1type ErrorResponse = {
2  message: string;
3  statusCode: number;
4  cause: string;
5};
6
7type SuccessResponse = {
8  statusCode: number;
9  data: Record<string, unknown>;
10};
11
12// Union type to represent the possible responses
13type Result = ErrorResponse | SuccessResponse;

1const fetchedData = await fetchData("https://example.com/api");
2
3// Check for a property that only exists in one variant
4if ("message" in fetchedData) {
5  // TypeScript narrows fetchedData to ErrorResponse
6  return { error: fetchedData.message };
7}
8
9// TypeScript narrows fetchedData to SuccessResponse
10const response = fetchedData.data;

1type ErrorResponse = {
2  message: string;
3  statusCode: number;
4  cause: string;
5  response_type: "error";  // The discriminant
6};
7
8type SuccessResponse = {
9  message: string;
10  statusCode: number;
11  data: Record<string, unknown>;
12  response_type: "success";  // The discriminant
13};
14
15type Result = ErrorResponse | SuccessResponse;

1const fetchedData = await fetchData2("https://example.com/api");
2
3if (fetchedData.response_type === "error") {
4  // TypeScript knows fetchedData is ErrorResponse
5  return { error: fetchedData.message };
6}
7
8// TypeScript knows fetchedData is SuccessResponse
9const response = fetchedData.data;