if confirmed, false if cancelled
  */
-function showConfirmDialog({ title, message, confirmText, cancelText, danger = true } = {}) {
+function showConfirmDialog({ title, message, confirmText, cancelText, danger = true }) {
     const ct = confirmText || i18n.t('actions.delete');
     const cc = cancelText || i18n.t('actions.cancel');
     const t = title || i18n.t('dialogs.confirm_title');
@@ -1674,8 +1693,8 @@ function showConfirmDialog({ title, message, confirmText, cancelText, danger = t
             resolve(result);
         };
 
-        overlay.querySelector('.confirm-dialog-cancel').addEventListener('click', () => cleanup(false));
-        overlay.querySelector('.confirm-dialog-ok').addEventListener('click', () => cleanup(true));
+        overlay.querySelector('.confirm-dialog-cancel')?.addEventListener('click', () => cleanup(false));
+        overlay.querySelector('.confirm-dialog-ok')?.addEventListener('click', () => cleanup(true));
         overlay.addEventListener('click', (e) => {
             if (e.target === overlay) cleanup(false);
         });
diff --git a/static/js/app/uiFileTypes.js b/static/js/app/uiFileTypes.js
index d547df46..497dc462 100644
--- a/static/js/app/uiFileTypes.js
+++ b/static/js/app/uiFileTypes.js
@@ -5,8 +5,162 @@
 
 import { isTextViewable } from '../core/formatters.js';
 
+/** @import {FileInfo} from '../core/types.js' */
+
+/** @type {Record<string, string>} */
+const ICON_CLASS_MAP = {
+    pdf: 'fas fa-file-pdf',
+    doc: 'fas fa-file-word',
+    docx: 'fas fa-file-word',
+    txt: 'fas fa-file-alt',
+    rtf: 'fas fa-file-alt',
+    odt: 'fas fa-file-alt',
+    xls: 'fas fa-file-excel',
+    xlsx: 'fas fa-file-excel',
+    csv: 'fas fa-file-excel',
+    ods: 'fas fa-file-excel',
+    ppt: 'fas fa-file-powerpoint',
+    pptx: 'fas fa-file-powerpoint',
+    odp: 'fas fa-file-powerpoint',
+    jpg: 'fas fa-file-image',
+    jpeg: 'fas fa-file-image',
+    png: 'fas fa-file-image',
+    gif: 'fas fa-file-image',
+    svg: 'fas fa-file-image',
+    webp: 'fas fa-file-image',
+    bmp: 'fas fa-file-image',
+    ico: 'fas fa-file-image',
+    mp4: 'fas fa-file-video',
+    avi: 'fas fa-file-video',
+    mov: 'fas fa-file-video',
+    mkv: 'fas fa-file-video',
+    webm: 'fas fa-file-video',
+    flv: 'fas fa-file-video',
+    mp3: 'fas fa-file-audio',
+    wav: 'fas fa-file-audio',
+    ogg: 'fas fa-file-audio',
+    flac: 'fas fa-file-audio',
+    aac: 'fas fa-file-audio',
+    m4a: 'fas fa-file-audio',
+    zip: 'fas fa-file-archive',
+    rar: 'fas fa-file-archive',
+    '7z': 'fas fa-file-archive',
+    tar: 'fas fa-file-archive',
+    gz: 'fas fa-file-archive',
+    js: 'fas fa-file-code',
+    ts: 'fas fa-file-code',
+    py: 'fas fa-file-code',
+    rs: 'fas fa-file-code',
+    java: 'fas fa-file-code',
+    html: 'fas fa-file-code',
+    css: 'fas fa-file-code',
+    json: 'fas fa-file-code',
+    xml: 'fas fa-file-code',
+    sh: 'fas fa-terminal',
+    bash: 'fas fa-terminal',
+    bat: 'fas fa-terminal',
+    md: 'fas fa-file-alt'
+};
+
+/** @type {Record<string, string>} */
+const ICON_SPECIAL_CLASS_MAP = {
+    pdf: 'pdf-icon',
+    doc: 'doc-icon',
+    docx: 'doc-icon',
+    odt: 'doc-icon',
+    rtf: 'doc-icon',
+    xls: 'spreadsheet-icon',
+    xlsx: 'spreadsheet-icon',
+    ods: 'spreadsheet-icon',
+    csv: 'spreadsheet-icon',
+    ppt: 'presentation-icon',
+    pptx: 'presentation-icon',
+    odp: 'presentation-icon',
+    key: 'presentation-icon',
+    jpg: 'image-icon',
+    jpeg: 'image-icon',
+    png: 'image-icon',
+    gif: 'image-icon',
+    svg: 'image-icon',
+    webp: 'image-icon',
+    bmp: 'image-icon',
+    ico: 'image-icon',
+    heic: 'image-icon',
+    heif: 'image-icon',
+    avif: 'image-icon',
+    tiff: 'image-icon',
+    mp4: 'video-icon',
+    avi: 'video-icon',
+    mkv: 'video-icon',
+    mov: 'video-icon',
+    wmv: 'video-icon',
+    flv: 'video-icon',
+    webm: 'video-icon',
+    m4v: 'video-icon',
+    mp3: 'audio-icon',
+    wav: 'audio-icon',
+    ogg: 'audio-icon',
+    flac: 'audio-icon',
+    aac: 'audio-icon',
+    wma: 'audio-icon',
+    m4a: 'audio-icon',
+    opus: 'audio-icon',
+    zip: 'archive-icon',
+    rar: 'archive-icon',
+    '7z': 'archive-icon',
+    tar: 'archive-icon',
+    gz: 'archive-icon',
+    bz2: 'archive-icon',
+    xz: 'archive-icon',
+    exe: 'installer-icon',
+    msi: 'installer-icon',
+    dmg: 'installer-icon',
+    deb: 'installer-icon',
+    rpm: 'installer-icon',
+    appimage: 'installer-icon',
+    py: 'code-icon py-icon',
+    rs: 'code-icon rust-icon',
+    go: 'code-icon go-icon',
+    js: 'code-icon js-icon',
+    jsx: 'code-icon js-icon',
+    mjs: 'code-icon js-icon',
+    ts: 'code-icon ts-icon',
+    tsx: 'code-icon ts-icon',
+    java: 'code-icon java-icon',
+    c: 'code-icon c-icon',
+    cpp: 'code-icon c-icon',
+    cs: 'code-icon cs-icon',
+    rb: 'code-icon ruby-icon',
+    php: 'code-icon php-icon',
+    swift: 'code-icon swift-icon',
+    html: 'code-icon html-icon',
+    htm: 'code-icon html-icon',
+    css: 'code-icon css-icon',
+    scss: 'code-icon css-icon',
+    json: 'code-icon json-icon',
+    xml: 'code-icon html-icon',
+    yaml: 'code-icon config-icon',
+    yml: 'code-icon config-icon',
+    toml: 'code-icon config-icon',
+    ini: 'code-icon config-icon',
+    sql: 'code-icon sql-icon',
+    vue: 'code-icon js-icon',
+    svelte: 'code-icon js-icon',
+    sh: 'script-icon',
+    bash: 'script-icon',
+    zsh: 'script-icon',
+    bat: 'script-icon',
+    md: 'code-icon md-icon',
+    txt: 'doc-icon'
+};
+
 const uiFileTypes = {
     // TODO: 'd better to use a canViw() method in inlineViewer
+    /**
+     *
+     * @param {FileInfo} file
+     * @returns {boolean}
+     */
     isViewableFile(file) {
         if (!file?.mime_type) return false;
         if (file.mime_type.startsWith('image/')) return true;
@@ -16,159 +170,28 @@ const uiFileTypes = {
         return isTextViewable(file.mime_type);
     },
 
+    /**
+     *
+     * @param {string} fileName
+     * @returns {string}
+     */
     getIconClass(fileName) {
         if (!fileName) return 'fas fa-file';
         const ext = (fileName.split('.').pop() || '').toLowerCase();
-        const map = {
-            pdf: 'fas fa-file-pdf',
-            doc: 'fas fa-file-word',
-            docx: 'fas fa-file-word',
-            txt: 'fas fa-file-alt',
-            rtf: 'fas fa-file-alt',
-            odt: 'fas fa-file-alt',
-            xls: 'fas fa-file-excel',
-            xlsx: 'fas fa-file-excel',
-            csv: 'fas fa-file-excel',
-            ods: 'fas fa-file-excel',
-            ppt: 'fas fa-file-powerpoint',
-            pptx: 'fas fa-file-powerpoint',
-            odp: 'fas fa-file-powerpoint',
-            jpg: 'fas fa-file-image',
-            jpeg: 'fas fa-file-image',
-            png: 'fas fa-file-image',
-            gif: 'fas fa-file-image',
-            svg: 'fas fa-file-image',
-            webp: 'fas fa-file-image',
-            bmp: 'fas fa-file-image',
-            ico: 'fas fa-file-image',
-            mp4: 'fas fa-file-video',
-            avi: 'fas fa-file-video',
-            mov: 'fas fa-file-video',
-            mkv: 'fas fa-file-video',
-            webm: 'fas fa-file-video',
-            flv: 'fas fa-file-video',
-            mp3: 'fas fa-file-audio',
-            wav: 'fas fa-file-audio',
-            ogg: 'fas fa-file-audio',
-            flac: 'fas fa-file-audio',
-            aac: 'fas fa-file-audio',
-            m4a: 'fas fa-file-audio',
-            zip: 'fas fa-file-archive',
-            rar: 'fas fa-file-archive',
-            '7z': 'fas fa-file-archive',
-            tar: 'fas fa-file-archive',
-            gz: 'fas fa-file-archive',
-            js: 'fas fa-file-code',
-            ts: 'fas fa-file-code',
-            py: 'fas fa-file-code',
-            rs: 'fas fa-file-code',
-            java: 'fas fa-file-code',
-            html: 'fas fa-file-code',
-            css: 'fas fa-file-code',
-            json: 'fas fa-file-code',
-            xml: 'fas fa-file-code',
-            sh: 'fas fa-terminal',
-            bash: 'fas fa-terminal',
-            bat: 'fas fa-terminal',
-            md: 'fas fa-file-alt'
-        };
-        return map[ext] || 'fas fa-file';
+
+        return ICON_CLASS_MAP[ext] || 'fas fa-file';
     },
 
+    /**
+     *
+     * @param {string} fileName
+     * @returns
+     */
     getIconSpecialClass(fileName) {
         if (!fileName) return '';
         const ext = (fileName.split('.').pop() || '').toLowerCase();
-        const map = {
-            pdf: 'pdf-icon',
-            doc: 'doc-icon',
-            docx: 'doc-icon',
-            odt: 'doc-icon',
-            rtf: 'doc-icon',
-            xls: 'spreadsheet-icon',
-            xlsx: 'spreadsheet-icon',
-            ods: 'spreadsheet-icon',
-            csv: 'spreadsheet-icon',
-            ppt: 'presentation-icon',
-            pptx: 'presentation-icon',
-            odp: 'presentation-icon',
-            key: 'presentation-icon',
-            jpg: 'image-icon',
-            jpeg: 'image-icon',
-            png: 'image-icon',
-            gif: 'image-icon',
-            svg: 'image-icon',
-            webp: 'image-icon',
-            bmp: 'image-icon',
-            ico: 'image-icon',
-            heic: 'image-icon',
-            heif: 'image-icon',
-            avif: 'image-icon',
-            tiff: 'image-icon',
-            mp4: 'video-icon',
-            avi: 'video-icon',
-            mkv: 'video-icon',
-            mov: 'video-icon',
-            wmv: 'video-icon',
-            flv: 'video-icon',
-            webm: 'video-icon',
-            m4v: 'video-icon',
-            mp3: 'audio-icon',
-            wav: 'audio-icon',
-            ogg: 'audio-icon',
-            flac: 'audio-icon',
-            aac: 'audio-icon',
-            wma: 'audio-icon',
-            m4a: 'audio-icon',
-            opus: 'audio-icon',
-            zip: 'archive-icon',
-            rar: 'archive-icon',
-            '7z': 'archive-icon',
-            tar: 'archive-icon',
-            gz: 'archive-icon',
-            bz2: 'archive-icon',
-            xz: 'archive-icon',
-            exe: 'installer-icon',
-            msi: 'installer-icon',
-            dmg: 'installer-icon',
-            deb: 'installer-icon',
-            rpm: 'installer-icon',
-            appimage: 'installer-icon',
-            py: 'code-icon py-icon',
-            rs: 'code-icon rust-icon',
-            go: 'code-icon go-icon',
-            js: 'code-icon js-icon',
-            jsx: 'code-icon js-icon',
-            mjs: 'code-icon js-icon',
-            ts: 'code-icon ts-icon',
-            tsx: 'code-icon ts-icon',
-            java: 'code-icon java-icon',
-            c: 'code-icon c-icon',
-            cpp: 'code-icon c-icon',
-            cs: 'code-icon cs-icon',
-            rb: 'code-icon ruby-icon',
-            php: 'code-icon php-icon',
-            swift: 'code-icon swift-icon',
-            html: 'code-icon html-icon',
-            htm: 'code-icon html-icon',
-            css: 'code-icon css-icon',
-            scss: 'code-icon css-icon',
-            json: 'code-icon json-icon',
-            xml: 'code-icon html-icon',
-            yaml: 'code-icon config-icon',
-            yml: 'code-icon config-icon',
-            toml: 'code-icon config-icon',
-            ini: 'code-icon config-icon',
-            sql: 'code-icon sql-icon',
-            vue: 'code-icon js-icon',
-            svelte: 'code-icon js-icon',
-            sh: 'script-icon',
-            bash: 'script-icon',
-            zsh: 'script-icon',
-            bat: 'script-icon',
-            md: 'code-icon md-icon',
-            txt: 'doc-icon'
-        };
-        return map[ext] || '';
+
+        return ICON_SPECIAL_CLASS_MAP[ext] || '';
     }
 };
 
diff --git a/static/js/app/uiNotifications.js b/static/js/app/uiNotifications.js
index 1e675c8f..75a5d3c1 100644
--- a/static/js/app/uiNotifications.js
+++ b/static/js/app/uiNotifications.js
@@ -6,6 +6,12 @@
 import { notifications } from '../core/notifications.js';
 
 const uiNotifications = {
+    /**
+     *
+     * @param {string} title
+     * @param {string} message
+     * @returns
+     */
     show(title, message) {
         const normalizedTitle = String(title || '').toLowerCase();
         let icon = 'fa-info-circle';
diff --git a/static/js/app/userMenu.js b/static/js/app/userMenu.js
index 81e106d4..ad4c8457 100644
--- a/static/js/app/userMenu.js
+++ b/static/js/app/userMenu.js
@@ -49,7 +49,7 @@ function setupUserMenu() {
     });
 
     document.addEventListener('click', (e) => {
-        if (wrapper.classList.contains('open') && !wrapper.contains(e.target)) {
+        if (wrapper.classList.contains('open') && !wrapper.contains(/** @type {Node|null} */ (e.target))) {
             wrapper.classList.remove('open');
         }
     });
@@ -137,7 +137,7 @@ function setupUserMenu() {
     const aboutOverlay = document.getElementById('about-modal-overlay');
     if (aboutCloseBtn) {
         aboutCloseBtn.addEventListener('click', () => {
-            aboutOverlay.classList.add('hidden');
+            aboutOverlay?.classList.add('hidden');
         });
     }
     if (aboutOverlay) {
@@ -242,7 +242,7 @@ function showUserProfileModal() {
     `;
 
     // Set dynamic bar width and color via JS property (CSP-safe)
-    const barFill = overlay.querySelector('#about-bar-fill');
+    const barFill = /** @type {HTMLDivElement} */ (overlay.querySelector('#about-bar-fill'));
     if (barFill) {
         barFill.style.width = `${percentage}%`;
         barFill.style.background = barColor;
@@ -251,7 +251,7 @@ function showUserProfileModal() {
     document.body.appendChild(overlay);
     requestAnimationFrame(() => overlay.classList.add('show'));
 
-    overlay.querySelector('#profile-modal-close').addEventListener('click', () => {
+    overlay.querySelector('#profile-modal-close')?.addEventListener('click', () => {
         overlay.classList.remove('show');
         setTimeout(() => overlay.remove(), 200);
     });
diff --git a/static/js/core/formatters.js b/static/js/core/formatters.js
index 72671db6..e24d99d6 100644
--- a/static/js/core/formatters.js
+++ b/static/js/core/formatters.js
@@ -3,11 +3,21 @@
  * Centralized global helpers for date/size/text formatting and XSS-safe escaping.
  */
 
+/**
+ *
+ * @param {string} str
+ * @returns {string}
+ */
 function escapeHtml(str) {
     if (typeof str !== 'string') return '';
     return str.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;').replace(/'/g, '&#039;');
 }
 
+/**
+ *
+ * @param {number} bytes
+ * @returns {string}
+ */
 function formatFileSize(bytes) {
     if (bytes === 0) return '0 Bytes';
 
@@ -19,11 +29,21 @@ function formatFileSize(bytes) {
 }
 
 /// Formats a byte count for quota display. When bytes is 0, returns "∞" (unlimited).
+/**
+ *
+ * @param {number} bytes
+ * @returns {string}
+ */
 function formatQuotaSize(bytes) {
     if (bytes === 0) return '∞';
     return formatFileSize(bytes);
 }
 
+/**
+ *
+ * @param {Date | number| null} value
+ * @returns {string}
+ */
 function formatDateTime(value) {
     if (!value) return '';
     let dateValue;
@@ -38,6 +58,11 @@ function formatDateTime(value) {
     return `${dateValue.toLocaleDateString()} ${dateValue.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' })}`;
 }
 
+/**
+ *
+ * @param {Date | number| null} value
+ * @returns {string}
+ */
 function formatDateShort(value) {
     if (!value) return 'N/A';
     const dateValue = typeof value === 'number' ? new Date(value * 1000) : new Date(value);
@@ -49,20 +74,27 @@ function formatDateShort(value) {
     });
 }
 
+const TEXT_TYPES = [
+    'application/json',
+    'application/xml',
+    'application/javascript',
+    'application/x-sh',
+    'application/x-yaml',
+    'application/toml',
+    'application/x-toml',
+    'application/sql'
+];
+// FIXME: move is to another file
+/**
+ *
+ * @param {string} mimeType
+ * @returns {boolean}
+ */
 function isTextViewable(mimeType) {
     if (!mimeType) return false;
     if (mimeType.startsWith('text/')) return true;
-    const textTypes = [
-        'application/json',
-        'application/xml',
-        'application/javascript',
-        'application/x-sh',
-        'application/x-yaml',
-        'application/toml',
-        'application/x-toml',
-        'application/sql'
-    ];
-    return textTypes.includes(mimeType);
+
+    return TEXT_TYPES.includes(mimeType);
 }
 
 export { escapeHtml, formatDateShort, formatDateTime, formatFileSize, formatQuotaSize, isTextViewable };
diff --git a/static/js/core/i18n.js b/static/js/core/i18n.js
index c46ad23a..55294e5a 100644
--- a/static/js/core/i18n.js
+++ b/static/js/core/i18n.js
@@ -18,6 +18,7 @@ if (!supportedLocales.includes(currentLocale)) {
 }
 
 // Cache for translations
+/** @type {Record<string, Object>} */
 const translations = {};
 
 /**
diff --git a/static/js/core/icons.js b/static/js/core/icons.js
index 9437a3b5..ecdc6470 100644
--- a/static/js/core/icons.js
+++ b/static/js/core/icons.js
@@ -450,7 +450,7 @@ function replaceIconsInElement(container) {
     if (!container) container = document.body;
     const icons = container.querySelectorAll('i[class*="fa-"]');
     for (let i = 0; i < icons.length; i++) {
-        const el = icons[i];
+        const el = /** @type {HTMLElement} */ (icons[i]);
         const classes = el.className.split(/\s+/);
 
         // Find the icon name (fa-xxx)
diff --git a/static/js/core/languageSelector.js b/static/js/core/languageSelector.js
index 6468f545..bfae22c0 100644
--- a/static/js/core/languageSelector.js
+++ b/static/js/core/languageSelector.js
@@ -96,7 +96,7 @@ function createLanguageSelector(containerId = 'language-selector') {
         option.className = `language-option${lang.code === currentLocale ? ' active' : ''}`;
         option.setAttribute('role', 'option');
         option.setAttribute('data-lang', lang.code);
-        option.setAttribute('aria-selected', lang.code === currentLocale);
+        option.setAttribute('aria-selected', String(lang.code === currentLocale));
         option.innerHTML = `
             <span class="lang-flag">${lang.flag}</span>
             <span class="lang-name">${lang.name}</span>
@@ -134,7 +134,7 @@ function createLanguageSelector(containerId = 'language-selector') {
 
     // Close dropdown when clicking outside
     document.addEventListener('click', (e) => {
-        if (!container.contains(e.target)) {
+        if (!container.contains(/** @type {Node | null } */ (e.target))) {
             closeDropdown(container);
         }
     });
diff --git a/static/js/core/modal.js b/static/js/core/modal.js
index 016f623d..c29e17cf 100644
--- a/static/js/core/modal.js
+++ b/static/js/core/modal.js
@@ -53,9 +53,9 @@ const Modal = {
         this.closeBtn = document.getElementById('modal-close-btn');
 
         // Event listeners
-        this.cancelBtn.addEventListener('click', () => this.close(false));
-        this.closeBtn.addEventListener('click', () => this.close(false));
-        this.confirmBtn.addEventListener('click', () => this.confirm());
+        this.cancelBtn?.addEventListener('click', () => this.close(false));
+        this.closeBtn?.addEventListener('click', () => this.close(false));
+        this.confirmBtn?.addEventListener('click', () => this.confirm());
 
         // Close on overlay click
         this.overlay.addEventListener('click', (e) => {
@@ -65,7 +65,7 @@ const Modal = {
         });
 
         // Handle Enter and Escape keys
-        this.input.addEventListener('keydown', (e) => {
+        this.input?.addEventListener('keydown', (e) => {
             if (e.key === 'Enter') {
                 e.preventDefault();
                 this.confirm();
diff --git a/static/js/core/notifications.js b/static/js/core/notifications.js
index 4fe484b8..5d46d767 100644
--- a/static/js/core/notifications.js
+++ b/static/js/core/notifications.js
@@ -34,7 +34,7 @@ const notifications = (() => {
 
         bellBtn.addEventListener('click', (e) => {
             e.stopPropagation();
-            const open = wrapper.classList.toggle('open');
+            const open = wrapper?.classList.toggle('open');
             bellBtn.classList.toggle('active', open);
 
             // Close user-menu if it's open
@@ -46,7 +46,7 @@ const notifications = (() => {
 
         // Close on outside click
         document.addEventListener('click', (e) => {
-            if (!wrapper.contains(e.target)) {
+            if (!wrapper?.contains(/** @type {Node | null} */ (e.target))) {
                 close();
             }
         });
@@ -63,8 +63,8 @@ const notifications = (() => {
     function close() {
         const bellBtn = $('notif-bell-btn');
         const wrapper = $('notif-wrapper');
-        wrapper.classList.remove('open');
-        bellBtn.classList.remove('active');
+        wrapper?.classList.remove('open');
+        bellBtn?.classList.remove('active');
     }
 
     /* ── badge helpers ──────────────────────────────────────── */
@@ -82,7 +82,7 @@ const notifications = (() => {
         if (!badge) return;
         if (_badgeCount > 0) {
             badge.classList.remove('hidden');
-            badge.textContent = _badgeCount > 99 ? '99+' : _badgeCount;
+            badge.textContent = _badgeCount > 99 ? '99+' : String(_badgeCount);
         } else {
             badge.classList.add('hidden');
         }
diff --git a/static/js/core/types.js b/static/js/core/types.js
new file mode 100644
index 00000000..4d51f773
--- /dev/null
+++ b/static/js/core/types.js
@@ -0,0 +1,55 @@
+/**
+ * @typedef {Object} FolderInfo
+ * @property {string} category
+ * @property {number} created_at - timestamp
+ * @property {string} icon_class
+ * @property {string} icon_special_class
+ * @property {string} id the uniq id of the folder
+ * @property {boolean} is_root
+ * @property {number} modified_at
+ * @property {string} name
+ * @property {string} owner_id
+ * @property {string|null} parent_id the folder parent (null if is_root)
+ * @property {string} path the full path
+ */
+
+/**
+ * @typedef {Object} FileInfo
+ * @property {string} category
+ * @property {number} created_at - timestamp
+ * @property {string} icon_class
+ * @property {string} icon_special_class
+ * @property {string} id the uniq id of the folder
+ * @property {string} mime_type
+ * @property {number} modified_at - timestamp
+ * @property {string} name
+ * @property {string} owner_id
+ * @property {string} folder_id the folder parent
+ * @property {string} path the full path
+ * @property {number} size
+ * @property {string} size_formatted
+ * @property {number} sort_date
+ */
+
+/**
+ * @typedef {Object} SharePermissions
+ * @property {boolean} read
+ * @property {boolean} reshare
+ * @property {boolean} write
+ */
+
+/**
+ * @typedef {Object} Share
+ * @property {number} access_count
+ * @property {number} created_at - timestamp
+ * @property {String} created_by
+ * @property {number} expires_at - timestamp
+ * @property {boolean} has_password
+ * @property {string} id
+ * @property {string} item_id
+ * @property {string} item_name
+ * @property {string} item_type
+ * @property {SharePermissions} permissions
+ * @property {string | null} token
+ * @property {string} url
+ */
diff --git a/static/js/features/auth/auth.js b/static/js/features/auth/auth.js
index 122ace65..4ccae22f 100644
--- a/static/js/features/auth/auth.js
+++ b/static/js/features/auth/auth.js
@@ -405,7 +405,7 @@ function initLanguageSelector() {
     const pickerName = document.getElementById('lang-picker-name');
     const searchInput = document.getElementById('lang-picker-search-input');
 
-    if (!languagePanel || !picker) return;
+    if (!languagePanel || !picker || !pickerFlag || !pickerName || !pickerList) return;
 
     // --- Auto-detect browser language ---
     const detected = detectBrowserLanguage();
diff --git a/static/js/features/files/fileOperations.js b/static/js/features/files/fileOperations.js
index 184c91b4..7296b3c8 100644
--- a/static/js/features/files/fileOperations.js
+++ b/static/js/features/files/fileOperations.js
@@ -271,7 +271,7 @@ const fileOps = {
 
     /**
      * Upload files to the server with real-time progress indication
-     * @param {FileList} files - Files to upload
+     * @param {FileList | File[]} files - Files to upload
      */
     async uploadFiles(files) {
         const originalFiles = Array.from(files || []);
diff --git a/static/js/features/files/search.js b/static/js/features/files/search.js
index e47ad8da..1f678c79 100644
--- a/static/js/features/files/search.js
+++ b/static/js/features/files/search.js
@@ -146,14 +146,14 @@ const search = {
         if (previousSearchHeader) {
             previousSearchHeader.replaceWith(searchHeader);
         } else {
-            pageStickyHeader.appendChild(searchHeader);
+            pageStickyHeader?.appendChild(searchHeader);
         }
 
         // Sort dropdown — re-searches with new sort order (server-side)
-        const sortSelect = document.getElementById('search-sort-select');
+        const sortSelect = /** @type {HTMLSelectElement} */ (document.getElementById('search-sort-select'));
         if (sortSelect) {
             sortSelect.addEventListener('change', () => {
-                const searchInput = document.querySelector('.search-container input');
+                const searchInput = /** @type {HTMLInputElement} */ (document.querySelector('.search-container input'));
                 if (searchInput?.value.trim()) {
                     const event = new CustomEvent('search-resort', {
                         detail: { sort_by: sortSelect.value }
@@ -167,11 +167,12 @@ const search = {
         const clearSearchBtn = document.getElementById('clear-search-btn');
         if (clearSearchBtn) {
             clearSearchBtn.addEventListener('click', () => {
-                document.querySelector('.search-container input').value = '';
+                const searchInput = /** @type {HTMLInputElement} */ (document.querySelector('.search-container input'));
+                searchInput.value = '';
                 app.currentPath = '';
                 app.isSearchMode = false;
                 document.querySelector('.search-results-header')?.remove();
-                ui.updateBreadcrumb('');
+                ui.updateBreadcrumb();
                 loadFiles();
             });
         }
diff --git a/static/js/features/files/wopiEditor.js b/static/js/features/files/wopiEditor.js
index 7c3f487d..8401fe47 100644
--- a/static/js/features/files/wopiEditor.js
+++ b/static/js/features/files/wopiEditor.js
@@ -6,6 +6,7 @@
  */
 
 import { loadFiles } from '../../app/filesView.js';
+import { ui } from '../../app/ui.js';
 
 class WopiEditor {
     constructor() {
@@ -46,9 +47,7 @@ class WopiEditor {
             window.open(hostUrl, '_blank');
         } catch (error) {
             console.error('Failed to open WOPI editor in tab:', error);
-            if (window.showNotification) {
-                window.showNotification('Could not open the document editor.', 'error');
-            }
+            ui.showNotification('Could not open the document editor.', 'error');
         }
     }
 
diff --git a/static/js/features/library/favorites.js b/static/js/features/library/favorites.js
index 79ab0e31..7fb19e63 100644
--- a/static/js/features/library/favorites.js
+++ b/static/js/features/library/favorites.js
@@ -171,7 +171,7 @@ const favorites = {
             // FIXME: this case is not easy to understand, should apply better implementation
             multiSelect.init();
 
-            ui.updateBreadcrumb('');
+            ui.updateBreadcrumb();
 
             if (this._cache.size === 0) {
                 ui.showError(`
diff --git a/static/js/features/library/photos.js b/static/js/features/library/photos.js
index eee64332..9b405779 100644
--- a/static/js/features/library/photos.js
+++ b/static/js/features/library/photos.js
@@ -214,7 +214,7 @@ const photosView = {
                 if (grid?.classList.contains('photos-grid')) {
                     grid.insertAdjacentHTML('beforeend', tilesHtml);
                     const countSpan = existingHeader.querySelector('.photos-day-count');
-                    if (countSpan) countSpan.textContent = grid.children.length;
+                    if (countSpan) countSpan.textContent = String(grid.children.length);
                 }
             } else {
                 // New group — insert header + grid before sentinel
@@ -269,11 +269,14 @@ const photosView = {
     /** @param {number} [startIndex=0] When > 0, only process video tiles
      *  for items[startIndex..] — avoids re-scanning the entire DOM. */
     _setupVideoThumbnails(startIndex = 0) {
-        const tiles = this._container.querySelectorAll('.photo-tile[data-mime^="video/"]');
+        const tiles = /** @type {NodeListOf<HTMLDivElement> */ (this._container?.querySelectorAll('.photo-tile[data-mime^="video/"]'));
         const newIds = startIndex > 0 ? new Set(this.items.slice(startIndex).map((f) => f.id)) : null;
 
+        if (!tiles) return;
+
         for (const tile of tiles) {
             const fileId = tile.dataset.id;
+            if (!fileId) continue;
             if (newIds && !newIds.has(fileId)) continue;
             if (this._videoThumbCache.has(fileId)) continue;
 
@@ -313,6 +316,8 @@ const photosView = {
     /** Extract a single frame from a video and display it as the tile
      *  thumbnail, then upload the JPEG to the server for caching. */
     _generateVideoThumbnail(tile, img) {
+        // TODO: use thumbnail.js s common lib
+
         const fileId = tile.dataset.id;
         const video = document.createElement('video');
         video.crossOrigin = 'anonymous';
@@ -341,7 +346,7 @@ const photosView = {
                 canvas.width = Math.round(video.videoWidth * scale);
                 canvas.height = Math.round(video.videoHeight * scale);
                 const ctx = canvas.getContext('2d');
-                ctx.drawImage(video, 0, 0, canvas.width, canvas.height);
+                ctx?.drawImage(video, 0, 0, canvas.width, canvas.height);
 
                 // JPEG: explicit quality control, universally supported,
                 // and server stores as-is when dimensions fit (zero re-encode).
@@ -362,7 +367,7 @@ const photosView = {
 
                         // Upload to server for permanent caching
                         const token = localStorage.getItem('token') || sessionStorage.getItem('token');
-                        const headers = { 'Content-Type': blob.type, ...getCsrfHeaders() };
+                        const headers = /** @type {Record<String, String>} */ ({ 'Content-Type': blob.type, ...getCsrfHeaders() });
                         if (token) headers.Authorization = `Bearer ${token}`;
 
                         fetch(`/api/files/${fileId}/thumbnail/preview`, {
@@ -426,6 +431,7 @@ const photosView = {
 
     /** Render empty state */
     _renderEmpty() {
+        if (!this._container) return;
         this._container.innerHTML = `
             <div class="photos-empty">
                 <i class="fas fa-images"></i>
@@ -526,44 +532,53 @@ const photosView = {
             <button id="photos-sel-clear" title="Clear"><i class="fas fa-times"></i></button>
         `;
 
-        bar.querySelector('#photos-sel-clear').onclick = () => {
-            this.selected.clear();
-            this._container.querySelectorAll('.photo-tile.selected').forEach((t) => {
-                t.classList.remove('selected');
-            });
-            this._hideSelectionBar();
-        };
+        const bar_clear = /** @type {HTMLButtonElement} */ (bar.querySelector('#photos-sel-clear'));
+        if (bar_clear) {
+            bar_clear.onclick = () => {
+                this.selected.clear();
+                this._container.querySelectorAll('.photo-tile.selected').forEach((t) => {
+                    t.classList.remove('selected');
+                });
+                this._hideSelectionBar();
+            };
+        }
 
-        bar.querySelector('#photos-sel-delete').onclick = async () => {
-            if (!confirm('Delete selected items?')) return;
-            for (const fid of this.selected) {
-                try {
-                    await fetch(`/api/files/${fid}`, {
-                        method: 'DELETE',
-                        credentials: 'include',
-                        headers: this._headers()
-                    });
-                } catch (err) {
-                    console.error('Delete failed:', fid, err);
+        const bar_delete = /** @type {HTMLButtonElement} */ (bar.querySelector('#photos-sel-delete'));
+        if (bar_delete) {
+            bar_delete.onclick = async () => {
+                if (!confirm('Delete selected items?')) return;
+                for (const fid of this.selected) {
+                    try {
+                        await fetch(`/api/files/${fid}`, {
+                            method: 'DELETE',
+                            credentials: 'include',
+                            headers: this._headers()
+                        });
+                    } catch (err) {
+                        console.error('Delete failed:', fid, err);
+                    }
                 }
-            }
-            this.items = this.items.filter((f) => !this.selected.has(f.id));
-            this.selected.clear();
-            this._hideSelectionBar();
-            this._renderedCount = 0;
-            this._renderFull();
-        };
+                this.items = this.items.filter((f) => !this.selected.has(f.id));
+                this.selected.clear();
+                this._hideSelectionBar();
+                this._renderedCount = 0;
+                this._renderFull();
+            };
+        }
 
-        bar.querySelector('#photos-sel-download').onclick = async () => {
-            for (const fid of this.selected) {
-                const a = document.createElement('a');
-                a.href = `/api/files/${fid}`;
-                a.download = '';
-                document.body.appendChild(a);
-                a.click();
-                a.remove();
-            }
-        };
+        const bar_download = /** @type {HTMLButtonElement} */ (bar.querySelector('#photos-sel-download'));
+        if (bar_download) {
+            bar_download.onclick = async () => {
+                for (const fid of this.selected) {
+                    const a = document.createElement('a');
+                    a.href = `/api/files/${fid}`;
+                    a.download = '';
+                    document.body.appendChild(a);
+                    a.click();
+                    a.remove();
+                }
+            };
+        }
 
         bar.style.display = 'flex';
     },
diff --git a/static/js/features/library/recent.js b/static/js/features/library/recent.js
index 792bed58..833b33a7 100644
--- a/static/js/features/library/recent.js
+++ b/static/js/features/library/recent.js
@@ -109,7 +109,7 @@ const recent = {
                 multiSelect.clear();
                 multiSelect.init(); // this will wire buttons & select-all-checkbox
             }
-            ui.updateBreadcrumb('');
+            ui.updateBreadcrumb();
 
             if (recentItems.length === 0) {
                 ui.showError(`
diff --git a/static/js/features/sharing/fileSharing.js b/static/js/features/sharing/fileSharing.js
index 54f72a45..150b8734 100644
--- a/static/js/features/sharing/fileSharing.js
+++ b/static/js/features/sharing/fileSharing.js
@@ -165,7 +165,7 @@ const fileSharing = {
 
     /**
      * Format expiration date for display (Unix timestamp in seconds or ISO string)
-     * @param {number|string} value
+     * @param {number|Date} value
      * @returns {string}
      */
     formatExpirationDate(value) {
@@ -177,7 +177,7 @@ const fileSharing = {
      * Send a notification about a shared resource (stub — no backend endpoint yet)
      * @param {string} shareUrl
      * @param {string} recipientEmail
-     * @param {string} message
+     * @param {string} _message
      * @returns {Promise<boolean>}
      */
     async sendShareNotification(shareUrl, recipientEmail, _message = '') {
diff --git a/static/js/views/device-verify/device-verify.js b/static/js/views/device-verify/device-verify.js
index 8983f1ac..3e0eb523 100644
--- a/static/js/views/device-verify/device-verify.js
+++ b/static/js/views/device-verify/device-verify.js
@@ -3,7 +3,7 @@ import { getCsrfHeaders } from '../../core/csrf.js';
 
 (() => {
     var API_BASE = window.location.origin;
-    var codeInput = document.getElementById('user-code');
+    var codeInput = /** @type {HTMLInputElement} */ (document.getElementById('user-code'));
     var deviceInfo = document.getElementById('device-info');
     var actionButtons = document.getElementById('action-buttons');
     var errorText = document.getElementById('error-text');
diff --git a/static/js/views/nextcloud/error.js b/static/js/views/nextcloud/error.js
index 85fe5322..405600f1 100644
--- a/static/js/views/nextcloud/error.js
+++ b/static/js/views/nextcloud/error.js
@@ -6,6 +6,10 @@ var errorTitle = document.getElementById('error-title');
 var errorMessage = document.getElementById('error-message');
 var errorAction = document.getElementById('error-action');
 
+if (!errorTitle || !errorMessage || !errorAction) {
+    throw new Error('missing html elements');
+}
+
 switch (errorType) {
     case 'invalid-credentials':
         errorTitle.textContent = 'Login Failed';
diff --git a/static/js/views/shared/sharedView.js b/static/js/views/shared/sharedView.js
index d252d750..bd47ca88 100644
--- a/static/js/views/shared/sharedView.js
+++ b/static/js/views/shared/sharedView.js
@@ -10,10 +10,14 @@ import { formatDateShort } from '../../core/formatters.js';
 import { i18n } from '../../core/i18n.js';
 import { fileSharing } from '../../features/sharing/fileSharing.js';
 
+/** @import {Share} from '../../core/types.js' */
+
 const TTL = 5 * 60 * 1000; // 5 min
 
 const sharedView = {
     // State
+
+    /** @type {Array<Share>} */
     items: [],
 
     _expires: 0,
@@ -21,6 +25,7 @@ const sharedView = {
     /** @type {Map<string, boolean>} key = "file:<id>" | "folder:<id>" */
     _knownItemsId: new Map(),
 
+    /** @type {Array<Share>} */
     filteredItems: [],
     currentItem: null,
 
@@ -67,7 +72,7 @@ const sharedView = {
      *
      * @param {boolean} force ignore cache
      */
-    async loadItems(force) {
+    async loadItems(force = false) {
         if (this._expires > Date.now() && !force) return;
 
         try {