Validation Rules and Error Messages
Apply built-in validation rules like required, minLength, and pattern in the rules prop, and render the error object beneath each field.
Validation Rules and Error Messages is a free React Native Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the React Native Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
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.
Frequently asked questions
Is the “Validation Rules and Error Messages” lesson free?
Yes — the full text of “Validation Rules and Error Messages” is free to read here on the web, and the React Native Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the React Native Academy course, upgrade to CoddyKit PRO.
What will I learn in “Validation Rules and Error Messages”?
Apply built-in validation rules like required, minLength, and pattern in the rules prop, and render the error object beneath each field. You practise React Native Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start React Native Academy?
No prior experience is required. React Native Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Validation Rules and Error Messages” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this React Native Academy lesson?
Yes. Every React Native Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Setting Up react-hook-form with Controller
- Validation Rules and Error Messages
- Multi-Step Forms with Form State
- Submitting and Resetting Forms