Node.js Lesson 11: Async/Await & Error Handling

Async code is everywhere in Node.js — file reads, database queries, HTTP calls. Mastering async/await and proper error handling separates professional Node.js code from amateur code.

Async Evolution

// Old way: Callback Hell
readFile("a.txt", (err, a) => {
  readFile("b.txt", (err, b) => {
    writeFile("out.txt", a + b, (err) => {
      console.log("Done! (maybe)");
    });
  });
});

// Modern way: async/await
async function combineFiles() {
  const a = await readFile("a.txt", "utf8");
  const b = await readFile("b.txt", "utf8");
  await writeFile("out.txt", a + b);
  console.log("Done!");
}

Error Handling Patterns

// Pattern 1: try/catch
async function fetchUser(id) {
  try {
    const user = await db.findUser(id);
    return user;
  } catch (err) {
    console.error("Failed to fetch user:", err.message);
    throw err; // re-throw to caller
  }
}

// Pattern 2: Express async wrapper
const asyncHandler = (fn) => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next);

app.get("/users/:id", asyncHandler(async (req, res) => {
  const user = await db.findUser(req.params.id);
  if (!user) return res.status(404).json({ error: "Not found" });
  res.json(user);
}));

// Pattern 3: Global error handler
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(err.status || 500).json({
    error: err.message || "Internal Server Error"
  });
});