TypeScript: Function Overloading, Objects Using Bracket Notation and Recursion

Question

I have been breaking my head over this for a few hours. How can I type this correctly. The purpose of this function is to recursively flatten an object as demonstrated on this stackOverflow thread. Below I have attached a text copy as well as a screenshot of the code in question and the typescript error for your viewing pleasure. Thanks in advance!

    private flattenObject = (originalJson: object): object => {
        return Object.assign(
          {},
          ...(function flatten(obj: object): Array<object> {
            return [].concat(
              ...Object.keys(obj).map((key: string) =>
                typeof obj[key] === 'object'
                  ? flatten(obj[key])
                  : { [key]: obj[key] },
              ),
            );
          })(originalJson),
        );
      };

ps. I do realize a Json can either be an array or an object but ive tried both and any and still get errors. I'm open to any suggestions at this point.