What is guard in dart , give some example ?

Overview

Dart (unlike Swift) does not have a built-in

guard

Guard Pattern in Dart (Early Return)

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// Swift's guard:
// guard let user = getUser() else { return }

// Dart equivalent — early return on invalid condition
void processUser(User? user) {
  // Guard: exit early if user is null
  if (user == null) return;

  // Guard: exit early if not active
  if (!user.isActive) {
    print('User is not active');
    return;
  }

  // Guard: exit early if missing email
  if (user.email.isEmpty) {
    throw ArgumentError('Email is required');
  }

  // Happy path — no nesting needed
  print('Processing user: ${user.name}');
  sendWelcomeEmail(user.email);
}

Null-aware Guard with ?? and !

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void fetchData(String? url) {
  // Guard with null coalescing
  final safeUrl = url ?? 'https://default.api.com

  // Guard with assertion (throws if null)
  final token = getToken()!; // Crashes if null — use carefully

  // Better: null check guard
  final token2 = getToken();
  if (token2 == null) {
    print('No token — user not logged in');
    return;
  }
  // token2 is String (non-null) here
}

Guard with Pattern Matching (Dart 3)

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// switch with guard clause using 'when'
String describeAge(int age) {
  return switch (age) {
    < 0 => 'Invalid age',
    < 13 => 'Child',
    int n when n < 18 => 'Teenager ($n)',  // Guard with 'when'
    int n when n < 65 => 'Adult ($n)',
    _ => 'Senior',
  };
}

Guard in Form Validation

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String? validateEmail(String? value) {
  // Guard chain
  if (value == null || value.isEmpty) return 'Email is required';
  if (!value.contains('@')) return 'Invalid email format';
  if (!value.contains('.')) return 'Invalid domain';
  return null; // Valid — no guard triggered
}

Future<void> loginUser(String email, String password) async {
  // Guard: validate before proceeding
  final emailError = validateEmail(email);
  if (emailError != null) throw ArgumentError(emailError);

  if (password.length < 8) throw ArgumentError('Password too short');

  // Happy path
  await authService.login(email, password);
}

Extension for Guard-like Syntax

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extension GuardExtension<T> on T? {
  T guard(String errorMessage) {
    if (this == null) throw StateError(errorMessage);
    return this!;
  }
}

// Usage
final user = getUser().guard('User not found');
final token = cache['token'].guard('Token missing');

Summary: Dart doesn't have

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guard

keyword, but the same pattern is achieved with early

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return

/

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throw

on invalid conditions. Always put guard conditions at the top of a function — this avoids deep nesting and keeps the "happy path" flat.