React Hook Form with async validation

                  import { useForm } from 'react-hook-form'
import { useState } from 'react'
import api from '@/services/api'

interface SignupFormData {
  email: string
  username: string
  password: string
}

export function SignupForm() {
  const [validatingUsername, setValidatingUsername] = useState(false)

  const {
    register,
    handleSubmit,
    formState: { errors, isSubmitting },
  } = useForm<SignupFormData>()

  const checkUsernameAvailability = async (username: string) => {
    if (username.length < 3) {
      return 'Username must be at least 3 characters'
    }

    setValidatingUsername(true)

    try {
      const { data } = await api.get(`/auth/check-username?username=${username}`)

      if (!data.available) {
        return 'Username is already taken'
      }

      return true
    } catch (error) {
      return 'Unable to verify username'
    } finally {
      setValidatingUsername(false)
    }
  }

  const onSubmit = async (data: SignupFormData) => {
    try {
      await api.post('/auth/signup', data)
      window.location.href = '/dashboard'
    } catch (error: any) {
      console.error('Signup failed:', error)
    }
  }

  return (
    <form onSubmit={handleSubmit(onSubmit)} className="space-y-4">
      <div>
        <label htmlFor="email">Email</label>
        <input
          {...register('email', {
            required: 'Email is required',
            pattern: {
              value: /^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}$/i,
              message: 'Invalid email address',
            },
          })}
          type="email"
          id="email"
          className="w-full px-3 py-2 border rounded"
        />
        {errors.email && <p className="text-red-600 text-sm mt-1">{errors.email.message}</p>}
      </div>

      <div>
        <label htmlFor="username">Username</label>
        <div className="relative">
          <input
            {...register('username', {
              required: 'Username is required',
              validate: checkUsernameAvailability,
            })}
            id="username"
            className="w-full px-3 py-2 border rounded"
          />
          {validatingUsername && (
            <div className="absolute right-3 top-1/2 transform -translate-y-1/2">
              <div className="animate-spin rounded-full h-4 w-4 border-b-2 border-blue-600" />
            </div>
          )}
        </div>
        {errors.username && <p className="text-red-600 text-sm mt-1">{errors.username.message}</p>}
      </div>

      <div>
        <label htmlFor="password">Password</label>
        <input
          {...register('password', {
            required: 'Password is required',
            minLength: { value: 8, message: 'Password must be at least 8 characters' },
          })}
          type="password"
          id="password"
          className="w-full px-3 py-2 border rounded"
        />
        {errors.password && <p className="text-red-600 text-sm mt-1">{errors.password.message}</p>}
      </div>

      <button
        type="submit"
        disabled={isSubmitting || validatingUsername}
        className="w-full px-4 py-2 bg-blue-600 text-white rounded hover:bg-blue-700 disabled:opacity-50"
      >
        {isSubmitting ? 'Creating account...' : 'Sign Up'}
      </button>
    </form>
  )
}