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React Native Academy · Aula

Regras de validação e mensagens de erro

Aplique regras de validação integradas, como required, minLength e pattern, na propriedade rules e renderize o objeto de erro abaixo de cada campo.

Regras de validação e mensagens de erro é uma aula grátis de React Native Academy no CoddyKit. Esta é a aula 2 de 4. Você pode ler a aula completa abaixo gratuitamente — depois pratica ao vivo no navegador com um editor de código integrado e um tutor de IA 24/7. Faz parte do caminho de aprendizado de React Native Academy, e seu progresso é sincronizado entre a web e o app CoddyKit. O curso de React Native Academy inclui 4 aulas no total.

Partes desta aula ainda não foram traduzidas e aparecem em inglês.

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.

Perguntas Frequentes

A aula “Regras de validação e mensagens de erro” é grátis?

Sim — o texto completo de “Regras de validação e mensagens de erro” é grátis para ler aqui na web. Para praticá-la interativamente (um editor de código integrado e um tutor de IA 24/7) e desbloquear o restante do curso de React Native Academy, atualize para CoddyKit PRO. O curso de React Native Academy inclui 4 aulas no total.

O que vou aprender em “Regras de validação e mensagens de erro”?

Aplique regras de validação integradas, como required, minLength e pattern, na propriedade rules e renderize o objeto de erro abaixo de cada campo. Você pratica React Native Academy com código prático que executa diretamente no navegador, e um tutor de IA 24/7 responde suas dúvidas enquanto trabalha na aula.

Preciso ter experiência prévia para começar React Native Academy?

Nenhuma experiência prévia é necessária. React Native Academy no CoddyKit é estruturado para alunos iniciantes até avançados, então você pode começar aqui ou desde o início e aprender no seu ritmo. Esta é a aula 2 de 4.

Quanto tempo leva a aula “Regras de validação e mensagens de erro”?

A maioria das aulas CoddyKit leva cerca de 5–10 minutos. Cada uma é compacta e interativa, então você faz progresso constante e retoma exatamente de onde parou entre web e app.

Posso escrever e executar código nesta aula de React Native Academy?

Sim. Cada aula de React Native Academy inclui um editor de código integrado, então você escreve e executa código real direto no navegador e recebe feedback de IA instantaneamente — nenhuma configuração local necessária.

Todas as aulas deste curso

  1. Configurando react-hook-form com Controller
  2. Regras de validação e mensagens de erro
  3. Formulários em várias etapas com estado do formulário
  4. Enviando e redefinindo formulários
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