TypeScript Tagged Union Type

type SuccessResult<T> = {
  status: "OK";
  value: T;
};

type ErrorResult = {
  status: "ERROR";
  message?: string;
};

function isEven(x: number): SuccessResult<number> | ErrorResult {
  if (x % 2 === 0) {
    const result: SuccessResult<number> = {
      status: "OK",
      value: 2,
    };
    return result;
  } else {
    const result: ErrorResult = {
      status: "ERROR",
      message: "Not Even!",
    };
  }
}

const r = isEven(2);

if (r.status === "OK") {
  console.log(`That is even ${r.value} ${r.value * 2}`);
} else {
  const msg = r.message ? r.message : "No Message Provided!";

  console.log(`It is not even. Message ${msg}`);
}

Extended Example

type SuccessResult<T> = {
  status: "OK";

  value: T;
};

type ErrorResult = {
  status: "ERROR";

  message?: string;
};

function isEven(x: number): SuccessResult<number> | ErrorResult {
  if (x % 2 === 0) {
    const result: SuccessResult<number> = {
      status: "OK",

      value: 2,
    };

    return result;
  } else {
    const result: ErrorResult = {
      status: "ERROR",

      message: "Not Even!",
    };
  }
}

const r = isEven(2);

if (r.status === "OK") {
  console.log(`That is even ${r.value} ${r.value * 2}`);
} else {
  const msg = r.message ? r.message : "No Message Provided!";

  console.log(`It is not even. Message ${msg}`);
}

interface CountData {
  current: number;

  by: 1 | 3 | 5;
}

const c: CountData = { current: 99, by: 3 };

localStorage.setItem("count", JSON.stringify(c));

function getCountData(): SuccessResult<CountData> | ErrorResult {
  const stored = localStorage.getItem("count");

  if (stored) {
    let data: any;

    try {
      data = JSON.parse(stored);
    } catch {
      return {
        status: "ERROR",

        message: "Not valid JSON",
      };
    }

    if (typeof data.by === "number" && typeof data.current === "number") {
      return {
        status: "OK",

        value: data as CountData,
      };
    } else {
      return {
        status: "ERROR",

        message: "Data not in correct format ",
      };
    }
  } else {
    return {
      status: "ERROR",

      message: "No Data Stored",
    };
  }
}

const countDataResult = getCountData();

switch (countDataResult.status) {
  case "OK": {
    console.log(countDataResult.value);

    break;
  }

  case "ERROR": {
    console.log(countDataResult.message);
  }
}