검증 규칙과 오류 메시지
rules prop에서 required, minLength, pattern과 같은 기본 제공 검증 규칙을 적용하고, 각 필드 아래에 오류 객체를 표시합니다.
검증 규칙과 오류 메시지은(는) CoddyKit의 무료 React Native Academy 강의입니다. 이것은 4개 중 2번째 강의입니다. 아래에서 전체 강의를 무료로 읽을 수 있으며, 내장 코드 에디터와 24/7 AI 튜터와 함께 브라우저에서 직접 실습할 수 있습니다. 이 강의는 React Native Academy 학습 경로의 일부이며, 진행 상황이 웹과 CoddyKit 앱에 동기화됩니다. React Native Academy 강의에는 총 4개의 강의가 포함되어 있습니다.
이 강의의 일부는 아직 번역되지 않았으며 영어로 표시됩니다.
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.
자주 묻는 질문
“검증 규칙과 오류 메시지” 강의는 무료인가요?
네 — “검증 규칙과 오류 메시지” 전체 내용을 이 웹사이트에서 무료로 읽을 수 있습니다. 인터랙티브하게 실습하려면(내장 코드 에디터와 24/7 AI 튜터), CoddyKit PRO로 업그레이드하면 React Native Academy 강의 전체를 잠금 해제할 수 있습니다. React Native Academy 강의에는 총 4개의 강의가 포함되어 있습니다.
“검증 규칙과 오류 메시지”에서 뭘 배우나요?
rules prop에서 required, minLength, pattern과 같은 기본 제공 검증 규칙을 적용하고, 각 필드 아래에 오류 객체를 표시합니다. 브라우저에서 직접 실행하는 실습 코드로 React Native Academy을(를) 배우며, 24/7 AI 튜터가 강의를 진행하면서 질문에 답변해줍니다.
React Native Academy을(를) 시작하는 데 경험이 필요한가요?
사전 경험은 필요하지 않습니다. CoddyKit의 React Native Academy은(는) 초급자부터 고급 학습자까지를 위해 구성되어 있으므로, 여기서 시작하거나 처음부터 시작할 수 있으며 자신의 속도대로 진행할 수 있습니다. 이것은 4개 중 2번째 강의입니다.
“검증 규칙과 오류 메시지” 강의는 얼마나 걸리나요?
대부분의 CoddyKit 강의는 약 5~10분이 소요됩니다. 각 강의는 간결하고 인터랙티브하여 꾸준한 진행이 가능하며, 웹과 앱에서 중단한 부분부터 바로 시작할 수 있습니다.
이 React Native Academy 강의에서 코드를 작성하고 실행할 수 있나요?
네. 모든 React Native Academy 강의에는 내장 코드 에디터가 포함되어 있으므로, 브라우저에서 바로 실제 코드를 작성하고 실행한 후 즉시 AI 피드백을 받을 수 있습니다 — 로컬 설정이 필요 없습니다.