In Dart, we have two types of optional parameters: named and positional.

• Dart’s optional parameters are optional in that the caller isn’t required to specify a value for the parameter when calling the function.
• Optional parameters can only be declared after any required parameters.
• Optional parameters can have a default value, which is used when a caller does not specify a value.

Optional parameters are parameters which don’t have to be specified when calling given function. Optional parameters must be declared after required parameters. Additionally, optional parameters can have a default value, which is used once the function invocation doesn’t specify it.

👉Dart Cheat Sheet and Quick Reference

Positional Parameters in Dart

Square brackets [] are used to specify optional, positional parameters in Dart.

void hamViDu(int a, [int b = 0, int c = 0]) {
  if (b != 0 && c != 0) {
    print('a:$a - b:$b - c:$c');
  } else if (b != 0) {
    print('a:$a - b:$b');
  } else {
    print('a:$a');
  }
}

void main() {
  hamViDu(5, 10, 15);

  print('----------------------');
  hamViDu(5);

  print('----------------------');
  hamViDu(5, 10);
}

For such declaration a is always required while b and c are optional. Also, b, c will default to 0 if not specified.

hamViDu(5, 10, 15); //print a:5 - b:10 - c:15

hamViDu(5); //print a:5

hamViDu(5, 10);//print a:5 - b:10

Additionally, the parameter names are specified and visible only at the level of the function declaration. The function caller must know which position corresponds to which parameter. This leads to a slightly less readable code.

Named parameters in Dart

Curly brackets {} are used to specify optional, named parameters in Dart.

readFile(String name, {String mode, String charset = 'utf-8'}) { 
//......
}

Named parameters are referenced by name, which means that they can be used during the function invocation in an order different from the function declaration.

void hamViDu({int a = 0, int b = 0, int c = 0}) {
  print('a:$a - b:$b - c:$c');
}

void main() {
  hamViDu();

  print('----------------------');
  hamViDu(a: 5, c: 15, b: 10);

  print('----------------------');
  hamViDu(c: 5);
  
  print('----------------------');
  hamViDu(b: 5, c: 10);
}

Checking if optional parameter was provided

Unfortunately, you cannot distinguish between the cases “an optional parameter was not provided” and “an optional parameter was provided with the default value”.

Note: You may use positional optional parameters or named optional parameters, but not both in the same function or method. The following is not allowed.

thisFunctionWontWork(String foo, [String positonal], {String named}) {
 // will not work! 
}