Narrowing a return type from a generic, discriminated union in TypeScript

Question

I have a class method which accepts a single argument as a string and returns an object which has the matching type property. This method is used to narrow a di

Answer 1

A little more verbose on the setup but we can achieve your desired API with type lookups:

interface Action {
  type: string;
}

interface Actions {
  [key: string]: Action;
}

interface ExampleAction extends Action {
  type: 'Example';
  example: true;
}

interface AnotherAction extends Action {
  type: 'Another';
  another: true;
}

type MyActions = {
  Another: AnotherAction;
  Example: ExampleAction;
};

declare class Example {
  doSomething(key: K): T[K];
}

const items = new Example();

const result1 = items.doSomething('Example');

console.log(result1.example);