TypeScript mapped type change value function's return type

Question

I have a TypeScript type like this:

type Api = {
  createItem: (name: string) => Promise<Item>;
  getItem: (id: number) => Promise<Item>;
  updateItem: (item: Item) => Promise<Item>;
  deleteItem: (id: number) => Promise<void>;
};

And I'm trying to figure out how to convert it to the following:

type Api = {
  createItem: (name: string) => { type: 'success'; result: Item; } | { type: 'error' };
  getItem: (id: number) => { type: 'success'; result: Item; } | { type: 'error' };
  updateItem: (item: Item) => { type: 'success'; result: Item; } | { type: 'error' };
  deleteItem: (id: number) => { type: 'success' } | { type: 'error' };
};

I think mapping types are a way to go here:

type NewApi = {
  [_ in keyof Api]: (???) => { type: 'success'; result: ??? } | { type: 'error' }
};

I'm unable to figure out how to fill in the blanks there though. I'm not even sure it is possible to do that.

The reason I need this type is that I am building a gateway proxy and the actual implementation of the type will be an actual Proxy object intercepting the member accesses, mocking their values with another proxy which traps function invocation for a change and returns the above union type.

The implementation part is fine, but I can't seem to be able to get the type for it to get Intellisense working.

Answer 1

You can make use of typescripts Parameters and ReturnType types

type NewApi = {
  [K in keyof Api]: (...args: Parameters<Api[K]>) => { type: 'success'; result: ReturnType<Api[K]> } | { type: 'error' }
};

Please see this playground .