Dart is a typed language, and it's often useful to assert that a value is of a correct type, or change the type of a value to a related type. Typecasting is quite a bit topic, but here we'll discuss the basics.

example "is" use:

// suppose your program has these three subclasses of User
class Customer extends User {...}
class Employee extends User {...}
class Boss extends User {...}
// And suppose there's a variable called 'user',
// and it could be any of the subclasses of 'User' 
// handle a customer
if (user is Customer) {
  // do something
}
// assert that the user is an employee or boss 
if (user is! Customer) {
  // do something
}

Intro to Typecasting

Typecasting can be useful in many ways, but as an intro to using as, let's suppose you have a class called User, and several subclasses of User. Each of these subclasses will have different properties from each other. You can use as to access these sub-class specific properties in your code.

class User {}
class Customer {
  String address;
}
class Employee {
  DateTime shiftStartTime;
}
// suppose you have a User object here,
// but you know in this function you'll always 
// be dealing with the "employee" sub-types
clockIn(User user) {
  // case the user object as it's subtype
  (user as Employee).shiftStartTime = now;
}
// NB: this is important because if you didn't use 'as'
// the Dart compiler doesn't know if the user is an Employee or Customer,
// and can't guarantee that the the `startShiftTime` property exists.
// example:
user.startShiftTime = now; // will this work? who knows!