In Dart, named functions are functions that are defined with a specific name and can be called multiple times in a program. Named functions can have parameters, return values, and can be defined at different levels within the program (globally, inside classes, or inside other functions).

Here’s a breakdown of how to work with named functions in Dart, including defining functions with parameters, using optional parameters, and specifying default values.

1. Basic Named Function

A basic named function in Dart is declared using the void keyword if it doesn’t return a value or by specifying a return type like int, String, etc.

void greet() {
  print("Hello, welcome to Dart!");
}

void main() {
  greet();
}

Output:

Hello, welcome to Dart!

2. Function with Parameters

You can define a function with parameters so that it can accept input values.

void greetUser(String name) {
  print("Hello, $name!");
}

void main() {
  greetUser("Alice");
}

Output:

Hello, Alice!

3. Function with Return Value

If a function needs to return a value, specify the return type instead of void, and use the return keyword to send a value back.

int add(int a, int b) {
  return a + b;
}

void main() {
  int result = add(5, 10);
  print("The sum is: $result");
}

Output:

The sum is: 15

4. Optional Positional Parameters

You can make parameters optional by placing them inside square brackets []. If not provided, optional parameters have a null value by default.

void greetUser(String name, [String? title]) {
  if (title != null) {
    print("Hello, $title $name!");
  } else {
    print("Hello, $name!");
  }
}

void main() {
  greetUser("Alice", "Dr.");
  greetUser("Bob");
}

Output:

Hello, Dr. Alice!
Hello, Bob!

5. Named Parameters

Dart also supports named parameters, which are defined inside curly braces {}. Named parameters make the code more readable, as you specify the parameter name when calling the function.

void greetUser({required String name, String? title}) {
  if (title != null) {
    print("Hello, $title $name!");
  } else {
    print("Hello, $name!");
  }
}

void main() {
  greetUser(name: "Alice", title: "Dr.");
  greetUser(name: "Bob");
}

Output:

Hello, Dr. Alice!
Hello, Bob!

6. Default Values for Named Parameters

You can set default values for named parameters to ensure they have a specific value if not explicitly provided.

void greetUser({required String name, String title = "Mr./Ms."}) {
  print("Hello, $title $name!");
}

void main() {
  greetUser(name: "Alice", title: "Dr.");
  greetUser(name: "Bob");
}

Output:

Hello, Dr. Alice!
Hello, Mr./Ms. Bob!

7. Anonymous Functions with Named Function Assignment

Dart allows you to define anonymous functions and assign them to variables. These can be passed as parameters or stored for later use.

void executeFunction(Function action) {
  action();
}

void main() {
  var sayHello = () => print("Hello from anonymous function!");

  executeFunction(sayHello);
}

Output:

Hello from anonymous function!

Summary

• Basic named functions: Defined with a name, can take parameters, and can return values.
• Optional parameters: Defined within [], are optional and default to null.
• Named parameters: Defined with {}, require explicit names when calling.
• Default values: Provide default values for parameters within the function definition.

Dart’s function flexibility helps make code concise and readable, offering control over function parameters and behavior in various scenarios.