diff --git a/deno_dist/context.ts b/deno_dist/context.ts
index 99fe18c98..ce647e4cf 100644
--- a/deno_dist/context.ts
+++ b/deno_dist/context.ts
@@ -1,6 +1,6 @@
 import type { HonoRequest } from './request.ts'
-import type { Env, FetchEventLike, NotFoundHandler, Input, TypedResponse } from './types.ts'
-import { resolveCallback, HtmlEscapedCallbackPhase } from './utils/html.ts'
+import type { Env, FetchEventLike, Input, NotFoundHandler, TypedResponse } from './types.ts'
+import { HtmlEscapedCallbackPhase, resolveCallback } from './utils/html.ts'
 import type { RedirectStatusCode, StatusCode } from './utils/http-status.ts'
 import type { JSONValue, JSONParsed, IsAny, Simplify } from './utils/types.ts'
 
@@ -224,7 +224,14 @@ export class Context<
    */
   render: Renderer = (...args) => this.renderer(...args)
 
-  setLayout = (layout: Layout<PropsForRenderer & { Layout: Layout }>) => (this.layout = layout)
+  setLayout = (
+    layout: Layout<PropsForRenderer & { Layout: Layout }>
+  ): Layout<
+    PropsForRenderer & {
+      Layout: Layout
+    }
+  > => (this.layout = layout)
+
   getLayout = () => this.layout
 
   /**
diff --git a/deno_dist/helper/accepts/accepts.ts b/deno_dist/helper/accepts/accepts.ts
index 9800874c0..bdf64fd93 100644
--- a/deno_dist/helper/accepts/accepts.ts
+++ b/deno_dist/helper/accepts/accepts.ts
@@ -25,7 +25,7 @@ export interface acceptsOptions extends acceptsConfig {
   match?: (accepts: Accept[], config: acceptsConfig) => string
 }
 
-export const parseAccept = (acceptHeader: string) => {
+export const parseAccept = (acceptHeader: string): Accept[] => {
   // Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
   const accepts = acceptHeader.split(',') // ['text/html', 'application/xhtml+xml', 'application/xml;q=0.9', 'image/webp', '*/*;q=0.8']
   return accepts.map((accept) => {
@@ -42,7 +42,7 @@ export const parseAccept = (acceptHeader: string) => {
   })
 }
 
-export const defaultMatch = (accepts: Accept[], config: acceptsConfig) => {
+export const defaultMatch = (accepts: Accept[], config: acceptsConfig): string => {
   const { supports, default: defaultSupport } = config
   const accept = accepts.sort((a, b) => b.q - a.q).find((accept) => supports.includes(accept.type))
   return accept ? accept.type : defaultSupport
@@ -61,7 +61,7 @@ export const defaultMatch = (accepts: Accept[], config: acceptsConfig) => {
  * })
  * ```
  */
-export const accepts = (c: Context, options: acceptsOptions) => {
+export const accepts = (c: Context, options: acceptsOptions): string => {
   const acceptHeader = c.req.header(options.header)
   if (!acceptHeader) {
     return options.default
diff --git a/deno_dist/helper/adapter/index.ts b/deno_dist/helper/adapter/index.ts
index abb5ece13..c9e46833a 100644
--- a/deno_dist/helper/adapter/index.ts
+++ b/deno_dist/helper/adapter/index.ts
@@ -30,7 +30,7 @@ export const env = <T extends Record<string, unknown>, C extends Context = Conte
   return runtimeEnvHandlers[runtime]()
 }
 
-export const getRuntimeKey = () => {
+export const getRuntimeKey = (): Runtime => {
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   const global = globalThis as any
 
diff --git a/deno_dist/helper/cookie/index.ts b/deno_dist/helper/cookie/index.ts
index ae2a84abb..7c3840fac 100644
--- a/deno_dist/helper/cookie/index.ts
+++ b/deno_dist/helper/cookie/index.ts
@@ -118,7 +118,7 @@ export const setSignedCookie = async (
   c.header('set-cookie', cookie, { append: true })
 }
 
-export const deleteCookie = (c: Context, name: string, opt?: CookieOptions) => {
+export const deleteCookie = (c: Context, name: string, opt?: CookieOptions): string | undefined => {
   const deletedCookie = getCookie(c, name)
   setCookie(c, name, '', { ...opt, maxAge: 0 })
   return deletedCookie
diff --git a/deno_dist/helper/css/common.ts b/deno_dist/helper/css/common.ts
index 46a02e9d8..aff38a280 100644
--- a/deno_dist/helper/css/common.ts
+++ b/deno_dist/helper/css/common.ts
@@ -3,12 +3,12 @@ export const PSEUDO_GLOBAL_SELECTOR = ':-hono-global'
 export const isPseudoGlobalSelectorRe = new RegExp(`^${PSEUDO_GLOBAL_SELECTOR}{(.*)}$`)
 export const DEFAULT_STYLE_ID = 'hono-css'
 
-export const SELECTOR = Symbol()
-export const CLASS_NAME = Symbol()
-export const STYLE_STRING = Symbol()
-export const SELECTORS = Symbol()
-export const EXTERNAL_CLASS_NAMES = Symbol()
-const CSS_ESCAPED = Symbol()
+export const SELECTOR: unique symbol = Symbol()
+export const CLASS_NAME: unique symbol = Symbol()
+export const STYLE_STRING: unique symbol = Symbol()
+export const SELECTORS: unique symbol = Symbol()
+export const EXTERNAL_CLASS_NAMES: unique symbol = Symbol()
+const CSS_ESCAPED: unique symbol = Symbol()
 
 export interface CssClassName {
   [SELECTOR]: string
@@ -49,12 +49,12 @@ const toHash = (str: string): string => {
   return 'css-' + out
 }
 
-const cssStringReStr = [
+const cssStringReStr: string = [
   '"(?:(?:\\\\[\\s\\S]|[^"\\\\])*)"', // double quoted string
   // eslint-disable-next-line quotes
   "'(?:(?:\\\\[\\s\\S]|[^'\\\\])*)'", // single quoted string
 ].join('|')
-const minifyCssRe = new RegExp(
+const minifyCssRe: RegExp = new RegExp(
   [
     '(' + cssStringReStr + ')', // $1: quoted string
 
diff --git a/deno_dist/helper/css/index.ts b/deno_dist/helper/css/index.ts
index 9b737e77f..33d53e619 100644
--- a/deno_dist/helper/css/index.ts
+++ b/deno_dist/helper/css/index.ts
@@ -24,12 +24,36 @@ type usedClassNameData = [
   Record<string, true> // class name already added
 ]
 
+interface CssType {
+  (strings: TemplateStringsArray, ...values: CssVariableType[]): Promise<string>
+}
+
+interface CxType {
+  (
+    ...args: (CssClassName | Promise<string> | string | boolean | null | undefined)[]
+  ): Promise<string>
+}
+
+interface KeyframesType {
+  (strings: TemplateStringsArray, ...values: CssVariableType[]): CssClassNameCommon
+}
+
+interface ViewTransitionType {
+  (strings: TemplateStringsArray, ...values: CssVariableType[]): Promise<string>
+  (content: Promise<string>): Promise<string>
+  (): Promise<string>
+}
+
+interface StyleType {
+  (args?: { children?: Promise<string> }): HtmlEscapedString
+}
+
 /**
  * @experimental
  * `createCssContext` is an experimental feature.
  * The API might be changed.
  */
-export const createCssContext = ({ id }: { id: Readonly<string> }) => {
+export const createCssContext = ({ id }: { id: Readonly<string> }): DefaultContextType => {
   const [cssJsxDomObject, StyleRenderToDom] = createCssJsxDomObjects({ id })
 
   const contextMap: WeakMap<object, usedClassNameData> = new WeakMap()
@@ -106,13 +130,11 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
     return promise
   }
 
-  const css = (strings: TemplateStringsArray, ...values: CssVariableType[]): Promise<string> => {
+  const css: CssType = (strings, ...values) => {
     return newCssClassNameObject(cssCommon(strings, values))
   }
 
-  const cx = (
-    ...args: (CssClassName | Promise<string> | string | boolean | null | undefined)[]
-  ): Promise<string> => {
+  const cx: CxType = (...args) => {
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     args = cxCommon(args as any) as any
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -121,20 +143,15 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
 
   const keyframes = keyframesCommon
 
-  type ViewTransitionType = {
-    (strings: TemplateStringsArray, ...values: CssVariableType[]): Promise<string>
-    (content: Promise<string>): Promise<string>
-    (): Promise<string>
-  }
   const viewTransition: ViewTransitionType = ((
     strings: TemplateStringsArray | Promise<string> | undefined,
     ...values: CssVariableType[]
-  ): Promise<string> => {
+  ) => {
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     return newCssClassNameObject(viewTransitionCommon(strings as any, values))
   }) as ViewTransitionType
 
-  const Style = ({ children }: { children?: Promise<string> } = {}) =>
+  const Style: StyleType = ({ children } = {}) =>
     children
       ? raw(`<style id="${id}">${(children as unknown as CssClassName)[STYLE_STRING]}</style>`)
       : raw(`<style id="${id}"></style>`)
@@ -150,7 +167,17 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
   }
 }
 
-const defaultContext = createCssContext({ id: DEFAULT_STYLE_ID })
+interface DefaultContextType {
+  css: CssType
+  cx: CxType
+  keyframes: KeyframesType
+  viewTransition: ViewTransitionType
+  Style: StyleType
+}
+
+const defaultContext: DefaultContextType = createCssContext({
+  id: DEFAULT_STYLE_ID,
+})
 
 /**
  * @experimental
diff --git a/deno_dist/helper/dev/index.ts b/deno_dist/helper/dev/index.ts
index 1898c16b8..7ba8613ee 100644
--- a/deno_dist/helper/dev/index.ts
+++ b/deno_dist/helper/dev/index.ts
@@ -15,7 +15,7 @@ interface RouteData {
   isMiddleware: boolean
 }
 
-const handlerName = (handler: Function) => {
+const handlerName = (handler: Function): string => {
   return handler.name || (isMiddleware(handler) ? '[middleware]' : '[handler]')
 }
 
@@ -68,7 +68,7 @@ export const showRoutes = <E extends Env>(hono: Hono<E>, opts?: ShowRoutesOption
     })
 }
 
-export const getRouterName = <E extends Env>(app: Hono<E>) => {
+export const getRouterName = <E extends Env>(app: Hono<E>): string => {
   app.router.match('GET', '/')
   return app.router.name
 }
diff --git a/deno_dist/helper/factory/index.ts b/deno_dist/helper/factory/index.ts
index b8fcf8b45..2478031a7 100644
--- a/deno_dist/helper/factory/index.ts
+++ b/deno_dist/helper/factory/index.ts
@@ -216,7 +216,7 @@ export class Factory<E extends Env = any, P extends string = any> {
    * @experimental
    * `createApp` is an experimental feature.
    */
-  createApp = () => {
+  createApp = (): Hono<E> => {
     const app = new Hono<E>()
     if (this.initApp) {
       this.initApp(app)
@@ -234,8 +234,8 @@ export class Factory<E extends Env = any, P extends string = any> {
 
 export const createFactory = <E extends Env = any, P extends string = any>(init?: {
   initApp?: InitApp<E>
-}) => new Factory<E, P>(init)
+}): Factory<E, P> => new Factory<E, P>(init)
 
 export const createMiddleware = <E extends Env = any, P extends string = any, I extends Input = {}>(
   middleware: MiddlewareHandler<E, P, I>
-) => createFactory<E, P>().createMiddleware<I>(middleware)
+): MiddlewareHandler<E, P, I> => createFactory<E, P>().createMiddleware<I>(middleware)
diff --git a/deno_dist/helper/streaming/sse.ts b/deno_dist/helper/streaming/sse.ts
index a459ec87c..4fa30be01 100644
--- a/deno_dist/helper/streaming/sse.ts
+++ b/deno_dist/helper/streaming/sse.ts
@@ -62,7 +62,7 @@ export const streamSSE = (
   c: Context,
   cb: (stream: SSEStreamingApi) => Promise<void>,
   onError?: (e: Error, stream: SSEStreamingApi) => Promise<void>
-) => {
+): Response => {
   const { readable, writable } = new TransformStream()
   const stream = new SSEStreamingApi(writable, readable)
 
diff --git a/deno_dist/helper/testing/index.ts b/deno_dist/helper/testing/index.ts
index 99ad7ddb9..b542ef5dd 100644
--- a/deno_dist/helper/testing/index.ts
+++ b/deno_dist/helper/testing/index.ts
@@ -1,6 +1,8 @@
 import { hc } from '../../client/index.ts'
+import type { Client } from '../../client/types.ts'
 import type { ExecutionContext } from '../../context.ts'
 import type { Hono } from '../../hono.ts'
+import type { UnionToIntersection } from '../../utils/types.ts'
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 type ExtractEnv<T> = T extends Hono<infer E, any, any> ? E : never
@@ -10,7 +12,7 @@ export const testClient = <T extends Hono<any, any, any>>(
   app: T,
   Env?: ExtractEnv<T>['Bindings'] | {},
   executionCtx?: ExecutionContext
-) => {
+): UnionToIntersection<Client<T>> => {
   const customFetch = (input: RequestInfo | URL, init?: RequestInit) => {
     return app.request(input, init, Env, executionCtx)
   }
diff --git a/deno_dist/hono-base.ts b/deno_dist/hono-base.ts
index b49a6fc33..5a69e8055 100644
--- a/deno_dist/hono-base.ts
+++ b/deno_dist/hono-base.ts
@@ -233,7 +233,7 @@ class Hono<
    * })
    * ```
    */
-  onError = (handler: ErrorHandler<E>) => {
+  onError = (handler: ErrorHandler<E>): Hono<E, S, BasePath> => {
     this.errorHandler = handler
     return this
   }
@@ -247,7 +247,7 @@ class Hono<
    * ```
    * @see https://hono.dev/api/hono#not-found
    */
-  notFound = (handler: NotFoundHandler<E>) => {
+  notFound = (handler: NotFoundHandler<E>): Hono<E, S, BasePath> => {
     this.notFoundHandler = handler
     return this
   }
@@ -396,7 +396,7 @@ class Hono<
     requestInit?: RequestInit,
     Env?: E['Bindings'] | {},
     executionCtx?: ExecutionContext
-  ) => {
+  ): Response | Promise<Response> => {
     if (input instanceof Request) {
       if (requestInit !== undefined) {
         input = new Request(input, requestInit)
diff --git a/deno_dist/jsx/dom/context.ts b/deno_dist/jsx/dom/context.ts
index 52fe061ba..0089d052b 100644
--- a/deno_dist/jsx/dom/context.ts
+++ b/deno_dist/jsx/dom/context.ts
@@ -5,7 +5,7 @@ import { globalContexts } from '../context.ts'
 import { Fragment } from './jsx-runtime.ts'
 import { setInternalTagFlag } from './utils.ts'
 
-export const createContextProviderFunction = <T>(values: T[]) =>
+export const createContextProviderFunction = <T>(values: T[]): Function =>
   setInternalTagFlag(({ value, children }: { value: T; children: Child[] }) => {
     if (!children) {
       return undefined
diff --git a/deno_dist/jsx/dom/css.ts b/deno_dist/jsx/dom/css.ts
index 29917d727..338c078d2 100644
--- a/deno_dist/jsx/dom/css.ts
+++ b/deno_dist/jsx/dom/css.ts
@@ -56,7 +56,16 @@ const splitRule = (rule: string): string[] => {
   return result
 }
 
-export const createCssJsxDomObjects = ({ id }: { id: Readonly<string> }) => {
+interface CreateCssJsxDomObjectsType {
+  (args: { id: Readonly<string> }): readonly [
+    {
+      toString(this: CssClassName): string
+    },
+    FC<PropsWithChildren<void>>
+  ]
+}
+
+export const createCssJsxDomObjects: CreateCssJsxDomObjectsType = ({ id }) => {
   let styleSheet: CSSStyleSheet | null | undefined = undefined
   const findStyleSheet = (): [CSSStyleSheet, Set<string>] | [] => {
     if (!styleSheet) {
diff --git a/deno_dist/jsx/dom/jsx-dev-runtime.ts b/deno_dist/jsx/dom/jsx-dev-runtime.ts
index 216aca4fc..fc1837012 100644
--- a/deno_dist/jsx/dom/jsx-dev-runtime.ts
+++ b/deno_dist/jsx/dom/jsx-dev-runtime.ts
@@ -28,4 +28,4 @@ export const jsxDEV = (tag: string | Function, props: Props, key?: string): JSXN
   ) as JSXNode
 }
 
-export const Fragment = (props: Record<string, unknown>) => jsxDEV('', props, undefined)
+export const Fragment = (props: Record<string, unknown>): JSXNode => jsxDEV('', props, undefined)
diff --git a/deno_dist/jsx/dom/utils.ts b/deno_dist/jsx/dom/utils.ts
index 5e2340d0e..803652155 100644
--- a/deno_dist/jsx/dom/utils.ts
+++ b/deno_dist/jsx/dom/utils.ts
@@ -1,6 +1,6 @@
 import { DOM_INTERNAL_TAG } from '../constants.ts'
 
-export const setInternalTagFlag = (fn: Function) => {
+export const setInternalTagFlag = (fn: Function): Function => {
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   ;(fn as any)[DOM_INTERNAL_TAG] = true
   return fn
diff --git a/deno_dist/jsx/hooks/index.ts b/deno_dist/jsx/hooks/index.ts
index 27566990e..93e63b22e 100644
--- a/deno_dist/jsx/hooks/index.ts
+++ b/deno_dist/jsx/hooks/index.ts
@@ -18,7 +18,10 @@ export type EffectData = [
   (() => void) | undefined // effect
 ]
 
-const resolvedPromiseValueMap = new WeakMap<Promise<unknown>, unknown>()
+const resolvedPromiseValueMap: WeakMap<Promise<unknown>, unknown> = new WeakMap<
+  Promise<unknown>,
+  unknown
+>()
 
 const isDepsChanged = (
   prevDeps: readonly unknown[] | undefined,
diff --git a/deno_dist/jsx/jsx-runtime.ts b/deno_dist/jsx/jsx-runtime.ts
index 61d7096ba..cf1c0c0a9 100644
--- a/deno_dist/jsx/jsx-runtime.ts
+++ b/deno_dist/jsx/jsx-runtime.ts
@@ -2,7 +2,11 @@ export { jsxDEV as jsx, Fragment } from './jsx-dev-runtime.ts'
 export { jsxDEV as jsxs } from './jsx-dev-runtime.ts'
 
 import { raw, html } from '../helper/html/index.ts'
+import type { HtmlEscapedString } from '../utils/html.ts'
 export { html as jsxTemplate }
-export const jsxAttr = (name: string, value: string | Promise<string>) =>
+export const jsxAttr = (
+  name: string,
+  value: string | Promise<string>
+): HtmlEscapedString | Promise<HtmlEscapedString> =>
   typeof value === 'string' ? raw(name + '="' + html`${value}` + '"') : html`${name}="${value}"`
 export const jsxEscape = (value: string) => value
diff --git a/deno_dist/middleware/jsx-renderer/index.ts b/deno_dist/middleware/jsx-renderer/index.ts
index 4f7dd75c4..0d76102e3 100644
--- a/deno_dist/middleware/jsx-renderer/index.ts
+++ b/deno_dist/middleware/jsx-renderer/index.ts
@@ -3,11 +3,13 @@ import type { Context, PropsForRenderer } from '../../context.ts'
 import { html, raw } from '../../helper/html/index.ts'
 import { jsx, createContext, useContext, Fragment } from '../../jsx/index.ts'
 import type { FC, PropsWithChildren, JSXNode } from '../../jsx/index.ts'
+import type { Context as JSXContext } from '../../jsx/index.ts'
 import { renderToReadableStream } from '../../jsx/streaming.ts'
 import type { Env, Input, MiddlewareHandler } from '../../types.ts'
 import type { HtmlEscapedString } from '../../utils/html.ts'
 
-export const RequestContext = createContext<Context | null>(null)
+export const RequestContext: JSXContext<Context<any, any, {}> | null> =
+  createContext<Context | null>(null)
 
 type RendererOptions = {
   docType?: boolean | string
diff --git a/deno_dist/middleware/timing/index.ts b/deno_dist/middleware/timing/index.ts
index 429929c32..3881f8ce4 100644
--- a/deno_dist/middleware/timing/index.ts
+++ b/deno_dist/middleware/timing/index.ts
@@ -24,7 +24,7 @@ interface TimingOptions {
   crossOrigin: boolean | string | ((c: Context) => boolean | string)
 }
 
-const getTime = () => {
+const getTime = (): number => {
   try {
     return performance.now()
   } catch {}
diff --git a/deno_dist/request.ts b/deno_dist/request.ts
index ea1825379..6fd80d42b 100644
--- a/deno_dist/request.ts
+++ b/deno_dist/request.ts
@@ -289,7 +289,7 @@ export class HonoRequest<P extends string = '/', I extends Input['out'] = {}> {
    * ```
    * @see https://hono.dev/api/request#url
    */
-  get url() {
+  get url(): string {
     return this.raw.url
   }
 
@@ -303,7 +303,7 @@ export class HonoRequest<P extends string = '/', I extends Input['out'] = {}> {
    * ```
    * @see https://hono.dev/api/request#method
    */
-  get method() {
+  get method(): string {
     return this.raw.method
   }
 
diff --git a/deno_dist/router/smart-router/router.ts b/deno_dist/router/smart-router/router.ts
index 9c35f2b70..f0ade4671 100644
--- a/deno_dist/router/smart-router/router.ts
+++ b/deno_dist/router/smart-router/router.ts
@@ -59,7 +59,7 @@ export class SmartRouter<T> implements Router<T> {
     return res as Result<T>
   }
 
-  get activeRouter() {
+  get activeRouter(): Router<T> {
     if (this.routes || this.routers.length !== 1) {
       throw new Error('No active router has been determined yet.')
     }
diff --git a/deno_dist/utils/buffer.ts b/deno_dist/utils/buffer.ts
index 075c3d5ec..5ba55a4e0 100644
--- a/deno_dist/utils/buffer.ts
+++ b/deno_dist/utils/buffer.ts
@@ -48,7 +48,10 @@ export const bufferToString = (buffer: ArrayBuffer): string => {
   return buffer
 }
 
-export const bufferToFormData = (arrayBuffer: ArrayBuffer, contentType: string) => {
+export const bufferToFormData = (
+  arrayBuffer: ArrayBuffer,
+  contentType: string
+): Promise<FormData> => {
   const response = new Response(arrayBuffer, {
     headers: {
       'Content-Type': contentType,
diff --git a/deno_dist/utils/color.ts b/deno_dist/utils/color.ts
index fd2f62b85..7f354e32d 100644
--- a/deno_dist/utils/color.ts
+++ b/deno_dist/utils/color.ts
@@ -1,4 +1,4 @@
-export function getColorEnabled() {
+export function getColorEnabled(): boolean {
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   const { process, Deno } = globalThis as any
 
diff --git a/deno_dist/utils/jwt/utf8.ts b/deno_dist/utils/jwt/utf8.ts
index 107407aee..01d1972a1 100644
--- a/deno_dist/utils/jwt/utf8.ts
+++ b/deno_dist/utils/jwt/utf8.ts
@@ -1,2 +1,2 @@
-export const utf8Encoder = new TextEncoder()
-export const utf8Decoder = new TextDecoder()
+export const utf8Encoder: TextEncoder = new TextEncoder()
+export const utf8Decoder: TextDecoder = new TextDecoder()
diff --git a/deno_dist/utils/stream.ts b/deno_dist/utils/stream.ts
index 4269a3598..b129caf91 100644
--- a/deno_dist/utils/stream.ts
+++ b/deno_dist/utils/stream.ts
@@ -30,7 +30,7 @@ export class StreamingApi {
     })
   }
 
-  async write(input: Uint8Array | string) {
+  async write(input: Uint8Array | string): Promise<StreamingApi> {
     try {
       if (typeof input === 'string') {
         input = this.encoder.encode(input)
@@ -42,12 +42,12 @@ export class StreamingApi {
     return this
   }
 
-  async writeln(input: string) {
+  async writeln(input: string): Promise<StreamingApi> {
     await this.write(input + '\n')
     return this
   }
 
-  sleep(ms: number) {
+  sleep(ms: number): Promise<unknown> {
     return new Promise((res) => setTimeout(res, ms))
   }
 
diff --git a/src/context.ts b/src/context.ts
index 8de2f9e30..405c841b2 100644
--- a/src/context.ts
+++ b/src/context.ts
@@ -1,6 +1,6 @@
 import type { HonoRequest } from './request'
-import type { Env, FetchEventLike, NotFoundHandler, Input, TypedResponse } from './types'
-import { resolveCallback, HtmlEscapedCallbackPhase } from './utils/html'
+import type { Env, FetchEventLike, Input, NotFoundHandler, TypedResponse } from './types'
+import { HtmlEscapedCallbackPhase, resolveCallback } from './utils/html'
 import type { RedirectStatusCode, StatusCode } from './utils/http-status'
 import type { JSONValue, JSONParsed, IsAny, Simplify } from './utils/types'
 
@@ -224,7 +224,14 @@ export class Context<
    */
   render: Renderer = (...args) => this.renderer(...args)
 
-  setLayout = (layout: Layout<PropsForRenderer & { Layout: Layout }>) => (this.layout = layout)
+  setLayout = (
+    layout: Layout<PropsForRenderer & { Layout: Layout }>
+  ): Layout<
+    PropsForRenderer & {
+      Layout: Layout
+    }
+  > => (this.layout = layout)
+
   getLayout = () => this.layout
 
   /**
diff --git a/src/helper/accepts/accepts.ts b/src/helper/accepts/accepts.ts
index 4a8338e66..b984e6f36 100644
--- a/src/helper/accepts/accepts.ts
+++ b/src/helper/accepts/accepts.ts
@@ -25,7 +25,7 @@ export interface acceptsOptions extends acceptsConfig {
   match?: (accepts: Accept[], config: acceptsConfig) => string
 }
 
-export const parseAccept = (acceptHeader: string) => {
+export const parseAccept = (acceptHeader: string): Accept[] => {
   // Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
   const accepts = acceptHeader.split(',') // ['text/html', 'application/xhtml+xml', 'application/xml;q=0.9', 'image/webp', '*/*;q=0.8']
   return accepts.map((accept) => {
@@ -42,7 +42,7 @@ export const parseAccept = (acceptHeader: string) => {
   })
 }
 
-export const defaultMatch = (accepts: Accept[], config: acceptsConfig) => {
+export const defaultMatch = (accepts: Accept[], config: acceptsConfig): string => {
   const { supports, default: defaultSupport } = config
   const accept = accepts.sort((a, b) => b.q - a.q).find((accept) => supports.includes(accept.type))
   return accept ? accept.type : defaultSupport
@@ -61,7 +61,7 @@ export const defaultMatch = (accepts: Accept[], config: acceptsConfig) => {
  * })
  * ```
  */
-export const accepts = (c: Context, options: acceptsOptions) => {
+export const accepts = (c: Context, options: acceptsOptions): string => {
   const acceptHeader = c.req.header(options.header)
   if (!acceptHeader) {
     return options.default
diff --git a/src/helper/adapter/index.ts b/src/helper/adapter/index.ts
index 0df451fd1..b8d544409 100644
--- a/src/helper/adapter/index.ts
+++ b/src/helper/adapter/index.ts
@@ -30,7 +30,7 @@ export const env = <T extends Record<string, unknown>, C extends Context = Conte
   return runtimeEnvHandlers[runtime]()
 }
 
-export const getRuntimeKey = () => {
+export const getRuntimeKey = (): Runtime => {
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   const global = globalThis as any
 
diff --git a/src/helper/cookie/index.ts b/src/helper/cookie/index.ts
index eeaba9d86..fd90704bb 100644
--- a/src/helper/cookie/index.ts
+++ b/src/helper/cookie/index.ts
@@ -118,7 +118,7 @@ export const setSignedCookie = async (
   c.header('set-cookie', cookie, { append: true })
 }
 
-export const deleteCookie = (c: Context, name: string, opt?: CookieOptions) => {
+export const deleteCookie = (c: Context, name: string, opt?: CookieOptions): string | undefined => {
   const deletedCookie = getCookie(c, name)
   setCookie(c, name, '', { ...opt, maxAge: 0 })
   return deletedCookie
diff --git a/src/helper/css/common.ts b/src/helper/css/common.ts
index 46a02e9d8..aff38a280 100644
--- a/src/helper/css/common.ts
+++ b/src/helper/css/common.ts
@@ -3,12 +3,12 @@ export const PSEUDO_GLOBAL_SELECTOR = ':-hono-global'
 export const isPseudoGlobalSelectorRe = new RegExp(`^${PSEUDO_GLOBAL_SELECTOR}{(.*)}$`)
 export const DEFAULT_STYLE_ID = 'hono-css'
 
-export const SELECTOR = Symbol()
-export const CLASS_NAME = Symbol()
-export const STYLE_STRING = Symbol()
-export const SELECTORS = Symbol()
-export const EXTERNAL_CLASS_NAMES = Symbol()
-const CSS_ESCAPED = Symbol()
+export const SELECTOR: unique symbol = Symbol()
+export const CLASS_NAME: unique symbol = Symbol()
+export const STYLE_STRING: unique symbol = Symbol()
+export const SELECTORS: unique symbol = Symbol()
+export const EXTERNAL_CLASS_NAMES: unique symbol = Symbol()
+const CSS_ESCAPED: unique symbol = Symbol()
 
 export interface CssClassName {
   [SELECTOR]: string
@@ -49,12 +49,12 @@ const toHash = (str: string): string => {
   return 'css-' + out
 }
 
-const cssStringReStr = [
+const cssStringReStr: string = [
   '"(?:(?:\\\\[\\s\\S]|[^"\\\\])*)"', // double quoted string
   // eslint-disable-next-line quotes
   "'(?:(?:\\\\[\\s\\S]|[^'\\\\])*)'", // single quoted string
 ].join('|')
-const minifyCssRe = new RegExp(
+const minifyCssRe: RegExp = new RegExp(
   [
     '(' + cssStringReStr + ')', // $1: quoted string
 
diff --git a/src/helper/css/index.ts b/src/helper/css/index.ts
index a77346836..0393e9e29 100644
--- a/src/helper/css/index.ts
+++ b/src/helper/css/index.ts
@@ -24,12 +24,36 @@ type usedClassNameData = [
   Record<string, true> // class name already added
 ]
 
+interface CssType {
+  (strings: TemplateStringsArray, ...values: CssVariableType[]): Promise<string>
+}
+
+interface CxType {
+  (
+    ...args: (CssClassName | Promise<string> | string | boolean | null | undefined)[]
+  ): Promise<string>
+}
+
+interface KeyframesType {
+  (strings: TemplateStringsArray, ...values: CssVariableType[]): CssClassNameCommon
+}
+
+interface ViewTransitionType {
+  (strings: TemplateStringsArray, ...values: CssVariableType[]): Promise<string>
+  (content: Promise<string>): Promise<string>
+  (): Promise<string>
+}
+
+interface StyleType {
+  (args?: { children?: Promise<string> }): HtmlEscapedString
+}
+
 /**
  * @experimental
  * `createCssContext` is an experimental feature.
  * The API might be changed.
  */
-export const createCssContext = ({ id }: { id: Readonly<string> }) => {
+export const createCssContext = ({ id }: { id: Readonly<string> }): DefaultContextType => {
   const [cssJsxDomObject, StyleRenderToDom] = createCssJsxDomObjects({ id })
 
   const contextMap: WeakMap<object, usedClassNameData> = new WeakMap()
@@ -106,13 +130,11 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
     return promise
   }
 
-  const css = (strings: TemplateStringsArray, ...values: CssVariableType[]): Promise<string> => {
+  const css: CssType = (strings, ...values) => {
     return newCssClassNameObject(cssCommon(strings, values))
   }
 
-  const cx = (
-    ...args: (CssClassName | Promise<string> | string | boolean | null | undefined)[]
-  ): Promise<string> => {
+  const cx: CxType = (...args) => {
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     args = cxCommon(args as any) as any
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -121,20 +143,15 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
 
   const keyframes = keyframesCommon
 
-  type ViewTransitionType = {
-    (strings: TemplateStringsArray, ...values: CssVariableType[]): Promise<string>
-    (content: Promise<string>): Promise<string>
-    (): Promise<string>
-  }
   const viewTransition: ViewTransitionType = ((
     strings: TemplateStringsArray | Promise<string> | undefined,
     ...values: CssVariableType[]
-  ): Promise<string> => {
+  ) => {
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     return newCssClassNameObject(viewTransitionCommon(strings as any, values))
   }) as ViewTransitionType
 
-  const Style = ({ children }: { children?: Promise<string> } = {}) =>
+  const Style: StyleType = ({ children } = {}) =>
     children
       ? raw(`<style id="${id}">${(children as unknown as CssClassName)[STYLE_STRING]}</style>`)
       : raw(`<style id="${id}"></style>`)
@@ -150,7 +167,17 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
   }
 }
 
-const defaultContext = createCssContext({ id: DEFAULT_STYLE_ID })
+interface DefaultContextType {
+  css: CssType
+  cx: CxType
+  keyframes: KeyframesType
+  viewTransition: ViewTransitionType
+  Style: StyleType
+}
+
+const defaultContext: DefaultContextType = createCssContext({
+  id: DEFAULT_STYLE_ID,
+})
 
 /**
  * @experimental
diff --git a/src/helper/dev/index.ts b/src/helper/dev/index.ts
index 3393aeacd..f27b1f3be 100644
--- a/src/helper/dev/index.ts
+++ b/src/helper/dev/index.ts
@@ -15,7 +15,7 @@ interface RouteData {
   isMiddleware: boolean
 }
 
-const handlerName = (handler: Function) => {
+const handlerName = (handler: Function): string => {
   return handler.name || (isMiddleware(handler) ? '[middleware]' : '[handler]')
 }
 
@@ -68,7 +68,7 @@ export const showRoutes = <E extends Env>(hono: Hono<E>, opts?: ShowRoutesOption
     })
 }
 
-export const getRouterName = <E extends Env>(app: Hono<E>) => {
+export const getRouterName = <E extends Env>(app: Hono<E>): string => {
   app.router.match('GET', '/')
   return app.router.name
 }
diff --git a/src/helper/factory/index.ts b/src/helper/factory/index.ts
index ecd1e6f64..faeec0812 100644
--- a/src/helper/factory/index.ts
+++ b/src/helper/factory/index.ts
@@ -216,7 +216,7 @@ export class Factory<E extends Env = any, P extends string = any> {
    * @experimental
    * `createApp` is an experimental feature.
    */
-  createApp = () => {
+  createApp = (): Hono<E> => {
     const app = new Hono<E>()
     if (this.initApp) {
       this.initApp(app)
@@ -234,8 +234,8 @@ export class Factory<E extends Env = any, P extends string = any> {
 
 export const createFactory = <E extends Env = any, P extends string = any>(init?: {
   initApp?: InitApp<E>
-}) => new Factory<E, P>(init)
+}): Factory<E, P> => new Factory<E, P>(init)
 
 export const createMiddleware = <E extends Env = any, P extends string = any, I extends Input = {}>(
   middleware: MiddlewareHandler<E, P, I>
-) => createFactory<E, P>().createMiddleware<I>(middleware)
+): MiddlewareHandler<E, P, I> => createFactory<E, P>().createMiddleware<I>(middleware)
diff --git a/src/helper/streaming/sse.ts b/src/helper/streaming/sse.ts
index bcadf1f43..e909fc274 100644
--- a/src/helper/streaming/sse.ts
+++ b/src/helper/streaming/sse.ts
@@ -62,7 +62,7 @@ export const streamSSE = (
   c: Context,
   cb: (stream: SSEStreamingApi) => Promise<void>,
   onError?: (e: Error, stream: SSEStreamingApi) => Promise<void>
-) => {
+): Response => {
   const { readable, writable } = new TransformStream()
   const stream = new SSEStreamingApi(writable, readable)
 
diff --git a/src/helper/testing/index.ts b/src/helper/testing/index.ts
index 23e879f4f..4920a5cbc 100644
--- a/src/helper/testing/index.ts
+++ b/src/helper/testing/index.ts
@@ -1,6 +1,8 @@
 import { hc } from '../../client'
+import type { Client } from '../../client/types'
 import type { ExecutionContext } from '../../context'
 import type { Hono } from '../../hono'
+import type { UnionToIntersection } from '../../utils/types'
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 type ExtractEnv<T> = T extends Hono<infer E, any, any> ? E : never
@@ -10,7 +12,7 @@ export const testClient = <T extends Hono<any, any, any>>(
   app: T,
   Env?: ExtractEnv<T>['Bindings'] | {},
   executionCtx?: ExecutionContext
-) => {
+): UnionToIntersection<Client<T>> => {
   const customFetch = (input: RequestInfo | URL, init?: RequestInit) => {
     return app.request(input, init, Env, executionCtx)
   }
diff --git a/src/hono-base.ts b/src/hono-base.ts
index cb4092e38..293d3874f 100644
--- a/src/hono-base.ts
+++ b/src/hono-base.ts
@@ -233,7 +233,7 @@ class Hono<
    * })
    * ```
    */
-  onError = (handler: ErrorHandler<E>) => {
+  onError = (handler: ErrorHandler<E>): Hono<E, S, BasePath> => {
     this.errorHandler = handler
     return this
   }
@@ -247,7 +247,7 @@ class Hono<
    * ```
    * @see https://hono.dev/api/hono#not-found
    */
-  notFound = (handler: NotFoundHandler<E>) => {
+  notFound = (handler: NotFoundHandler<E>): Hono<E, S, BasePath> => {
     this.notFoundHandler = handler
     return this
   }
@@ -396,7 +396,7 @@ class Hono<
     requestInit?: RequestInit,
     Env?: E['Bindings'] | {},
     executionCtx?: ExecutionContext
-  ) => {
+  ): Response | Promise<Response> => {
     if (input instanceof Request) {
       if (requestInit !== undefined) {
         input = new Request(input, requestInit)
diff --git a/src/jsx/dom/context.ts b/src/jsx/dom/context.ts
index c1a3d8b33..d39f52f8c 100644
--- a/src/jsx/dom/context.ts
+++ b/src/jsx/dom/context.ts
@@ -5,7 +5,7 @@ import { globalContexts } from '../context'
 import { Fragment } from './jsx-runtime'
 import { setInternalTagFlag } from './utils'
 
-export const createContextProviderFunction = <T>(values: T[]) =>
+export const createContextProviderFunction = <T>(values: T[]): Function =>
   setInternalTagFlag(({ value, children }: { value: T; children: Child[] }) => {
     if (!children) {
       return undefined
diff --git a/src/jsx/dom/css.ts b/src/jsx/dom/css.ts
index 4629ef9c5..5a145ad7f 100644
--- a/src/jsx/dom/css.ts
+++ b/src/jsx/dom/css.ts
@@ -56,7 +56,16 @@ const splitRule = (rule: string): string[] => {
   return result
 }
 
-export const createCssJsxDomObjects = ({ id }: { id: Readonly<string> }) => {
+interface CreateCssJsxDomObjectsType {
+  (args: { id: Readonly<string> }): readonly [
+    {
+      toString(this: CssClassName): string
+    },
+    FC<PropsWithChildren<void>>
+  ]
+}
+
+export const createCssJsxDomObjects: CreateCssJsxDomObjectsType = ({ id }) => {
   let styleSheet: CSSStyleSheet | null | undefined = undefined
   const findStyleSheet = (): [CSSStyleSheet, Set<string>] | [] => {
     if (!styleSheet) {
diff --git a/src/jsx/dom/jsx-dev-runtime.ts b/src/jsx/dom/jsx-dev-runtime.ts
index 28b9acdc9..a4c439b16 100644
--- a/src/jsx/dom/jsx-dev-runtime.ts
+++ b/src/jsx/dom/jsx-dev-runtime.ts
@@ -28,4 +28,4 @@ export const jsxDEV = (tag: string | Function, props: Props, key?: string): JSXN
   ) as JSXNode
 }
 
-export const Fragment = (props: Record<string, unknown>) => jsxDEV('', props, undefined)
+export const Fragment = (props: Record<string, unknown>): JSXNode => jsxDEV('', props, undefined)
diff --git a/src/jsx/dom/utils.ts b/src/jsx/dom/utils.ts
index cf4c58072..fef0c3c05 100644
--- a/src/jsx/dom/utils.ts
+++ b/src/jsx/dom/utils.ts
@@ -1,6 +1,6 @@
 import { DOM_INTERNAL_TAG } from '../constants'
 
-export const setInternalTagFlag = (fn: Function) => {
+export const setInternalTagFlag = (fn: Function): Function => {
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   ;(fn as any)[DOM_INTERNAL_TAG] = true
   return fn
diff --git a/src/jsx/hooks/index.ts b/src/jsx/hooks/index.ts
index 60e2d240a..d142d44af 100644
--- a/src/jsx/hooks/index.ts
+++ b/src/jsx/hooks/index.ts
@@ -18,7 +18,10 @@ export type EffectData = [
   (() => void) | undefined // effect
 ]
 
-const resolvedPromiseValueMap = new WeakMap<Promise<unknown>, unknown>()
+const resolvedPromiseValueMap: WeakMap<Promise<unknown>, unknown> = new WeakMap<
+  Promise<unknown>,
+  unknown
+>()
 
 const isDepsChanged = (
   prevDeps: readonly unknown[] | undefined,
diff --git a/src/jsx/jsx-runtime.ts b/src/jsx/jsx-runtime.ts
index a20521e29..d75da1697 100644
--- a/src/jsx/jsx-runtime.ts
+++ b/src/jsx/jsx-runtime.ts
@@ -2,7 +2,11 @@ export { jsxDEV as jsx, Fragment } from './jsx-dev-runtime'
 export { jsxDEV as jsxs } from './jsx-dev-runtime'
 
 import { raw, html } from '../helper/html'
+import type { HtmlEscapedString } from '../utils/html'
 export { html as jsxTemplate }
-export const jsxAttr = (name: string, value: string | Promise<string>) =>
+export const jsxAttr = (
+  name: string,
+  value: string | Promise<string>
+): HtmlEscapedString | Promise<HtmlEscapedString> =>
   typeof value === 'string' ? raw(name + '="' + html`${value}` + '"') : html`${name}="${value}"`
 export const jsxEscape = (value: string) => value
diff --git a/src/middleware/jsx-renderer/index.ts b/src/middleware/jsx-renderer/index.ts
index 1a3f261bf..e42b9868a 100644
--- a/src/middleware/jsx-renderer/index.ts
+++ b/src/middleware/jsx-renderer/index.ts
@@ -3,11 +3,13 @@ import type { Context, PropsForRenderer } from '../../context'
 import { html, raw } from '../../helper/html'
 import { jsx, createContext, useContext, Fragment } from '../../jsx'
 import type { FC, PropsWithChildren, JSXNode } from '../../jsx'
+import type { Context as JSXContext } from '../../jsx'
 import { renderToReadableStream } from '../../jsx/streaming'
 import type { Env, Input, MiddlewareHandler } from '../../types'
 import type { HtmlEscapedString } from '../../utils/html'
 
-export const RequestContext = createContext<Context | null>(null)
+export const RequestContext: JSXContext<Context<any, any, {}> | null> =
+  createContext<Context | null>(null)
 
 type RendererOptions = {
   docType?: boolean | string
diff --git a/src/middleware/timing/index.ts b/src/middleware/timing/index.ts
index 504331b15..2675b9e5b 100644
--- a/src/middleware/timing/index.ts
+++ b/src/middleware/timing/index.ts
@@ -24,7 +24,7 @@ interface TimingOptions {
   crossOrigin: boolean | string | ((c: Context) => boolean | string)
 }
 
-const getTime = () => {
+const getTime = (): number => {
   try {
     return performance.now()
   } catch {}
diff --git a/src/request.ts b/src/request.ts
index b5fd8546e..7c028aee5 100644
--- a/src/request.ts
+++ b/src/request.ts
@@ -289,7 +289,7 @@ export class HonoRequest<P extends string = '/', I extends Input['out'] = {}> {
    * ```
    * @see https://hono.dev/api/request#url
    */
-  get url() {
+  get url(): string {
     return this.raw.url
   }
 
@@ -303,7 +303,7 @@ export class HonoRequest<P extends string = '/', I extends Input['out'] = {}> {
    * ```
    * @see https://hono.dev/api/request#method
    */
-  get method() {
+  get method(): string {
     return this.raw.method
   }
 
diff --git a/src/router/smart-router/router.ts b/src/router/smart-router/router.ts
index 7b0510d87..aa1201122 100644
--- a/src/router/smart-router/router.ts
+++ b/src/router/smart-router/router.ts
@@ -59,7 +59,7 @@ export class SmartRouter<T> implements Router<T> {
     return res as Result<T>
   }
 
-  get activeRouter() {
+  get activeRouter(): Router<T> {
     if (this.routes || this.routers.length !== 1) {
       throw new Error('No active router has been determined yet.')
     }
diff --git a/src/utils/buffer.ts b/src/utils/buffer.ts
index fecfd82eb..6d24f692f 100644
--- a/src/utils/buffer.ts
+++ b/src/utils/buffer.ts
@@ -48,7 +48,10 @@ export const bufferToString = (buffer: ArrayBuffer): string => {
   return buffer
 }
 
-export const bufferToFormData = (arrayBuffer: ArrayBuffer, contentType: string) => {
+export const bufferToFormData = (
+  arrayBuffer: ArrayBuffer,
+  contentType: string
+): Promise<FormData> => {
   const response = new Response(arrayBuffer, {
     headers: {
       'Content-Type': contentType,
diff --git a/src/utils/color.ts b/src/utils/color.ts
index fd2f62b85..7f354e32d 100644
--- a/src/utils/color.ts
+++ b/src/utils/color.ts
@@ -1,4 +1,4 @@
-export function getColorEnabled() {
+export function getColorEnabled(): boolean {
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   const { process, Deno } = globalThis as any
 
diff --git a/src/utils/jwt/utf8.ts b/src/utils/jwt/utf8.ts
index 107407aee..01d1972a1 100644
--- a/src/utils/jwt/utf8.ts
+++ b/src/utils/jwt/utf8.ts
@@ -1,2 +1,2 @@
-export const utf8Encoder = new TextEncoder()
-export const utf8Decoder = new TextDecoder()
+export const utf8Encoder: TextEncoder = new TextEncoder()
+export const utf8Decoder: TextDecoder = new TextDecoder()
diff --git a/src/utils/stream.ts b/src/utils/stream.ts
index 4269a3598..b129caf91 100644
--- a/src/utils/stream.ts
+++ b/src/utils/stream.ts
@@ -30,7 +30,7 @@ export class StreamingApi {
     })
   }
 
-  async write(input: Uint8Array | string) {
+  async write(input: Uint8Array | string): Promise<StreamingApi> {
     try {
       if (typeof input === 'string') {
         input = this.encoder.encode(input)
@@ -42,12 +42,12 @@ export class StreamingApi {
     return this
   }
 
-  async writeln(input: string) {
+  async writeln(input: string): Promise<StreamingApi> {
     await this.write(input + '\n')
     return this
   }
 
-  sleep(ms: number) {
+  sleep(ms: number): Promise<unknown> {
     return new Promise((res) => setTimeout(res, ms))
   }