Typescript: How to use a generic parameter as object key

You’re close. This falls under the category of Mapped Types. You need to make two changes:

1. QueryKey extends string
2. key in QueryKey

interface GraphQLResponse<QueryKey extends string, ResponseType> {
    data: {
        [key in QueryKey]: ResponseType;
    }
}

interface User {
    username: string;
    id: number;
}

type UserByIdResponse = GraphQLResponse<'userById', User>;
type UserByUsernameResponse = GraphQLResponse<'userByUsername', User>;

Example Usage

const userByIdResult: UserByIdResponse = {
    data: {
        userById: {
            id: 123,
            username: 'joseph'
        }
    }
}

const userByUsernameResult: UserByUsernameResponse = {
    data: {
        userByUsername: {
            id: 123,
            username: 'joseph'
        }
    }
}

const userByIdResultBoom: UserByIdResponse = {
    data: {
        userByUsername: {
            id: 123,
            username: 'joseph'
        }
    }
}