Optional Positional Parameters

• Wrapping a set of function parameters in [] marks them as optional positional parameters. If you don't provide a default value, their types must be nullable as their default value will be null:

•     String say(String from, String msg, [String? device]) {
        var result = '$from says $msg';
        if (device != null) {
          result = '$result with a $device';
        }
        return result;
      }
  

Default Optional Positional Parameters

• To define a default value for an optional positional parameter besides null, use = to specify a default value. The specified value must be a compile-time constant.

•   String say(String from, String msg, [String device = 'carrier pigeon']) {
      var result = '$from says $msg with a $device';
      return result;
    }
  
    assert(say('Bob', 'Howdy') == 'Bob says Howdy with a carrier pigeon');