Next.js: How to handle expected errors

For expected errors like validation issues, you can model the errors as return values to be used by your components:

'use server'
 
export async function createPost(prevState: any, formData: FormData) {
  const title = formData.get('title')
  const content = formData.get('content')
 
  const res = await fetch('https://api.vercel.app/posts', {
    method: 'POST',
    body: { title, content },
  })
  const json = await res.json()
 
  if (!res.ok) {
    return { message: 'Failed to create post' }
  }
}

'use client'
 
import { useActionState } from 'react'
import { createPost } from '@/app/actions'
 
const initialState = {
  message: '',
}
 
export function Form() {
  const [state, formAction, pending] = useActionState(createPost, initialState)
 
  return (
    <form action={formAction}>
      <label htmlFor="title">Title</label>
      <input type="text" id="title" name="title" required />
      <label htmlFor="content">Content</label>
      <textarea id="content" name="content" required />
      {state?.message && <p aria-live="polite">{state.message}</p>}
      <button disabled={pending}>Create Post</button>
    </form>
  )
}