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

As of TypeScript 2.8, you can accomplish this via conditional types.

// Narrows a Union type base on N
// e.g. NarrowAction would produce ExampleAction
type NarrowAction = T extends { type: N } ? T : never;

interface Action {
    type: string;
}

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

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

type MyActions =
    | ExampleAction
    | AnotherAction;

declare class Example {
    doSomething(key: K): NarrowAction
}

const items = new Example();

// Inferred ExampleAction works
const result1 = items.doSomething('Example');

NOTE: Credit to @jcalz for the idea of the NarrowAction type from this answer https://stackoverflow.com/a/50125960/20489