select.ts

type propertyValuType<T, Key, KeyOriginal> = Key extends keyof T
	? KeyOriginal[] extends Key[]
		? T[Key]
		: never
	: never;

type PropertyValueType<T, Key> = propertyValuType<T, Key, Key>;

type isPropertyKey<T, Key, KeyOriginal> = Key extends keyof T
	? KeyOriginal[] extends Key[]
		? true
		: false
	: false;
type IsPropertyKey<T, Key> = isPropertyKey<T, Key, Key>;

type AreSameType<T, U> = T extends U ? (U extends T ? true : false) : false;

function select<T extends object, K extends keyof T, L extends keyof T>(
	obj: T,
	k: IsPropertyKey<T, K> extends true
		? AreSameType<T[K], T[L]> extends true
			? K
			: never
		: never,
	l: IsPropertyKey<T, L> extends true
		? AreSameType<T[K], T[L]> extends true
			? L
			: never
		: never
): [T[K], T[K]] {
	return [obj[k], obj[l]] as any;
}

interface EventPayloads {
	start: {
		user: string;
	};
	stop: {
		user: string;
		after: number;
	};
	end: {};
}

class EventEmitter<E> {
	emit<Ev extends keyof E>(eventName: Ev, payload: PropertyValueType<E, Ev>) {
		console.log(eventName, payload);
	}
}

interface FooFoo {
	id: number;
	name: string;
	hoge: string;
}

function exampleFooFoo(): number {
	const foo: FooFoo = { id: 1, name: "hello", hoge: "world" };
	const x: [string, string] = select(foo, "name", "hoge");
	console.log(x);
	return 1;
}
exampleFooFoo();