diff --git a/types.d.ts b/types.d.ts
index ddfe90407fc32..3da7cbe050394 100644
--- a/types.d.ts
+++ b/types.d.ts
@@ -10,6 +10,10 @@ interface Window {
   $: typeof import('@types/jquery'),
   jQuery: typeof import('@types/jquery'),
   htmx: typeof import('htmx.org'),
+  _globalHandlerErrors: Array<ErrorEvent & PromiseRejectionEvent> & {
+    _inited: boolean,
+    push: (e: ErrorEvent & PromiseRejectionEvent) => void | number,
+  },
 }
 
 declare module 'htmx.org/dist/htmx.esm.js' {
diff --git a/web_src/js/bootstrap.ts b/web_src/js/bootstrap.ts
index 26627dfdedbb0..9e41673b86c86 100644
--- a/web_src/js/bootstrap.ts
+++ b/web_src/js/bootstrap.ts
@@ -1,12 +1,13 @@
 // DO NOT IMPORT window.config HERE!
 // to make sure the error handler always works, we should never import `window.config`, because
 // some user's custom template breaks it.
+import type {Intent} from './types.ts';
 
 // This sets up the URL prefix used in webpack's chunk loading.
 // This file must be imported before any lazy-loading is being attempted.
 __webpack_public_path__ = `${window.config?.assetUrlPrefix ?? '/assets'}/`;
 
-function shouldIgnoreError(err) {
+function shouldIgnoreError(err: Error) {
   const ignorePatterns = [
     '/assets/js/monaco.', // https://github.com/go-gitea/gitea/issues/30861 , https://github.com/microsoft/monaco-editor/issues/4496
   ];
@@ -16,14 +17,14 @@ function shouldIgnoreError(err) {
   return false;
 }
 
-export function showGlobalErrorMessage(msg, msgType = 'error') {
+export function showGlobalErrorMessage(msg: string, msgType: Intent = 'error') {
   const msgContainer = document.querySelector('.page-content') ?? document.body;
   const msgCompact = msg.replace(/\W/g, '').trim(); // compact the message to a data attribute to avoid too many duplicated messages
-  let msgDiv = msgContainer.querySelector(`.js-global-error[data-global-error-msg-compact="${msgCompact}"]`);
+  let msgDiv = msgContainer.querySelector<HTMLDivElement>(`.js-global-error[data-global-error-msg-compact="${msgCompact}"]`);
   if (!msgDiv) {
     const el = document.createElement('div');
     el.innerHTML = `<div class="ui container js-global-error tw-my-[--page-spacing]"><div class="ui ${msgType} message tw-text-center tw-whitespace-pre-line"></div></div>`;
-    msgDiv = el.childNodes[0];
+    msgDiv = el.childNodes[0] as HTMLDivElement;
   }
   // merge duplicated messages into "the message (count)" format
   const msgCount = Number(msgDiv.getAttribute(`data-global-error-msg-count`)) + 1;
@@ -33,18 +34,7 @@ export function showGlobalErrorMessage(msg, msgType = 'error') {
   msgContainer.prepend(msgDiv);
 }
 
-/**
- * @param {ErrorEvent|PromiseRejectionEvent} event - Event
- * @param {string} event.message - Only present on ErrorEvent
- * @param {string} event.error - Only present on ErrorEvent
- * @param {string} event.type - Only present on ErrorEvent
- * @param {string} event.filename - Only present on ErrorEvent
- * @param {number} event.lineno - Only present on ErrorEvent
- * @param {number} event.colno - Only present on ErrorEvent
- * @param {string} event.reason - Only present on PromiseRejectionEvent
- * @param {number} event.promise - Only present on PromiseRejectionEvent
- */
-function processWindowErrorEvent({error, reason, message, type, filename, lineno, colno}) {
+function processWindowErrorEvent({error, reason, message, type, filename, lineno, colno}: ErrorEvent & PromiseRejectionEvent) {
   const err = error ?? reason;
   const assetBaseUrl = String(new URL(__webpack_public_path__, window.location.origin));
   const {runModeIsProd} = window.config ?? {};
@@ -90,7 +80,8 @@ function initGlobalErrorHandler() {
   }
   // then, change _globalHandlerErrors to an object with push method, to process further error
   // events directly
-  window._globalHandlerErrors = {_inited: true, push: (e) => processWindowErrorEvent(e)};
+  // @ts-expect-error -- this should be refactored to not use a fake array
+  window._globalHandlerErrors = {_inited: true, push: (e: ErrorEvent & PromiseRejectionEvent) => processWindowErrorEvent(e)};
 }
 
 initGlobalErrorHandler();
diff --git a/web_src/js/modules/fetch.ts b/web_src/js/modules/fetch.ts
index b70a4cb3048b4..90ff7184986c1 100644
--- a/web_src/js/modules/fetch.ts
+++ b/web_src/js/modules/fetch.ts
@@ -1,4 +1,5 @@
 import {isObject} from '../utils.ts';
+import type {RequestData, RequestOpts} from '../types.ts';
 
 const {csrfToken} = window.config;
 
@@ -8,8 +9,9 @@ const safeMethods = new Set(['GET', 'HEAD', 'OPTIONS', 'TRACE']);
 // fetch wrapper, use below method name functions and the `data` option to pass in data
 // which will automatically set an appropriate headers. For json content, only object
 // and array types are currently supported.
-export function request(url, {method = 'GET', data, headers = {}, ...other} = {}) {
-  let body, contentType;
+export function request(url: string, {method = 'GET', data, headers = {}, ...other}: RequestOpts = {}) {
+  let body: RequestData;
+  let contentType: string;
   if (data instanceof FormData || data instanceof URLSearchParams) {
     body = data;
   } else if (isObject(data) || Array.isArray(data)) {
@@ -34,8 +36,8 @@ export function request(url, {method = 'GET', data, headers = {}, ...other} = {}
   });
 }
 
-export const GET = (url, opts) => request(url, {method: 'GET', ...opts});
-export const POST = (url, opts) => request(url, {method: 'POST', ...opts});
-export const PATCH = (url, opts) => request(url, {method: 'PATCH', ...opts});
-export const PUT = (url, opts) => request(url, {method: 'PUT', ...opts});
-export const DELETE = (url, opts) => request(url, {method: 'DELETE', ...opts});
+export const GET = (url: string, opts?: RequestOpts) => request(url, {method: 'GET', ...opts});
+export const POST = (url: string, opts?: RequestOpts) => request(url, {method: 'POST', ...opts});
+export const PATCH = (url: string, opts?: RequestOpts) => request(url, {method: 'PATCH', ...opts});
+export const PUT = (url: string, opts?: RequestOpts) => request(url, {method: 'PUT', ...opts});
+export const DELETE = (url: string, opts?: RequestOpts) => request(url, {method: 'DELETE', ...opts});
diff --git a/web_src/js/modules/toast.ts b/web_src/js/modules/toast.ts
index cded48e6c2cfb..264ccbbdce77f 100644
--- a/web_src/js/modules/toast.ts
+++ b/web_src/js/modules/toast.ts
@@ -2,8 +2,19 @@ import {htmlEscape} from 'escape-goat';
 import {svg} from '../svg.ts';
 import {animateOnce, showElem} from '../utils/dom.ts';
 import Toastify from 'toastify-js'; // don't use "async import", because when network error occurs, the "async import" also fails and nothing is shown
+import type {Intent} from '../types.ts';
+import type {SvgName} from '../svg.ts';
+import type {Options} from 'toastify-js';
 
-const levels = {
+type ToastLevels = {
+  [intent in Intent]: {
+    icon: SvgName,
+    background: string,
+    duration: number,
+  }
+}
+
+const levels: ToastLevels = {
   info: {
     icon: 'octicon-check',
     background: 'var(--color-green)',
@@ -21,8 +32,13 @@ const levels = {
   },
 };
 
+type ToastOpts = {
+  useHtmlBody?: boolean,
+  preventDuplicates?: boolean,
+} & Options;
+
 // See https://github.com/apvarun/toastify-js#api for options
-function showToast(message, level, {gravity, position, duration, useHtmlBody, preventDuplicates = true, ...other} = {}) {
+function showToast(message: string, level: Intent, {gravity, position, duration, useHtmlBody, preventDuplicates = true, ...other}: ToastOpts = {}) {
   const body = useHtmlBody ? String(message) : htmlEscape(message);
   const key = `${level}-${body}`;
 
@@ -59,14 +75,14 @@ function showToast(message, level, {gravity, position, duration, useHtmlBody, pr
   return toast;
 }
 
-export function showInfoToast(message, opts) {
+export function showInfoToast(message: string, opts?: ToastOpts) {
   return showToast(message, 'info', opts);
 }
 
-export function showWarningToast(message, opts) {
+export function showWarningToast(message: string, opts?: ToastOpts) {
   return showToast(message, 'warning', opts);
 }
 
-export function showErrorToast(message, opts) {
+export function showErrorToast(message: string, opts?: ToastOpts) {
   return showToast(message, 'error', opts);
 }
diff --git a/web_src/js/svg.ts b/web_src/js/svg.ts
index a0fe52a7be20a..6a4bfafc9244c 100644
--- a/web_src/js/svg.ts
+++ b/web_src/js/svg.ts
@@ -146,17 +146,19 @@ const svgs = {
   'octicon-x-circle-fill': octiconXCircleFill,
 };
 
+export type SvgName = keyof typeof svgs;
+
 // TODO: use a more general approach to access SVG icons.
 //  At the moment, developers must check, pick and fill the names manually,
 //  most of the SVG icons in assets couldn't be used directly.
 
 // retrieve an HTML string for given SVG icon name, size and additional classes
-export function svg(name, size = 16, className = '') {
+export function svg(name: SvgName, size = 16, className = '') {
   if (!(name in svgs)) throw new Error(`Unknown SVG icon: ${name}`);
   if (size === 16 && !className) return svgs[name];
 
   const document = parseDom(svgs[name], 'image/svg+xml');
-  const svgNode = document.firstChild;
+  const svgNode = document.firstChild as SVGElement;
   if (size !== 16) {
     svgNode.setAttribute('width', String(size));
     svgNode.setAttribute('height', String(size));
@@ -165,7 +167,7 @@ export function svg(name, size = 16, className = '') {
   return serializeXml(svgNode);
 }
 
-export function svgParseOuterInner(name) {
+export function svgParseOuterInner(name: SvgName) {
   const svgStr = svgs[name];
   if (!svgStr) throw new Error(`Unknown SVG icon: ${name}`);
 
@@ -179,7 +181,7 @@ export function svgParseOuterInner(name) {
   const svgInnerHtml = svgStr.slice(p1 + 1, p2);
   const svgOuterHtml = svgStr.slice(0, p1 + 1) + svgStr.slice(p2);
   const svgDoc = parseDom(svgOuterHtml, 'image/svg+xml');
-  const svgOuter = svgDoc.firstChild;
+  const svgOuter = svgDoc.firstChild as SVGElement;
   return {svgOuter, svgInnerHtml};
 }
 
diff --git a/web_src/js/types.ts b/web_src/js/types.ts
index f69aa92768027..3bd1c072a8a90 100644
--- a/web_src/js/types.ts
+++ b/web_src/js/types.ts
@@ -21,3 +21,11 @@ export type Config = {
   mermaidMaxSourceCharacters: number,
   i18n: Record<string, string>,
 }
+
+export type Intent = 'error' | 'warning' | 'info';
+
+export type RequestData = string | FormData | URLSearchParams;
+
+export type RequestOpts = {
+  data?: RequestData,
+} & RequestInit;