Dart · Optional Function Parameters

In Dart, there are two ways to specify optional parameters: they can be either positional or named.

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.

Positional Parameters in Dart

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

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

For such declaration name is always required while mode and charset are optional. Also, charset will default to utf-8 if not specified. Those parameters are positional, because you cannot omit mode if you want to specify the file charset.

readFile('hello.dart');
readFile('hello.dart', 'w+');
readFile('hello.dart', 'w+', 'iso8859-1');

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'}) {
  // empty
}

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.

readFile('hello.dart');
readFile('hello.dart', mode: 'w+');
readFile('hello.dart', charset: 'iso8859-1');
readFile('hello.dart', charset: 'iso8859-1', mode: 'w+');
readFile('hello.dart', mode: 'w+', charset: 'iso8859-1');