TypeScript Generics and Parametric Polymorphism Explained Through an Olive Young Case Study

Subtype polymorphism alone has limitations when it comes to applying the same logic to types that are otherwise unrelated. For example, to create the same 'get first element' function for data models with nothing in common—such as products, reviews, and order history—you would have to define a separate function for each.

Generics are a tool that takes the type itself as a parameter, allowing the same logic to work across various types. Using generics prevents the loss of type information that occurs when using any, and lets the original type's properties remain intact—and protected by type checking—even after passing through a function.

In practice, generics really show their value when defining API response structures. If only the actual data (data) portion is treated as a type variable within a common structure that includes things like status codes and messages, countless API response types can be unified and managed under a single interface, maximizing maintainability.