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React Native Academy · Pelajaran

Aturan Validasi dan Pesan Kesalahan

Terapkan aturan validasi bawaan seperti required, minLength, dan pattern pada prop rules, lalu tampilkan objek error di bawah setiap bidang.

Aturan Validasi dan Pesan Kesalahan adalah pelajaran React Native Academy gratis di CoddyKit. Ini adalah pelajaran 2 dari 4. Kamu bisa membaca pelajaran lengkapnya di bawah secara gratis — lalu praktikkan langsung di browser dengan editor kode bawaan dan tutor AI 24/7. Ini adalah bagian dari jalur belajar React Native Academy, dan progresmu tersinkronisasi di web dan aplikasi CoddyKit. Kursus React Native Academy mencakup 4 pelajaran total.

Bagian dari pelajaran ini belum diterjemahkan dan ditampilkan dalam bahasa Inggris.

Built-In Validation Rules Overview

react-hook-form provides several built-in validation rules that cover the most common requirements without writing custom code. They are: required, minLength, maxLength, min (for numeric minimum), max (for numeric maximum), and pattern (regex match). Each rule can take a simple value or an object with value and message for a custom error message.

required Rule

The required rule ensures a field is not left empty. You can pass a boolean true (which uses a generic error message) or a string (which becomes the error message). For required fields in mobile forms, always pass a descriptive message so users immediately know what is missing without guessing.

// Simple boolean — uses generic message
rules={{ required: true }}

// String shorthand — message is the string
rules={{ required: 'Please enter your email' }}

// Full object form
rules={{ required: { value: true, message: 'Email is required' } }}

minLength and maxLength Rules

Use minLength to enforce a minimum number of characters and maxLength to cap input length. These are essential for username fields (minimum 3 characters), passwords (minimum 8 characters), and bio fields (maximum 200 characters). Both rules compare the string's length property against the value you specify.

rules={{
  required: 'Username is required',
  minLength: {
    value: 3,
    message: 'Username must be at least 3 characters',
  },
  maxLength: {
    value: 20,
    message: 'Username cannot exceed 20 characters',
  },
}}

pattern Rule for Regex Validation

The pattern rule tests the field value against a regular expression. Use it for email format, phone numbers, postal codes, or any structured text format. Pass the regex as the value property. React-hook-form tests the entire string against the pattern, so you typically do not need anchors.

// Email validation
rules={{
  pattern: {
    value: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/,
    message: 'Please enter a valid email address',
  },
}}

// Phone number (digits only, 10-15 chars)
rules={{
  pattern: {
    value: /^[0-9]{10,15}$/,
    message: 'Enter a valid phone number (digits only)',
  },
}}

min and max Rules for Numbers

For numeric inputs — quantities, ages, prices — use min and max rules. They compare the numeric value of the input string, not its length. Set keyboardType='numeric' on the TextInput to help users enter numbers. Combine with pattern to ensure only digits are entered.

rules={{
  required: 'Age is required',
  min: { value: 18, message: 'You must be at least 18 years old' },
  max: { value: 120, message: 'Please enter a valid age' },
  pattern: {
    value: /^[0-9]+$/,
    message: 'Age must be a whole number',
  },
}}

Custom Validation with validate

The validate rule accepts a function (or an object of named functions) that receives the current field value and returns true if valid or an error message string if invalid. Use it for complex requirements that built-in rules cannot express, such as confirming passwords match or checking whether a username is already taken via an async API call.

// Single custom validator
rules={{
  validate: (value) =>
    value.includes('@') || 'Email must contain an @ symbol',
}}

// Multiple named validators
rules={{
  validate: {
    notEmpty: (v) => v.trim().length > 0 || 'Cannot be blank',
    noSpaces: (v) => !v.includes(' ') || 'No spaces allowed',
  },
}}

Async validate for Server-Side Checks

The validate function can be async. This is useful for checking whether a username or email is already taken. The function receives the input value, makes an API call, and returns an error string if the value is already in use or true (or undefined) if it is available. The input is marked invalid until the async check resolves.

rules={{
  validate: async (email) => {
    try {
      const { available } = await api.checkEmailAvailability(email);
      return available || 'This email is already registered';
    } catch {
      return true; // Allow if check fails — server will catch it later
    }
  },
}}

Accessing Errors from formState

formState.errors is a nested object where each key is a field name and the value is an error object with a type (which rule failed) and a message (the error string). Access a field's error with errors.fieldName?.message — the optional chaining prevents runtime errors when the field has no error.

const { formState: { errors } } = useForm();

// Accessing errors:
console.log(errors.email?.type);    // 'required' | 'pattern' | 'validate'
console.log(errors.email?.message); // 'Email is required'

// Checking if any error exists:
const hasError = !!errors.email;

// Getting error from a named validate:
console.log(errors.username?.type); // 'noSpaces'

Styling Inputs Based on Error State

Provide immediate visual feedback by changing the input's border color when it has an error. Pass the error state as a conditional style using an array of styles. A red border on the invalid field plus a red error message text below creates a clear, standard pattern that users recognize from web forms.

const styles = StyleSheet.create({
  input: {
    borderWidth: 1,
    borderColor: '#ccc',
    borderRadius: 8,
    padding: 12,
  },
  inputError: {
    borderColor: '#d32f2f',
  },
  errorText: {
    color: '#d32f2f',
    fontSize: 12,
    marginTop: 4,
  },
});

<TextInput style={[styles.input, errors.email && styles.inputError]} />
{errors.email && <Text style={styles.errorText}>{errors.email.message}</Text>}

Controlling When Errors Appear

By default, errors only show after the first submit attempt. After that, react-hook-form re-validates in real time as the user fixes each field. You can change this with the mode option passed to useForm. The 'onBlur' mode shows errors as soon as a field loses focus, which gives earlier feedback without distracting red messages while the user is still typing.

const form = useForm<SignUpForm>({
  mode: 'onBlur',        // validate on field blur
  reValidateMode: 'onChange', // re-validate on every change after first error
  defaultValues: { name: '', email: '', password: '' },
});

Server-Side Errors with setError

After the form passes client-side validation and the API call fails with a server error — like 'Email already exists' — use setError to inject an error into a specific field. This re-triggers the error display for that field, giving the user precise feedback without re-running all validation rules from scratch.

const { setError, handleSubmit } = useForm<RegisterForm>();

async function onSubmit(data: RegisterForm) {
  try {
    await api.register(data);
    navigation.replace('Home');
  } catch (err: any) {
    if (err.code === 'EMAIL_EXISTS') {
      setError('email', {
        type: 'server',
        message: 'This email is already registered. Please log in.',
      });
    }
  }
}

Quick Check

Test your understanding of React Native Mobile Development concepts from this lesson.

Lesson Recap

In this lesson you learned: built-in rules (required, minLength, maxLength, pattern, min, max) cover most validation needs, validate handles complex and async custom checks, and setError injects server-side errors into specific fields after a failed API call. Next up we explore building multi-step forms with form state.

Pertanyaan yang Sering Diajukan

Apakah pelajaran “Aturan Validasi dan Pesan Kesalahan” gratis?

Ya — teks lengkap “Aturan Validasi dan Pesan Kesalahan” gratis dibaca di sini di web. Untuk praktiknya secara interaktif (editor kode bawaan dan tutor AI 24/7) dan buka sisa kursus React Native Academy, upgrade ke CoddyKit PRO. Kursus React Native Academy mencakup 4 pelajaran total.

Apa yang akan aku pelajari di “Aturan Validasi dan Pesan Kesalahan”?

Terapkan aturan validasi bawaan seperti required, minLength, dan pattern pada prop rules, lalu tampilkan objek error di bawah setiap bidang. Kamu berlatih React Native Academy dengan kode praktik yang langsung kamu jalankan di browser, dan tutor AI 24/7 menjawab pertanyaanmu saat kamu mengerjakan pelajaran ini.

Apakah aku perlu pengalaman untuk memulai React Native Academy?

Tidak diperlukan pengalaman sebelumnya. React Native Academy di CoddyKit dirancang untuk pemula hingga pelajar tingkat lanjut, jadi kamu bisa memulai di sini atau dari awal dan belajar sesuai kecepatan kamu sendiri. Ini adalah pelajaran 2 dari 4.

Berapa lama pelajaran “Aturan Validasi dan Pesan Kesalahan” memakan waktu?

Sebagian besar pelajaran CoddyKit memakan waktu sekitar 5–10 menit. Setiap pelajaran ringkas dan interaktif, jadi kamu membuat kemajuan stabil dan melanjutkan dari tempat kamu tinggalkan di web dan aplikasi.

Bisakah aku menulis dan menjalankan kode dalam pelajaran React Native Academy ini?

Ya. Setiap pelajaran React Native Academy menyertakan editor kode bawaan, jadi kamu menulis dan menjalankan kode nyata langsung di browser dan mendapatkan umpan balik AI instan — tidak diperlukan penyiapan lokal.

Semua pelajaran dalam kursus ini

  1. Menyiapkan react-hook-form dengan Controller
  2. Aturan Validasi dan Pesan Kesalahan
  3. Formulir Multi-Langkah dengan Status Formulir
  4. Mengirim dan Mengatur Ulang Formulir
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