How do you add synchronous and asynchronous field validation in TanStack Form?

Validators run on onChange, onBlur, or onSubmit, synchronously or asynchronously. Async validators support debounce to avoid API calls on every keystroke.

<script setup lang="ts">
import { useForm }       from "@tanstack/vue-form"
import { zodValidator } from "@tanstack/zod-form-adapter"
import { z }            from "zod"

const form=useForm({
  defaultValues:    { username:"", email:"" },
  validatorAdapter: zodValidator(),
  onSubmit: async ({ value })=>{ await register(value) },
})
</script>
<template>
  <form @submit.prevent="form.handleSubmit()">

    <!-- Sync Zod validation -->
    <form.Field name="username"
      :validators="{
        onChange: z.string().min(3,'Min 3 characters'),
        onBlur:   z.string().regex(/^[a-z0-9]+$/,'Lowercase/numbers only'),
      }"
    >
      <template #default="{ field }">
        <input :value="field.state.value" @blur="field.handleBlur"
               @input="field.handleChange(($event.target as HTMLInputElement).value)" />
        <p v-for="e in field.state.meta.errors" :key="e">{{e}}</p>
      </template>
    </form.Field>

    <!-- Async validation with debounce -->
    <form.Field name="email"
      :validators="{
        onChangeAsync: async ({ value })=>{
          const taken=await checkEmailTaken(value)
          return taken ? 'Email already registered' : undefined
        },
        onChangeAsyncDebounceMs: 500,
      }"
    >
      <template #default="{ field }">
        <input :value="field.state.value"
               @input="field.handleChange(($event.target as HTMLInputElement).value)" />
        <span v-if="field.state.meta.isValidating">Checking...</span>
        <p v-for="e in field.state.meta.errors" :key="e">{{e}}</p>
      </template>
    </form.Field>
    <button type="submit">Register</button>
  </form>
</template>

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