Function Parameters

Dart offers various types of function parameters to provide flexibility in function design and call structure. Here’s a detailed explanation of each type:

1. Positional Parameters

Positional parameters are defined by their position in the function declaration, meaning their order matters when the function is called. By default, positional parameters are mandatory unless we make them optional.

void describePerson(String name, int age) {
  print('$name is $age years old.');
}

describePerson('Alice', 30); // Correct usage
// describePerson(30, 'Alice'); // Error: Wrong order of arguments

2. Named Parameters

Named parameters allow us to specify arguments by name, making it clear which parameter is being set. Named parameters in Dart are defined using curly braces {} in the function signature. They’re optional by default unless marked as required.

void describePerson({String name, int age}) {
  print('$name is $age years old.');
}

// Calling with named parameters
describePerson(name: 'Alice', age: 30);
describePerson(age: 25, name: 'Bob'); // Order does not matter

3. Optional Parameters

Optional parameters can be either positional or named, and they aren’t required when calling the function. We can provide default values for them to ensure the function behaves predictably even when those arguments aren’t provided.

Optional Positional Parameters

To make positional parameters optional, wrap them in square brackets []:

void describePerson(String name, [int age = 18]) {
  print('$name is $age years old.');
}

describePerson('Alice');       // Prints: Alice is 18 years old.
describePerson('Bob', 30);     // Prints: Bob is 30 years old.

Optional Named Parameters

Named parameters are optional by default unless marked as required, so we can define default values directly:

void describePerson({String name = 'Anonymous', int age = 18}) {
  print('$name is $age years old.');
}

describePerson();                 // Prints: Anonymous is 18 years old.
describePerson(name: 'Alice');     // Prints: Alice is 18 years old.
describePerson(age: 30);           // Prints: Anonymous is 30 years old.

4. Required Parameters

To make a named parameter mandatory, use the required keyword. This enforces that the caller must provide a value for these parameters, helping avoid issues caused by missing values.

void describePerson({required String name, required int age}) {
  print('$name is $age years old.');
}

describePerson(name: 'Alice', age: 30); // Correct usage
// describePerson(name: 'Alice');       // Error: Missing required argument: 'age'

Summary of Syntax for Different Parameters

Parameter Type	Syntax	Example Call
Positional	(String name, int age)	describePerson('Alice', 30);
Named	({String name, int age})	describePerson(name: 'Alice');
Optional Positional	(String name, [int age = 18])	describePerson('Alice');
Optional Named	({String name = 'Anonymous', int age = 18})	describePerson(age: 30);
Required Named	({required String name, required int age})	describePerson(name: 'Alice', age: 30);

Dart’s parameter types make function calls more flexible and readable, improving code clarity and robustness.