TypeScript `getProperty` for safe index lookups in records

foo.ts

{
    // TS doesn't return the correct type for index lookups. It returns `T` when it might actually be `T
    // | undefined`. These helpers give us the correct type.
    // https://github.com/Microsoft/TypeScript/issues/13778

    type IndexLookup<Key extends string, V, K extends Key> = string extends Key
        ? Record<Key, V>[K] | undefined
        : Record<Key, V>[K];

    // $ExpectType string | undefined
    type Test1 = IndexLookup<string, string, string>;
    // $ExpectType string | undefined
    type Test2 = IndexLookup<string, string, 'foo'>;
    // $ExpectType string
    type Test3 = IndexLookup<'foo', string, 'foo'>;

    const getProperty = <Key extends string, V, K extends Key>(
        obj: Record<Key, V>,
        key: K,
    ): IndexLookup<Key, V, K> => obj[key] as any;

    {
        // If we don't know which keys exist, it should return undefined.
        declare const myRecord: Record<string, string>;
        // $ExpectType string | undefined
        const result = getProperty(myRecord, 'whatever');
    }

    {
        // If we do know which keys exist, it should not return undefined.
        declare const myRecord: Record<'foo', string>;
        // $ExpectType string
        const result = getProperty(myRecord, 'foo');
    }
}