Add types to various low-level functions (#31781)

Adds types to various low-level modules. All changes are type-only, no
runtime changes. `tsc` now reports 38 less errors.

One problem was that `@types/sortablejs` does not accept promise return
in its functions which triggered the linter, so I disabled the rules on
those line.

13 files changed, 123 insertions, 77 deletions

diff --git a/package-lock.json b/package-lock.json
index 846cf6f838..5c56531ec0 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -108,6 +108,7 @@
         "stylelint-declaration-strict-value": "1.10.6",
         "stylelint-value-no-unknown-custom-properties": "6.0.1",
         "svgo": "3.3.2",
+        "type-fest": "4.23.0",
         "updates": "16.3.7",
         "vite-string-plugin": "1.3.4",
         "vitest": "2.0.5"
@@ -7439,6 +7440,19 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/globals/node_modules/type-fest": {
+      "version": "0.20.2",
+      "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-0.20.2.tgz",
+      "integrity": "sha512-Ne+eE4r0/iWnpAxD852z3A+N0Bt5RN//NjJwRd2VFHEmrywxf5vsZlh4R6lixl6B+wz/8d+maTSAkN1FIkI3LQ==",
+      "dev": true,
+      "license": "(MIT OR CC0-1.0)",
+      "engines": {
+        "node": ">=10"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/sindresorhus"
+      }
+    },
     "node_modules/globby": {
       "version": "11.1.0",
       "resolved": "https://registry.npmjs.org/globby/-/globby-11.1.0.tgz",
@@ -12287,13 +12301,13 @@
       }
     },
     "node_modules/type-fest": {
-      "version": "0.20.2",
-      "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-0.20.2.tgz",
-      "integrity": "sha512-Ne+eE4r0/iWnpAxD852z3A+N0Bt5RN//NjJwRd2VFHEmrywxf5vsZlh4R6lixl6B+wz/8d+maTSAkN1FIkI3LQ==",
+      "version": "4.23.0",
+      "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-4.23.0.tgz",
+      "integrity": "sha512-ZiBujro2ohr5+Z/hZWHESLz3g08BBdrdLMieYFULJO+tWc437sn8kQsWLJoZErY8alNhxre9K4p3GURAG11n+w==",
       "dev": true,
       "license": "(MIT OR CC0-1.0)",
       "engines": {
-        "node": ">=10"
+        "node": ">=16"
       },
       "funding": {
         "url": "https://github.com/sponsors/sindresorhus"
diff --git a/package.json b/package.json
index 730c47f90d..d1a624f116 100644
--- a/package.json
+++ b/package.json
@@ -107,6 +107,7 @@
     "stylelint-declaration-strict-value": "1.10.6",
     "stylelint-value-no-unknown-custom-properties": "6.0.1",
     "svgo": "3.3.2",
+    "type-fest": "4.23.0",
     "updates": "16.3.7",
     "vite-string-plugin": "1.3.4",
     "vitest": "2.0.5"
diff --git a/types.d.ts b/types.d.ts
index 3da7cbe050..a8dc09e064 100644
--- a/types.d.ts
+++ b/types.d.ts
@@ -3,6 +3,11 @@ declare module '*.svg' {
   export default value;
 }
 
+declare module '*.css' {
+  const value: string;
+  export default value;
+}
+
 declare let __webpack_public_path__: string;
 
 interface Window {
@@ -20,3 +25,7 @@ declare module 'htmx.org/dist/htmx.esm.js' {
   const value = await import('htmx.org');
   export default value;
 }
+
+interface Element {
+  _tippy: import('tippy.js').Instance;
+}
diff --git a/web_src/js/features/dropzone.ts b/web_src/js/features/dropzone.ts
index 392bc1db66..f652af0456 100644
--- a/web_src/js/features/dropzone.ts
+++ b/web_src/js/features/dropzone.ts
@@ -52,7 +52,7 @@ function addCopyLink(file) {
   copyLinkEl.addEventListener('click', async (e) => {
     e.preventDefault();
     const success = await clippie(generateMarkdownLinkForAttachment(file));
-    showTemporaryTooltip(e.target, success ? i18n.copy_success : i18n.copy_error);
+    showTemporaryTooltip(e.target as Element, success ? i18n.copy_success : i18n.copy_error);
   });
   file.previewTemplate.append(copyLinkEl);
 }
diff --git a/web_src/js/features/repo-issue-list.ts b/web_src/js/features/repo-issue-list.ts
index 1e4a880f2e..134304617b 100644
--- a/web_src/js/features/repo-issue-list.ts
+++ b/web_src/js/features/repo-issue-list.ts
@@ -196,7 +196,7 @@ async function initIssuePinSort() {
 
   createSortable(pinDiv, {
     group: 'shared',
-    onEnd: pinMoveEnd,
+    onEnd: pinMoveEnd, // eslint-disable-line @typescript-eslint/no-misused-promises
   });
 }
 
diff --git a/web_src/js/features/repo-projects.ts b/web_src/js/features/repo-projects.ts
index 950d78fec7..bc2bb69a33 100644
--- a/web_src/js/features/repo-projects.ts
+++ b/web_src/js/features/repo-projects.ts
@@ -60,7 +60,7 @@ async function initRepoProjectSortable() {
     handle: '.project-column-header',
     delayOnTouchOnly: true,
     delay: 500,
-    onSort: async () => {
+    onSort: async () => { // eslint-disable-line @typescript-eslint/no-misused-promises
       boardColumns = mainBoard.querySelectorAll('.project-column');
 
       const columnSorting = {
@@ -84,8 +84,8 @@ async function initRepoProjectSortable() {
     const boardCardList = boardColumn.querySelectorAll('.cards')[0];
     createSortable(boardCardList, {
       group: 'shared',
-      onAdd: moveIssue,
-      onUpdate: moveIssue,
+      onAdd: moveIssue, // eslint-disable-line @typescript-eslint/no-misused-promises
+      onUpdate: moveIssue, // eslint-disable-line @typescript-eslint/no-misused-promises
       delayOnTouchOnly: true,
       delay: 500,
     });
diff --git a/web_src/js/features/stopwatch.ts b/web_src/js/features/stopwatch.ts
index d89aa4bfac..af52be4e24 100644
--- a/web_src/js/features/stopwatch.ts
+++ b/web_src/js/features/stopwatch.ts
@@ -27,7 +27,7 @@ export function initStopwatch() {
     stopwatchEl.removeAttribute('href'); // intended for noscript mode only
 
     createTippy(stopwatchEl, {
-      content: stopwatchPopup.cloneNode(true),
+      content: stopwatchPopup.cloneNode(true) as Element,
       placement: 'bottom-end',
       trigger: 'click',
       maxWidth: 'none',
diff --git a/web_src/js/modules/dirauto.ts b/web_src/js/modules/dirauto.ts
index 855bae1ca8..db45a9cd17 100644
--- a/web_src/js/modules/dirauto.ts
+++ b/web_src/js/modules/dirauto.ts
@@ -1,7 +1,9 @@
 import {isDocumentFragmentOrElementNode} from '../utils/dom.ts';
 
+type DirElement = HTMLInputElement | HTMLTextAreaElement;
+
 // for performance considerations, it only uses performant syntax
-function attachDirAuto(el) {
+function attachDirAuto(el: DirElement) {
   if (el.type !== 'hidden' &&
       el.type !== 'checkbox' &&
       el.type !== 'radio' &&
@@ -18,10 +20,12 @@ export function initDirAuto() {
       const mutation = mutationList[i];
       const len = mutation.addedNodes.length;
       for (let i = 0; i < len; i++) {
-        const addedNode = mutation.addedNodes[i];
+        const addedNode = mutation.addedNodes[i] as HTMLElement;
         if (!isDocumentFragmentOrElementNode(addedNode)) continue;
-        if (addedNode.nodeName === 'INPUT' || addedNode.nodeName === 'TEXTAREA') attachDirAuto(addedNode);
-        const children = addedNode.querySelectorAll('input, textarea');
+        if (addedNode.nodeName === 'INPUT' || addedNode.nodeName === 'TEXTAREA') {
+          attachDirAuto(addedNode as DirElement);
+        }
+        const children = addedNode.querySelectorAll<DirElement>('input, textarea');
         const len = children.length;
         for (let childIdx = 0; childIdx < len; childIdx++) {
           attachDirAuto(children[childIdx]);
@@ -30,7 +34,7 @@ export function initDirAuto() {
     }
   });
 
-  const docNodes = document.querySelectorAll('input, textarea');
+  const docNodes = document.querySelectorAll<DirElement>('input, textarea');
   const len = docNodes.length;
   for (let i = 0; i < len; i++) {
     attachDirAuto(docNodes[i]);
diff --git a/web_src/js/modules/sortable.ts b/web_src/js/modules/sortable.ts
index 1c9adb6d72..460f4c6d91 100644
--- a/web_src/js/modules/sortable.ts
+++ b/web_src/js/modules/sortable.ts
@@ -1,4 +1,6 @@
-export async function createSortable(el, opts = {}) {
+import type {SortableOptions} from 'sortablejs';
+
+export async function createSortable(el, opts: {handle?: string} & SortableOptions = {}) {
   const {Sortable} = await import(/* webpackChunkName: "sortablejs" */'sortablejs');
 
   return new Sortable(el, {
@@ -15,5 +17,5 @@ export async function createSortable(el, opts = {}) {
       opts.onUnchoose?.(e);
     },
     ...opts,
-  });
+  } satisfies SortableOptions);
 }
diff --git a/web_src/js/modules/stores.ts b/web_src/js/modules/stores.ts
index 1a0ed7eda1..942a7bc508 100644
--- a/web_src/js/modules/stores.ts
+++ b/web_src/js/modules/stores.ts
@@ -1,6 +1,7 @@
 import {reactive} from 'vue';
+import type {Reactive} from 'vue';
 
-let diffTreeStoreReactive;
+let diffTreeStoreReactive: Reactive<Record<string, any>>;
 export function diffTreeStore() {
   if (!diffTreeStoreReactive) {
     diffTreeStoreReactive = reactive(window.config.pageData.diffFileInfo);
diff --git a/web_src/js/modules/tippy.ts b/web_src/js/modules/tippy.ts
index a18bad5db7..375d816c6b 100644
--- a/web_src/js/modules/tippy.ts
+++ b/web_src/js/modules/tippy.ts
@@ -1,16 +1,22 @@
 import tippy, {followCursor} from 'tippy.js';
 import {isDocumentFragmentOrElementNode} from '../utils/dom.ts';
 import {formatDatetime} from '../utils/time.ts';
+import type {Content, Instance, Props} from 'tippy.js';
 
-const visibleInstances = new Set();
+type TippyOpts = {
+  role?: string,
+  theme?: 'default' | 'tooltip' | 'menu' | 'box-with-header' | 'bare',
+} & Partial<Props>;
+
+const visibleInstances = new Set<Instance>();
 const arrowSvg = `<svg width="16" height="7"><path d="m0 7 8-7 8 7Z" class="tippy-svg-arrow-outer"/><path d="m0 8 8-7 8 7Z" class="tippy-svg-arrow-inner"/></svg>`;
 
-export function createTippy(target, opts = {}) {
+export function createTippy(target: Element, opts: TippyOpts = {}) {
   // the callback functions should be destructured from opts,
   // because we should use our own wrapper functions to handle them, do not let the user override them
   const {onHide, onShow, onDestroy, role, theme, arrow, ...other} = opts;
 
-  const instance = tippy(target, {
+  const instance: Instance = tippy(target, {
     appendTo: document.body,
     animation: false,
     allowHTML: false,
@@ -18,15 +24,15 @@ export function createTippy(target, opts = {}) {
     interactiveBorder: 20,
     ignoreAttributes: true,
     maxWidth: 500, // increase over default 350px
-    onHide: (instance) => {
+    onHide: (instance: Instance) => {
       visibleInstances.delete(instance);
       return onHide?.(instance);
     },
-    onDestroy: (instance) => {
+    onDestroy: (instance: Instance) => {
       visibleInstances.delete(instance);
       return onDestroy?.(instance);
     },
-    onShow: (instance) => {
+    onShow: (instance: Instance) => {
       // hide other tooltip instances so only one tooltip shows at a time
       for (const visibleInstance of visibleInstances) {
         if (visibleInstance.props.role === 'tooltip') {
@@ -43,7 +49,7 @@ export function createTippy(target, opts = {}) {
     theme: theme || role || 'default',
     plugins: [followCursor],
     ...other,
-  });
+  } satisfies Partial<Props>);
 
   if (role === 'menu') {
     target.setAttribute('aria-haspopup', 'true');
@@ -58,12 +64,8 @@ export function createTippy(target, opts = {}) {
  * If the target element has no content, then no tooltip will be attached, and it returns null.
  *
  * Note: "tooltip" doesn't equal to "tippy". "tooltip" means a auto-popup content, it just uses tippy as the implementation.
- *
- * @param target {HTMLElement}
- * @param content {null|string}
- * @returns {null|tippy}
  */
-function attachTooltip(target, content = null) {
+function attachTooltip(target: Element, content: Content = null) {
   switchTitleToTooltip(target);
 
   content = content ?? target.getAttribute('data-tooltip-content');
@@ -84,7 +86,7 @@ function attachTooltip(target, content = null) {
     placement: target.getAttribute('data-tooltip-placement') || 'top-start',
     followCursor: target.getAttribute('data-tooltip-follow-cursor') || false,
     ...(target.getAttribute('data-tooltip-interactive') === 'true' ? {interactive: true, aria: {content: 'describedby', expanded: false}} : {}),
-  };
+  } as TippyOpts;
 
   if (!target._tippy) {
     createTippy(target, props);
@@ -94,7 +96,7 @@ function attachTooltip(target, content = null) {
   return target._tippy;
 }
 
-function switchTitleToTooltip(target) {
+function switchTitleToTooltip(target: Element) {
   let title = target.getAttribute('title');
   if (title) {
     // apply custom formatting to relative-time's tooltips
@@ -118,16 +120,15 @@ function switchTitleToTooltip(target) {
  * According to https://www.w3.org/TR/DOM-Level-3-Events/#events-mouseevent-event-order , mouseover event is fired before mouseenter event
  * Some browsers like PaleMoon don't support "addEventListener('mouseenter', capture)"
  * The tippy by default uses "mouseenter" event to show, so we use "mouseover" event to switch to tippy
- * @param e {Event}
  */
-function lazyTooltipOnMouseHover(e) {
+function lazyTooltipOnMouseHover(e: MouseEvent) {
   e.target.removeEventListener('mouseover', lazyTooltipOnMouseHover, true);
   attachTooltip(this);
 }
 
 // Activate the tooltip for current element.
 // If the element has no aria-label, use the tooltip content as aria-label.
-function attachLazyTooltip(el) {
+function attachLazyTooltip(el: Element) {
   el.addEventListener('mouseover', lazyTooltipOnMouseHover, {capture: true});
 
   // meanwhile, if the element has no aria-label, use the tooltip content as aria-label
@@ -140,15 +141,15 @@ function attachLazyTooltip(el) {
 }
 
 // Activate the tooltip for all children elements.
-function attachChildrenLazyTooltip(target) {
-  for (const el of target.querySelectorAll('[data-tooltip-content]')) {
+function attachChildrenLazyTooltip(target: Element) {
+  for (const el of target.querySelectorAll<Element>('[data-tooltip-content]')) {
     attachLazyTooltip(el);
   }
 }
 
 export function initGlobalTooltips() {
   // use MutationObserver to detect new "data-tooltip-content" elements added to the DOM, or attributes changed
-  const observerConnect = (observer) => observer.observe(document, {
+  const observerConnect = (observer: MutationObserver) => observer.observe(document, {
     subtree: true,
     childList: true,
     attributeFilter: ['data-tooltip-content', 'title'],
@@ -159,7 +160,7 @@ export function initGlobalTooltips() {
     for (const mutation of [...mutationList, ...pending]) {
       if (mutation.type === 'childList') {
         // mainly for Vue components and AJAX rendered elements
-        for (const el of mutation.addedNodes) {
+        for (const el of mutation.addedNodes as NodeListOf<Element>) {
           if (!isDocumentFragmentOrElementNode(el)) continue;
           attachChildrenLazyTooltip(el);
           if (el.hasAttribute('data-tooltip-content')) {
@@ -167,7 +168,7 @@ export function initGlobalTooltips() {
           }
         }
       } else if (mutation.type === 'attributes') {
-        attachTooltip(mutation.target);
+        attachTooltip(mutation.target as Element);
       }
     }
     observerConnect(observer);
@@ -177,7 +178,7 @@ export function initGlobalTooltips() {
   attachChildrenLazyTooltip(document.documentElement);
 }
 
-export function showTemporaryTooltip(target, content) {
+export function showTemporaryTooltip(target: Element, content: Content) {
   // if the target is inside a dropdown, don't show the tooltip because when the dropdown
   // closes, the tippy would be pushed unsightly to the top-left of the screen like seen
   // on the issue comment menu.
diff --git a/web_src/js/utils/color.ts b/web_src/js/utils/color.ts
index 198f97c454..3ee32395fb 100644
--- a/web_src/js/utils/color.ts
+++ b/web_src/js/utils/color.ts
@@ -1,26 +1,27 @@
 import tinycolor from 'tinycolor2';
+import type {ColorInput} from 'tinycolor2';
 
 // Returns relative luminance for a SRGB color - https://en.wikipedia.org/wiki/Relative_luminance
 // Keep this in sync with modules/util/color.go
-function getRelativeLuminance(color) {
+function getRelativeLuminance(color: ColorInput) {
   const {r, g, b} = tinycolor(color).toRgb();
   return (0.2126729 * r + 0.7151522 * g + 0.072175 * b) / 255;
 }
 
-function useLightText(backgroundColor) {
+function useLightText(backgroundColor: ColorInput) {
   return getRelativeLuminance(backgroundColor) < 0.453;
 }
 
 // Given a background color, returns a black or white foreground color that the highest
 // contrast ratio. In the future, the APCA contrast function, or CSS `contrast-color` will be better.
 // https://github.com/color-js/color.js/blob/eb7b53f7a13bb716ec8b28c7a56f052cd599acd9/src/contrast/APCA.js#L42
-export function contrastColor(backgroundColor) {
+export function contrastColor(backgroundColor: ColorInput) {
   return useLightText(backgroundColor) ? '#fff' : '#000';
 }
 
-function resolveColors(obj) {
+function resolveColors(obj: Record<string, string>) {
   const styles = window.getComputedStyle(document.documentElement);
-  const getColor = (name) => styles.getPropertyValue(name).trim();
+  const getColor = (name: string) => styles.getPropertyValue(name).trim();
   return Object.fromEntries(Object.entries(obj).map(([key, value]) => [key, getColor(value)]));
 }
 
diff --git a/web_src/js/utils/dom.ts b/web_src/js/utils/dom.ts
index 82e7b755a5..5fc2183194 100644
--- a/web_src/js/utils/dom.ts
+++ b/web_src/js/utils/dom.ts
@@ -1,14 +1,21 @@
 import {debounce} from 'throttle-debounce';
+import type {Promisable} from 'type-fest';
+import type $ from 'jquery';
 
-function elementsCall(el, func, ...args) {
+type ElementArg = Element | string | NodeListOf<Element> | Array<Element> | ReturnType<typeof $>;
+type ElementsCallback = (el: Element) => Promisable<any>;
+type ElementsCallbackWithArgs = (el: Element, ...args: any[]) => Promisable<any>;
+type IterableElements = NodeListOf<Element> | Array<Element>;
+
+function elementsCall(el: ElementArg, func: ElementsCallbackWithArgs, ...args: any[]) {
   if (typeof el === 'string' || el instanceof String) {
-    el = document.querySelectorAll(el);
+    el = document.querySelectorAll(el as string);
   }
   if (el instanceof Node) {
     func(el, ...args);
   } else if (el.length !== undefined) {
     // this works for: NodeList, HTMLCollection, Array, jQuery
-    for (const e of el) {
+    for (const e of (el as IterableElements)) {
       func(e, ...args);
     }
   } else {
@@ -17,10 +24,10 @@ function elementsCall(el, func, ...args) {
 }
 
 /**
- * @param el string (selector), Node, NodeList, HTMLCollection, Array or jQuery
+ * @param el Element
  * @param force force=true to show or force=false to hide, undefined to toggle
  */
-function toggleShown(el, force) {
+function toggleShown(el: Element, force: boolean) {
   if (force === true) {
     el.classList.remove('tw-hidden');
   } else if (force === false) {
@@ -32,26 +39,26 @@ function toggleShown(el, force) {
   }
 }
 
-export function showElem(el) {
+export function showElem(el: ElementArg) {
   elementsCall(el, toggleShown, true);
 }
 
-export function hideElem(el) {
+export function hideElem(el: ElementArg) {
   elementsCall(el, toggleShown, false);
 }
 
-export function toggleElem(el, force) {
+export function toggleElem(el: ElementArg, force?: boolean) {
   elementsCall(el, toggleShown, force);
 }
 
-export function isElemHidden(el) {
-  const res = [];
+export function isElemHidden(el: ElementArg) {
+  const res: boolean[] = [];
   elementsCall(el, (e) => res.push(e.classList.contains('tw-hidden')));
   if (res.length > 1) throw new Error(`isElemHidden doesn't work for multiple elements`);
   return res[0];
 }
 
-function applyElemsCallback(elems, fn) {
+function applyElemsCallback(elems: IterableElements, fn?: ElementsCallback) {
   if (fn) {
     for (const el of elems) {
       fn(el);
@@ -60,20 +67,22 @@ function applyElemsCallback(elems, fn) {
   return elems;
 }
 
-export function queryElemSiblings(el, selector = '*', fn) {
-  return applyElemsCallback(Array.from(el.parentNode.children).filter((child) => child !== el && child.matches(selector)), fn);
+export function queryElemSiblings(el: Element, selector = '*', fn?: ElementsCallback) {
+  return applyElemsCallback(Array.from(el.parentNode.children).filter((child: Element) => {
+    return child !== el && child.matches(selector);
+  }), fn);
 }
 
 // it works like jQuery.children: only the direct children are selected
-export function queryElemChildren(parent, selector = '*', fn) {
+export function queryElemChildren(parent: Element | ParentNode, selector = '*', fn?: ElementsCallback) {
   return applyElemsCallback(parent.querySelectorAll(`:scope > ${selector}`), fn);
 }
 
-export function queryElems(selector, fn) {
+export function queryElems(selector: string, fn?: ElementsCallback) {
   return applyElemsCallback(document.querySelectorAll(selector), fn);
 }
 
-export function onDomReady(cb) {
+export function onDomReady(cb: () => Promisable<void>) {
   if (document.readyState === 'loading') {
     document.addEventListener('DOMContentLoaded', cb);
   } else {
@@ -83,7 +92,7 @@ export function onDomReady(cb) {
 
 // checks whether an element is owned by the current document, and whether it is a document fragment or element node
 // if it is, it means it is a "normal" element managed by us, which can be modified safely.
-export function isDocumentFragmentOrElementNode(el) {
+export function isDocumentFragmentOrElementNode(el: Element) {
   try {
     return el.ownerDocument === document && el.nodeType === Node.ELEMENT_NODE || el.nodeType === Node.DOCUMENT_FRAGMENT_NODE;
   } catch {
@@ -108,12 +117,15 @@ export function isDocumentFragmentOrElementNode(el) {
 // The above copyright notice and this permission notice shall be
 // included in all copies or substantial portions of the Software.
 // ---------------------------------------------------------------------
-export function autosize(textarea, {viewportMarginBottom = 0} = {}) {
+export function autosize(textarea: HTMLTextAreaElement, {viewportMarginBottom = 0}: {viewportMarginBottom?: number} = {}) {
   let isUserResized = false;
   // lastStyleHeight and initialStyleHeight are CSS values like '100px'
-  let lastMouseX, lastMouseY, lastStyleHeight, initialStyleHeight;
+  let lastMouseX: number;
+  let lastMouseY: number;
+  let lastStyleHeight: string;
+  let initialStyleHeight: string;
 
-  function onUserResize(event) {
+  function onUserResize(event: MouseEvent) {
     if (isUserResized) return;
     if (lastMouseX !== event.clientX || lastMouseY !== event.clientY) {
       const newStyleHeight = textarea.style.height;
@@ -133,7 +145,7 @@ export function autosize(textarea, {viewportMarginBottom = 0} = {}) {
 
     while (el !== document.body && el !== null) {
       offsetTop += el.offsetTop || 0;
-      el = el.offsetParent;
+      el = el.offsetParent as HTMLTextAreaElement;
     }
 
     const top = offsetTop - document.defaultView.scrollY;
@@ -213,14 +225,15 @@ export function autosize(textarea, {viewportMarginBottom = 0} = {}) {
   };
 }
 
-export function onInputDebounce(fn) {
+export function onInputDebounce(fn: () => Promisable<any>) {
   return debounce(300, fn);
 }
 
+type LoadableElement = HTMLEmbedElement | HTMLIFrameElement | HTMLImageElement | HTMLScriptElement | HTMLTrackElement;
+
 // Set the `src` attribute on an element and returns a promise that resolves once the element
-// has loaded or errored. Suitable for all elements mention in:
-// https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/load_event
-export function loadElem(el, src) {
+// has loaded or errored.
+export function loadElem(el: LoadableElement, src: string) {
   return new Promise((resolve) => {
     el.addEventListener('load', () => resolve(true), {once: true});
     el.addEventListener('error', () => resolve(false), {once: true});
@@ -256,14 +269,14 @@ export function initSubmitEventPolyfill() {
  * @param {HTMLElement} element The element to check.
  * @returns {boolean} True if the element is visible.
  */
-export function isElemVisible(element) {
+export function isElemVisible(element: HTMLElement) {
   if (!element) return false;
 
   return Boolean(element.offsetWidth || element.offsetHeight || element.getClientRects().length);
 }
 
 // replace selected text in a textarea while preserving editor history, e.g. CTRL-Z works after this
-export function replaceTextareaSelection(textarea, text) {
+export function replaceTextareaSelection(textarea: HTMLTextAreaElement, text: string) {
   const before = textarea.value.slice(0, textarea.selectionStart ?? undefined);
   const after = textarea.value.slice(textarea.selectionEnd ?? undefined);
   let success = true;
@@ -287,13 +300,13 @@ export function replaceTextareaSelection(textarea, text) {
 }
 
 // Warning: Do not enter any unsanitized variables here
-export function createElementFromHTML(htmlString) {
+export function createElementFromHTML(htmlString: string) {
   const div = document.createElement('div');
   div.innerHTML = htmlString.trim();
-  return div.firstChild;
+  return div.firstChild as Element;
 }
 
-export function createElementFromAttrs(tagName, attrs) {
+export function createElementFromAttrs(tagName: string, attrs: Record<string, any>) {
   const el = document.createElement(tagName);
   for (const [key, value] of Object.entries(attrs)) {
     if (value === undefined || value === null) continue;
@@ -307,7 +320,7 @@ export function createElementFromAttrs(tagName, attrs) {
   return el;
 }
 
-export function animateOnce(el, animationClassName) {
+export function animateOnce(el: Element, animationClassName: string): Promise<void> {
   return new Promise((resolve) => {
     el.addEventListener('animationend', function onAnimationEnd() {
       el.classList.remove(animationClassName);