Optional parameters

Learn optional and named parameters.

Optional parameters

Dart offers two ways to make a parameter optional. Positional: wrap in square brackets, void greet(String name, [String? title]) { ... }. The caller may omit title.

Named: wrap in curly braces, void greet({required String name, String? title}) { ... }. The caller says which is which: greet(name: 'Ayşe');.

For named parameters, required means mandatory, otherwise optional. You can also give a default: String title = 'Mr.'.

Default values

You can give an optional parameter a default. Positional: void greet(String name, [String title = 'Mr.']) { ... }. If the caller omits title, 'Mr.' is used.

The same holds for named: {String title = 'Mr.'}. With a default the parameter cannot be null; therefore no ? is added.

The default must be a constant literal; it cannot be a value computed at runtime. This requires values known at compile time.

Exercise Greet by name

Define a greet function with the named parameter {required String name} that does print('Hello, $name'). Add a greet(name: 'Ayşe') call inside main.

Show example solution

void greet({required String name}) {
  print('Hello, $name');
}

void main() {
  greet(name: 'Ayşe');
}

Test yourself

Match the parameter types

[int? b]

Optional positional
{int b}

Optional named
{required int b}

Required named
int b

Required positional
How is a named parameter called?
- greet('Ayşe')
- greet(name: 'Ayşe')
- greet(name = 'Ayşe')
- greet(name -> 'Ayşe')
Show answer

greet(name: 'Ayşe') — named parameters are called as name: value.

Named parameter definition

void greet({____}) { print(name); }

Show answer

void greet({required String name}) { print(name); }

Next lesson: First-class functions

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