Understanding `is` and `as` in Dart — Type Checking & Casting Explained

In Dart, is and as are type operators used for type checking and type casting. Think of is as asking "Is this object of this type?" and as as saying "Treat this object as that type".

is — Type Checking

Use is when you want to check whether an object is a specific type. The result is a boolean (true or false).

// Example: is operator
void main() {
  var data = 'Hello Dart';

  print(data is String); // true
  print(data is int);    // false
}

There's also is! which checks the opposite (object is not of that type):

print(data is! String); // false

as — Casting

Use as to cast (convert) an object to a specific type. It's common to use is first to make sure the cast is safe.

// Example: as operator
void main() {
  dynamic data = 'Hello Dart';

  String text = data as String; // cast to String
  print(text.toUpperCase());    // HELLO DART
}

Warning: If you cast to the wrong type, Dart throws a TypeError.

// This will throw a TypeError:
dynamic number = 42;
String text = number as String; // runtime error

Combining is and as

To avoid runtime errors, check the type first with is, then cast:

void main() {
  dynamic data = 'Hello Dart';

  if (data is String) {
    // inside this block, Dart already treats `data` as String for type promotion
    print(data.length); // safe
    // you could still explicitly cast:
    String text = data as String;
    print(text.toUpperCase());
  }
}

Note: Dart performs type promotion in many cases, so once you check data is String inside the if, you often don't need an explicit as cast.

Quick Reference

Operator	Purpose	Result
is	Check if an object is of a type	true / false
is!	Check if an object is NOT of a type	true / false
as	Cast object to a type (may throw at runtime if wrong)	Typed object or TypeError

Practical Tips

• Prefer type-safe code (use strong typing) to minimize casts.
• Use is before as if the source might be multiple types.
• Take advantage of Dart's type promotion to reduce explicit casts.