diff --git a/.changeset/six-radios-attack.md b/.changeset/six-radios-attack.md
new file mode 100644
index 00000000..e0a6f913
--- /dev/null
+++ b/.changeset/six-radios-attack.md
@@ -0,0 +1,5 @@
+---
+'@hono/auth-js': patch
+---
+
+Add explicit return types
diff --git a/packages/auth-js/src/client.ts b/packages/auth-js/src/client.ts
index 6ece911d..6551ab1f 100644
--- a/packages/auth-js/src/client.ts
+++ b/packages/auth-js/src/client.ts
@@ -134,7 +134,7 @@ export async function fetchData<T = any>(
   }
 }
 
-export function useOnline() {
+export function useOnline(): boolean {
   const [isOnline, setIsOnline] = useState(
     typeof navigator !== 'undefined' ? navigator.onLine : false
   )
@@ -157,11 +157,19 @@ export function useOnline() {
   return isOnline
 }
 
-export function now() {
+export function now(): number {
   return Math.floor(Date.now() / 1000)
 }
 
-export function parseUrl(url?: string) {
+interface ParsedUrl {
+  origin: string
+  host: string
+  path: string
+  base: string
+  toString: () => string
+}
+
+export function parseUrl(url?: string): ParsedUrl {
   const defaultUrl = 'http://localhost:3000/api/auth'
   const parsedUrl = new URL(url ? (url.startsWith('http') ? url : `https://${url}`) : defaultUrl)
   const path = parsedUrl.pathname === '/' ? '/api/auth' : parsedUrl.pathname.replace(/\/$/, '')
diff --git a/packages/auth-js/src/index.ts b/packages/auth-js/src/index.ts
index 0944203c..97d2e182 100644
--- a/packages/auth-js/src/index.ts
+++ b/packages/auth-js/src/index.ts
@@ -31,12 +31,12 @@ export interface AuthConfig extends Omit<AuthConfigCore, 'raw'> {}
 
 export type ConfigHandler = (c: Context) => AuthConfig
 
-export function setEnvDefaults(env: AuthEnv, config: AuthConfig) {
+export function setEnvDefaults(env: AuthEnv, config: AuthConfig): void {
   config.secret ??= env.AUTH_SECRET
   coreSetEnvDefaults(env, config)
 }
 
-export function reqWithEnvUrl(req: Request, authUrl?: string) {
+export function reqWithEnvUrl(req: Request, authUrl?: string): Request {
   if (authUrl) {
     const reqUrlObj = new URL(req.url)
     const authUrlObj = new URL(authUrl)
diff --git a/packages/auth-js/src/react.tsx b/packages/auth-js/src/react.tsx
index 4484e4b0..62e79f7b 100644
--- a/packages/auth-js/src/react.tsx
+++ b/packages/auth-js/src/react.tsx
@@ -65,9 +65,11 @@ class AuthConfigManager {
   }
 }
 
-export const authConfigManager = AuthConfigManager.getInstance()
+export const authConfigManager: AuthConfigManager = AuthConfigManager.getInstance()
 
-export const SessionContext = React.createContext<SessionContextValue | undefined>(undefined)
+export const SessionContext: React.Context<SessionContextValue | undefined> = React.createContext<
+  SessionContextValue | undefined
+>(undefined)
 
 function useInitializeSession(hasInitialSession: boolean, initialSession: Session | null) {
   const authConfig = authConfigManager.getConfig()
@@ -148,7 +150,7 @@ function useRefetchInterval(
   }, [refetchInterval, shouldRefetch])
 }
 
-export async function getSession(params?: GetSessionParams) {
+export async function getSession(params?: GetSessionParams): Promise<Session | null> {
   const { baseUrl, basePath, credentials } = authConfigManager.getConfig()
   const session = await fetchData<Session>(
     'session',
@@ -163,7 +165,7 @@ export async function getSession(params?: GetSessionParams) {
   return session
 }
 
-export async function getCsrfToken() {
+export async function getCsrfToken(): Promise<string> {
   const { baseUrl, basePath, credentials } = authConfigManager.getConfig()
   const response = await fetchData<{ csrfToken: string }>(
     'csrf',
@@ -177,7 +179,7 @@ export async function getCsrfToken() {
   return response?.csrfToken ?? ''
 }
 
-export function SessionProvider(props: SessionProviderProps) {
+export function SessionProvider(props: SessionProviderProps): React.JSX.Element {
   if (!SessionContext) {
     throw new Error('React Context is unavailable in Server Components')
   }
@@ -274,7 +276,7 @@ export function useSession<R extends boolean>(
 
 type ProvidersType = Record<LiteralUnion<BuiltInProviderType>, ClientSafeProvider>
 
-export async function getProviders() {
+export async function getProviders(): Promise<ProvidersType | null> {
   return fetchData<ProvidersType>('providers', authConfigManager.getConfig(), logger)
 }
 
@@ -395,10 +397,14 @@ interface PopupLoginOptions extends Partial<Omit<WindowProps, 'url'>> {
   callbackUrl?: string
 }
 
+interface LoginState extends AuthState {
+  popUpSignin: () => Promise<void>
+}
+
 export const useOauthPopupLogin = (
   provider: Parameters<typeof signIn>[0],
   options: PopupLoginOptions = {}
-) => {
+): LoginState => {
   const { width = 500, height = 500, title = 'Signin', onSuccess, callbackUrl = '/' } = options
 
   const [externalWindow, setExternalWindow] = useState<Window | null>()
diff --git a/packages/auth-js/tsconfig.build.json b/packages/auth-js/tsconfig.build.json
index 44b204b9..402da835 100644
--- a/packages/auth-js/tsconfig.build.json
+++ b/packages/auth-js/tsconfig.build.json
@@ -1,9 +1,14 @@
 {
   "extends": "../../tsconfig.base.json",
   "compilerOptions": {
+    "rootDir": "src",
     "outDir": "dist",
+    "tsBuildInfoFile": "dist/tsconfig.build.tsbuildinfo",
+    "emitDeclarationOnly": false,
+    "isolatedDeclarations": true,
     "jsx": "react"
   },
   "include": ["src/**/*.ts", "src/**/*.tsx"],
+  "exclude": ["**/*.test.ts"],
   "references": []
 }