How do you add behavior to an existing type using an extension in Dart?

Short answer

An extension adds methods or getters to an existing type without modifying the original class or creating a subclass, and calls are resolved by the static type.

Explanation

Extensions are useful for grouping a small conversion or behavior close to the type it works on. They do not add state and do not actually change the class's interface, and since choosing an extension depends on the static type, do not use them to build polymorphism that needs runtime dispatch.

Code example

extension StringParsing on String {
  int? toSafeInt() => int.tryParse(this);
}

final count = "42".toSafeInt();

Common mistakes

• Expecting an extension to add fields or state to the object.
• Forgetting the effect of the static type when an extension with the same name exists.

Follow-up questions

• What is the difference between final and const in Dart?

Sources

• Dart language — Extension methods