diff --git a/deno_dist/context.ts b/deno_dist/context.ts
index 99fe18c98..be844d62e 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 +224,18 @@ 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
+    }
+  > = (layout) => (this.layout = layout)
+
   getLayout = () => this.layout
 
   /**
diff --git a/deno_dist/hono-base.ts b/deno_dist/hono-base.ts
index b49a6fc33..ed719bda0 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> = (handler) => {
     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> = (handler) => {
     this.notFoundHandler = handler
     return this
   }
@@ -391,12 +391,12 @@ class Hono<
    * ```
    * @see https://hono.dev/api/hono#request
    */
-  request = (
+  request: (
     input: RequestInfo | URL,
     requestInit?: RequestInit,
     Env?: E['Bindings'] | {},
     executionCtx?: ExecutionContext
-  ) => {
+  ) => Response | Promise<Response> = (input, requestInit, Env, executionCtx) => {
     if (input instanceof Request) {
       if (requestInit !== undefined) {
         input = new Request(input, requestInit)
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/src/context.ts b/src/context.ts
index 8de2f9e30..7cb776f8a 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 +224,18 @@ 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
+    }
+  > = (layout) => (this.layout = layout)
+
   getLayout = () => this.layout
 
   /**
diff --git a/src/hono-base.ts b/src/hono-base.ts
index cb4092e38..970666cc9 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> = (handler) => {
     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> = (handler) => {
     this.notFoundHandler = handler
     return this
   }
@@ -391,12 +391,12 @@ class Hono<
    * ```
    * @see https://hono.dev/api/hono#request
    */
-  request = (
+  request: (
     input: RequestInfo | URL,
     requestInit?: RequestInit,
     Env?: E['Bindings'] | {},
     executionCtx?: ExecutionContext
-  ) => {
+  ) => Response | Promise<Response> = (input, requestInit, Env, executionCtx) => {
     if (input instanceof Request) {
       if (requestInit !== undefined) {
         input = new Request(input, requestInit)
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,