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Validierungsregeln und Fehlermeldungen

Wenden Sie integrierte Validierungsregeln wie required, minLength und pattern in der rules-Prop an und rendern Sie das error-Objekt unter jedem Feld.

Validierungsregeln und Fehlermeldungen ist eine kostenlose React Native Academy-Lektion auf CoddyKit. Dies ist Lektion 2 von 4. Du kannst die komplette Lektion unten kostenlos lesen – dann übst du sie direkt im Browser mit einem integrierten Code-Editor und einem KI-Tutor rund um die Uhr. Sie ist Teil des React Native Academy-Lernpfads, und dein Fortschritt wird über Web und CoddyKit-App synchronisiert. Der React Native Academy-Kurs umfasst insgesamt 4 Lektionen.

Teile dieser Lektion wurden noch nicht übersetzt und werden auf Englisch angezeigt.

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.

Häufig gestellte Fragen

Ist die Lektion „Validierungsregeln und Fehlermeldungen“ kostenlos?

Ja — der vollständige Text von „Validierungsregeln und Fehlermeldungen“ ist hier im Web kostenlos zu lesen. Um sie interaktiv zu üben (integrierter Code-Editor und 24/7 KI-Tutor) und den Rest des React Native Academy-Kurses freizuschalten, upgrade auf CoddyKit PRO. Der React Native Academy-Kurs umfasst insgesamt 4 Lektionen.

Was lerne ich in „Validierungsregeln und Fehlermeldungen“?

Wenden Sie integrierte Validierungsregeln wie required, minLength und pattern in der rules-Prop an und rendern Sie das error-Objekt unter jedem Feld. Du übst React Native Academy mit praktischem Code, den du direkt im Browser ausführst, und ein 24/7 KI-Tutor beantwortet deine Fragen während du die Lektion bearbeitest.

Brauche ich Erfahrung, um React Native Academy zu starten?

Keine Vorkenntnisse erforderlich. React Native Academy auf CoddyKit ist für Anfänger bis fortgeschrittene Lernende strukturiert, sodass du hier starten oder von Anfang an beginnen und in deinem eigenen Tempo voranschreiten kannst. Dies ist Lektion 2 von 4.

Wie lange dauert die Lektion „Validierungsregeln und Fehlermeldungen“?

Die meisten CoddyKit-Lektionen dauern etwa 5–10 Minuten. Jede ist kompakt und interaktiv, sodass du stetig Fortschritte machst und genau dort weitermachst, wo du aufgehört hast – im Web und in der App.

Kann ich in dieser React Native Academy-Lektion Code schreiben und ausführen?

Ja. Jede React Native Academy-Lektion enthält einen integrierten Code-Editor, sodass du echten Code direkt in deinem Browser schreibst und ausführst und sofort KI-Feedback erhältst — ohne lokale Einrichtung erforderlich.

Alle Lektionen in diesem Kurs

  1. react-hook-form mit Controller einrichten
  2. Validierungsregeln und Fehlermeldungen
  3. Mehrstufige Formulare mit Formularstatus
  4. Formulare absenden und zurücksetzen
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