The author shares their experience of first encountering the term “homomorphic” in the TypeScript handbook and finding the explanation unclear. They explain that homomorphic mapped types in TypeScript refer to mapped types that preserve the original type’s structure. The author dives into the technical details of how TypeScript handles homomorphic mapped types, including the function that helps determine if a mapped type is homomorphic and the function used to instantiate a mapped type. They discuss the special properties of homomorphic mapped types, such as their behavior with primitive types, union types, arrays, and tuples. The author also mentions that homomorphic mapped types can preserve modifiers and symlinks. They note that while using the “as” clause in a mapped type might affect certain features, it doesn’t disqualify it from being homomorphic. The author concludes by advising developers to strive for homomorphism when crafting mapped types.
https://andreasimonecosta.dev/posts/what-the-heck-is-a-homomorphic-mapped-type/