Conditional Type Checking based on function parameter - reactjs

Question

Im writing a custom Hook that can take either one or two strings, or an object with more granular parameters. I wanted to add a conditional type check to check for a type of that param and do some logic based on it. Here are the relevant snippets:
// The hook itself
const [error, loading, data] = useFirestore('posts', 'test'); // in a string version
const [error, loading, data] = useFirestore({...someProps}); // in a object version version
// The types that i defined for them
type queryType<T> = T extends string ? string : documentQueryType;
type docType<T> = T extends string ? string : never;
type documentQueryType = {
collection: string;
query: string[] | string[][];
limit: number;
orderBy: string; // todo limit this to be only special words
order: string; // todo same as above
startAt: number;
endAt: number;
};
// The function that is in the question
export const useFirestore = <T>(query: queryType<T>, doc?: docType<T>) => {...rest of the function
How would I make the last snippet work so when passed an object it sets doc to never, and when passed a string sets the doc to string?

Answer 1

This can be partially achieved with conditional types, but it may not be 100% type-safe. Since doc is optional, it will not be required when the query is a string, and will still allow undefined when query is an object.
However, if these two scenarios are not an issue, this can be achieved with conditional types:
// Simplified type
type DocumentQueryType = {
collection: string;
};
// The two types of queries that are accepted
type QueryTypes = string | DocumentQueryType;
// Given the query type, infer the doc type
type InferDocType<QueryType> = QueryType extends string ? string : never;
const useFirestore = <QueryType extends QueryTypes>(query: QueryType, doc?: InferDocType<QueryType>) => { }
// Valid Examples
useFirestore('posts', 'test');
useFirestore({ collection: "" });
// Valid Examples (may not want these?)
useFirestore('posts');
useFirestore({ collection: "" }, undefined);
// Invalid Examples
useFirestore({ collection: "" }, "test");
// Argument of type '"test"' is not assignable to parameter of type 'undefined'.(2345)
useFirestore('posts', null);
// Argument of type 'null' is not assignable to parameter of type 'string | undefined'.(2345)