Dart functions allow positional parameters, named parameters, and optional positional and named parameters, or a combination of all of them. Positional parameters are the basic parameters, and they're required.

void debugger(String message, int lineNum) {
  // ...
}

To call that function, you must pass in a String and an int, in that order:

debugger('A bug!', 55);

Named parameters

Dart supports named parameters. Named means that when you call a function, you attach the argument to a label.

Named parameters are written a bit differently. When defining the function, wrap any named parameters in curly braces ({ }). This line defines a function with named parameters:

void debugger({String message, int lineNum}) {}

Calling that function would look like this:

debugger(message: 'A bug!', lineNum: 44);

Named parameters, by default, are optional. But, using the meta package, you can annotate them and make them required:

import 'package:meta/meta.dart'
Widget build({@required Widget child}) {
  //...
}

The pattern you see here will become familiar when we start writing Flutter apps. For now, don't worry too much about annotations.

Positional optional parameters

Finally, you can pass positional parameters that are optional, using [ ]:

int addSomeNums(int x, int y, [int z]) {
  int sum = x + y;
  if (z != null) {
    sum += z;
  }
  return sum;
}

You call that function like this:

addSomeNums(5, 4); // okay, because the third parameter is optional 
addSomeNums(5, 4, 3); // also okay

Default parameter values

You can define default values for parameters with the = operator in the function signature. Default parameter values tell the function to use the defined default only if nothing is passed in.

// function signature 
int addSomeNums(int x, int y, [int z = 5]) => x + y + z;
// calling that function without passing in the third argument
int sum = addSomeNums(5, 6);
assert(sum == 16); // 5 + 6 + 5
// calling that function with passing in the third argument
int sum2 = addSomeNums(5, 6, 10);
assert(sum2 == 21); // 5 + 6 + 10

Default parameter values only work with optional parameters.