read-file-with-result-type.ts

const readFile = async (file: string): Promise<string | Error> => {
  try {
    return await Deno.readTextFile(file);
  } catch (err) {
    return err;
  }
};

const main = async () => {
  const file = Deno.env.get("FILE");
  if (!file) {
    console.log("file name required.");
    return;
  }

  console.log(`reading ${file} ...`);

  const text = await readFile(file);
  if (text instanceof Error) {
    console.error(`failed to read ${file}: ${text}`);
    return;
  }

  console.log(`file content:\n${text}`);
};

main();