Alternate solution to @ThePrimeagen's typescript problem in AOC day 2, 2022 stream

example.ts

interface FooEvent {
    ack: { ack: string };
    bash: { bash: string };
    bar: { bar: string };
    buzz: { buzz: string };
}

interface BarEvent {
    ack: { ack: string };
    bash: { bash: string };
    buzz: { buzz: string };
}

// as opposed to type guards, casting, or this shit: https://www.totaltypescript.com/tips/use-generics-to-dynamically-specify-the-number-and-type-of-arguments-to-functions
// when your types don't have a natural distriminator field imo the best option
// is to define a new type with a discriminator field and a field with your type
type TypeToEvent =
    | {
          type: "bar";
          event: BarEvent;
      }
    | {
          type: "foo";
          event: FooEvent;
      };

export function write({ type, event }: TypeToEvent) {
    if (type === "foo") {
        console.log(type, event.bar);
    }
}

write({
    type: "foo",
    event: {
        ack: { ack: "ack" },
        bash: { bash: "bash" },
        // bar: { bar: "bar" },
        buzz: { buzz: "buzz" },
    },
});

original.ts

interface FooEvent {
    ack: { ack: string };
    bash: { bash: string };
    bar: { bar: string };
    buzz: { buzz: string };
}

interface BarEvent {
    ack: { ack: string };
    bash: { bash: string };
    buzz: { buzz: string };
}

interface TypeToEvent {
    foo: FooEvent;
    bar: BarEvent;
}

export function write(
    type: keyof TypeToEvent,
    event: TypeToEvent[keyof TypeToEvent],
) {
    if (type === "foo") {
        console.log(type, event.bar);
    }
}

write("foo", {
    ack: { ack: "ack" },
    bash: { bash: "bash" },
    buzz: { buzz: "buzz" },
});