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A type alias (declared using the typedef keyword) is a compile-time construct that provides an alternative identifier for an existing type. It does not introduce a new, distinct type into the Dart type system; instead, it acts as a symbolic reference that the compiler strictly resolves to the underlying type during static analysis.

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Syntax

The modern Dart syntax (introduced in Dart 2.13) allows aliasing of any type, utilizing an assignment-style declaration. 
typedef AliasName<TypeParameters> = ExistingType<TypeParameters>;

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Technical Characteristics

  • Type Equivalence: Because a type alias is structurally identical to its underlying type, they are completely interchangeable. Type checks (is) and runtime type evaluations (.runtimeType) will always evaluate to the original, underlying type.
  • Generic Parameterization: Type aliases can declare type parameters, allowing partial or complete application of generics to the underlying type.
  • Compile-Time Resolution: Aliases are stripped away during compilation. The Dart Virtual Machine (VM) and compiled JavaScript/machine code only possess knowledge of the underlying types.

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Structural Examples

1. Aliasing Generic Types You can alias complex generic structures. The alias can bind specific type arguments, pass them through, or partially apply them. 
// Fully bound type alias
typedef StringMap = Map<String, String>;

// Partially bound generic type alias
typedef JsonMap<T> = Map<String, T>;

// Pass-through generic type alias
typedef Matrix<T> = List<List<T>>;
2. Aliasing Function Types While Dart historically used a specific, C-style syntax for function aliases, modern Dart uses the standard assignment syntax for function signatures. 
// Modern function type alias
typedef StringTransformer = String Function(String input);

// Legacy function type alias (supported but discouraged)
typedef String LegacyTransformer(String input);
3. Aliasing Primitive Types Aliases can be applied directly to primitive or scalar types. 
typedef Identifier = int;

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Type Identity Demonstration

The following code illustrates that the Dart analyzer and runtime treat the alias and the base type as the exact same entity: 
typedef Score = double;

void main() {
  Score myScore = 95.5;
  double standardDouble = 95.5;

  // Evaluates to true. The runtime type is 'double', not 'Score'.
  print(myScore.runtimeType == double); 
  
  // Evaluates to true. They are structurally identical.
  print(myScore.runtimeType == standardDouble.runtimeType); 
  
  // Valid assignment: The compiler sees 'Score' as 'double'.
  standardDouble = myScore; 
}
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