Typedefs

A type alias—often called a typedef because it's declared with the keyword typedef—is a concise way to refer to a type. Here's an example of declaring and using a type alias named IntList:

typedef IntList = List<int>;
IntList il = [1, 2, 3];

A type alias can have type parameters:

typedef ListMapper<X> = Map<X, List<X>>;
Map<String, List<String>> m1 = {}; // Verbose.
ListMapper<String> m2 = {}; // Same thing but shorter and clearer.

We recommend using inline function types instead of typedefs for functions, in most situations. However, function typedefs can still be useful:

typedef Compare<T> = int Function(T a, T b);

int sort(int a, int b) => a - b;

void main() {
  assert(sort is Compare<int>); // True!
}

Unless stated otherwise, the documentation on this site reflects Dart 3.6.0. Page last updated on 2024-12-10. View source or report an issue.