Required Keyword in Dart

What is required?

The required keyword marks a named parameter as mandatory — the caller must provide it. Without required, named parameters are optional by default. With null safety, required is essential for non-nullable named parameters.

Syntax

void greet({required String name, required int age}) {
  print('Hello $name, you are $age years old!');
}

void main() {
  greet(name: 'Alice', age: 25);  // OK
  // greet(name: 'Bob');           ← COMPILE ERROR: age is required
}

required vs Optional Parameters

required in Classes

Use required in class constructors to enforce mandatory fields:

class User {
  final String name;
  final String email;
  final int age;

  User({
    required this.name,
    required this.email,
    required this.age,
  });

  @override
  String toString() => 'User($name, $email, age=$age)';
}

void main() {
  var user = User(name: 'Alice', email: 'alice@example.com', age: 28);
  print(user);
  // User(Alice, alice@example.com, age=28)
}

Example

class Product {
  final String id;
  final String name;
  final double price;
  final String? description;  // optional

  Product({
    required this.id,
    required this.name,
    required this.price,
    this.description,          // no required — defaults to null
  });
}

void main() {
  var p1 = Product(id: 'P001', name: 'Laptop', price: 999.99);
  var p2 = Product(
    id: 'P002',
    name: 'Phone',
    price: 499.99,
    description: 'Latest model smartphone',
  );
  print('$${p1.name}: \$$${p1.price}');
  print('$${p2.name}: $${p2.description}');
}

Laptop: $999.99
Phone: Latest model smartphone

🧠 Quiz

Summary

The required keyword enforces that named parameters must be provided by the caller. Omitting a required parameter is a compile-time error. Use it in functions and class constructors to make the API clear and safe, especially for non-nullable named parameters.