Hono RPC Type Safety

Patterns for type-safe client-server communication with Hono.

Server Setup

Export App Type

// src/index.ts
import { Hono } from 'hono'
import { zValidator } from '@hono/zod-validator'
import { z } from 'zod'

const app = new Hono()

// Define routes with validators for full type inference
const routes = app
  .get('/users', async (c) => {
    return c.json({ users: [{ id: '1', name: 'John' }] })
  })
  .post('/users',
    zValidator('json', z.object({
      email: z.string().email(),
      name: z.string()
    })),
    async (c) => {
      const data = c.req.valid('json')
      return c.json({ id: crypto.randomUUID(), ...data }, 201)
    }
  )
  .get('/users/:id', async (c) => {
    const id = c.req.param('id')
    return c.json({ id, name: 'John' })
  })

export default routes

// CRITICAL: Export type for client
export type AppType = typeof routes

Route Chaining for Type Inference

// Chain routes to preserve types
const userRoutes = new Hono()
  .get('/', async (c) => c.json({ users: [] }))
  .post('/',
    zValidator('json', createUserSchema),
    async (c) => c.json({ id: '1' }, 201)
  )

const postRoutes = new Hono()
  .get('/', async (c) => c.json({ posts: [] }))
  .post('/',
    zValidator('json', createPostSchema),
    async (c) => c.json({ id: '1' }, 201)
  )

// Compose and export
const app = new Hono()
  .route('/users', userRoutes)
  .route('/posts', postRoutes)

export type AppType = typeof app

Client Setup

Basic Client

// client.ts
import { hc } from 'hono/client'
import type { AppType } from './server'

// Create typed client
const client = hc<AppType>('http://localhost:8787')

// Type-safe requests
const res = await client.users.$get()
const data = await res.json() // Typed!

Client Factory

// lib/api-client.ts
import { hc } from 'hono/client'
import type { AppType } from '@/server'

export function createClient(baseUrl: string, token?: string) {
  return hc<AppType>(baseUrl, {
    headers: token ? { Authorization: `Bearer ${token}` } : undefined
  })
}

// Default instance
export const api = createClient(
  process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8787'
)

Request Patterns

GET Requests

// Simple GET
const res = await api.users.$get()
const { users } = await res.json()

// GET with query params
const res = await api.users.$get({
  query: {
    page: 1,
    limit: 20,
    search: 'john'
  }
})

// GET with path params
const res = await api.users[':id'].$get({
  param: { id: 'user-123' }
})

POST Requests

// POST with JSON body
const res = await api.users.$post({
  json: {
    email: 'user@example.com',
    name: 'New User'
  }
})

if (res.ok) {
  const user = await res.json()
  console.log(user.id) // Typed!
}

PUT/PATCH Requests

const res = await api.users[':id'].$put({
  param: { id: 'user-123' },
  json: {
    name: 'Updated Name'
  }
})

DELETE Requests

const res = await api.users[':id'].$delete({
  param: { id: 'user-123' }
})

if (res.status === 204) {
  console.log('Deleted successfully')
}

Type Utilities

InferRequestType / InferResponseType

import { hc, InferRequestType, InferResponseType } from 'hono/client'
import type { AppType } from './server'

// Get specific endpoint type
type CreateUserRequest = InferRequestType<typeof api.users.$post>['json']
// { email: string; name: string }

type UserResponse = InferResponseType<typeof api.users[':id'].$get>
// { id: string; name: string }

// Use in functions
async function createUser(data: CreateUserRequest): Promise<UserResponse> {
  const res = await api.users.$post({ json: data })
  return res.json()
}

URL Generation

// Generate URL without making request
const url = api.users[':id'].$url({
  param: { id: 'user-123' }
})
// 'http://localhost:8787/users/user-123'

// Useful for links, prefetching, etc.

Error Handling

Response Status Checking

async function getUser(id: string) {
  const res = await api.users[':id'].$get({ param: { id } })

  if (!res.ok) {
    if (res.status === 404) {
      throw new Error('User not found')
    }
    throw new Error(`API error: ${res.status}`)
  }

  return res.json()
}

Typed Error Responses

// Server defines error format
app.get('/users/:id', async (c) => {
  const user = await getUser(c.req.param('id'))

  if (!user) {
    return c.json({ error: 'User not found' }, 404)
  }

  return c.json(user)
})

// Client handles typed errors
const res = await api.users[':id'].$get({ param: { id } })

if (res.status === 404) {
  const { error } = await res.json()
  console.log(error) // 'User not found'
}

React Query Integration

Query Hooks

import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
import { api } from '@/lib/api-client'

// List query
export function useUsers(page = 1) {
  return useQuery({
    queryKey: ['users', page],
    queryFn: async () => {
      const res = await api.users.$get({ query: { page } })
      if (!res.ok) throw new Error('Failed to fetch users')
      return res.json()
    }
  })
}

// Single item query
export function useUser(id: string) {
  return useQuery({
    queryKey: ['users', id],
    queryFn: async () => {
      const res = await api.users[':id'].$get({ param: { id } })
      if (!res.ok) throw new Error('User not found')
      return res.json()
    },
    enabled: !!id
  })
}

Mutation Hooks

export function useCreateUser() {
  const queryClient = useQueryClient()

  return useMutation({
    mutationFn: async (data: { email: string; name: string }) => {
      const res = await api.users.$post({ json: data })
      if (!res.ok) throw new Error('Failed to create user')
      return res.json()
    },
    onSuccess: () => {
      queryClient.invalidateQueries({ queryKey: ['users'] })
    }
  })
}

export function useUpdateUser() {
  const queryClient = useQueryClient()

  return useMutation({
    mutationFn: async ({ id, data }: { id: string; data: { name: string } }) => {
      const res = await api.users[':id'].$put({ param: { id }, json: data })
      if (!res.ok) throw new Error('Failed to update user')
      return res.json()
    },
    onSuccess: (_, { id }) => {
      queryClient.invalidateQueries({ queryKey: ['users', id] })
    }
  })
}

Component Usage

function UserList() {
  const { data, isLoading, error } = useUsers()
  const createUser = useCreateUser()

  if (isLoading) return <div>Loading...</div>
  if (error) return <div>Error: {error.message}</div>

  return (
    <div>
      {data?.users.map(user => (
        <div key={user.id}>{user.name}</div>
      ))}

      <button
        onClick={() => createUser.mutate({
          email: 'new@example.com',
          name: 'New User'
        })}
      >
        Add User
      </button>
    </div>
  )
}

Configuration Requirements

TypeScript Config

Both server and client need strict mode:

{
  "compilerOptions": {
    "strict": true,
    "moduleResolution": "bundler"
  }
}

Status Codes for Type Discrimination

// Server: Explicit status codes enable type inference
app.post('/users', async (c) => {
  return c.json({ id: '1', name: 'User' }, 201) // 201 for created
})

app.get('/users/:id', async (c) => {
  const user = await getUser(id)
  if (!user) {
    return c.json({ error: 'Not found' }, 404) // 404 for not found
  }
  return c.json(user, 200) // 200 for success
})

Best Practices

1. Always export AppType from server
2. Chain routes for proper type inference
3. Use zValidator for automatic request type inference
4. Specify status codes for response type discrimination
5. Use strict: true in both server and client tsconfig
6. Create typed error handlers for consistent error responses
7. Wrap in React Query for caching and state management
8. Use InferRequestType/InferResponseType for complex types