> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # Dart Type Alias 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. ## Syntax The modern Dart syntax (introduced in Dart 2.13) allows aliasing of any type, utilizing an assignment-style declaration. ```dart theme={"dark"} typedef AliasName = ExistingType; ``` ## 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. ## 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. ```dart theme={"dark"} // Fully bound type alias typedef StringMap = Map; // Partially bound generic type alias typedef JsonMap = Map; // Pass-through generic type alias typedef Matrix = List>; ``` **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. ```dart theme={"dark"} // 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. ```dart theme={"dark"} typedef Identifier = int; ``` ## 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: ```dart theme={"dark"} 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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