Is it possible in Typescript to reference type variables within another type variable?

Currently, I am working with two generic types - Client<T> and MockClient<T>. Now, I want to introduce a third generic type called Mocked<C extends Client>. This new type should be a specialized version of MockClient that corresponds to a specific specialization of Client. For example:

type PersonClient = Client<Person>;

// the following two types should be equivalent
type MockPersonClient = Mocked<PersonClient>;
type MockPersonClient = MockClient<Person>;

In essence, I aim to define a type like:

type Mocked<C extends Client<T>> = MockClient<T>;

My question is whether it is feasible to achieve this in Typescript. If not, is there any existing proposal for such functionality (I have no clue what this concept might be named), or is what I am proposing ambiguous or inherently complex to implement within a type system?

typescriptgenerics

Answer №1

In my opinion, the concept you are aiming for is quite specific, but currently not supported in TypeScript (up to version 2.5). A possible solution that could help achieve this is through implementing type-level function application or utilizing the more general extended typeof operator. The inclusion of these features in TypeScript remains uncertain.

There are alternative approaches available. One method that I have utilized involves adding a redundant/phantom property of type T to your Client<T> type. If Client<T> is defined as a class, the implementation would look like:

class Client<T> {
   // phantom property
   _clientType: T = void 0 as any;

   // rest of class definition
} 
type Mocked<C extends Client<{}>> = MockClient<C['_clientType']>

While it needs to be a public property, you can designate it with an underscore to indicate internal use. At runtime, it will remain undefined and won't impact users of Client. However, if Client is an interface used to generate an instance from an object literal, there may be complications related to the absence of the _clientType property.

Ultimately, the decision to modify your class in this manner is up to you. Other individuals might offer different workarounds to consider. Best of luck!

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