sample.ts

sample.ts

type ResponseBase = {};
type SuccessResponse = ResponseBase & { data: any };
type ErrorResponse = ResponseBase & { error: string };
type Callback = (response: SuccessResponse | ErrorResponse) => void;

function call(callback: Callback) {
  callback(<SuccessResponse> { data: {} });
}

call(response => {
  if ('error' in response) {
    console.log(response.error);
  }
});