Async error handling in routes in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

Why do we need special error handling for async routes in Express?

Because async functions return promises, errors inside them won't be caught by Express's default error handler unless we pass errors to next(). We need to handle errors explicitly to avoid unhandled promise rejections.

beginner

What does the 'next' function do in Express route handlers?

The 'next' function passes control to the next middleware or error handler. When called with an error, it triggers Express's error handling middleware.

intermediate

How can you catch errors in async route handlers without try/catch blocks everywhere?

You can create a wrapper function that catches errors from async functions and passes them to next(), so you don't repeat try/catch in every route.

beginner

Show a simple example of an async route handler with error forwarding using next().

app.get('/data', async (req, res, next) => {
  try {
    const data = await getData();
    res.json(data);
  } catch (err) {
    next(err); // Pass error to Express error handler
  }
});

intermediate

What is the benefit of using an async error handler wrapper function in Express?

It reduces repetitive try/catch blocks in each route, making code cleaner and easier to maintain by automatically forwarding errors to Express's error handler.

What happens if an error occurs inside an async route handler but you don't call next(err)?

A The error is ignored and the request hangs or crashes the server.

B The error is logged but the response continues normally.

C Express automatically catches the error and sends a response.

D The server restarts automatically.

Without calling next(err), Express does not catch async errors automatically, which can cause the request to hang or crash.

Which of these is a common pattern to handle async errors in Express routes?

A Returning errors directly from the async function.

B Ignoring errors and letting the server crash.

C Using synchronous code only.

D Using try/catch inside each async route and calling next(err) on error.

Using try/catch and calling next(err) ensures errors are passed to Express's error handler.

What does a wrapper function for async error handling do?

A It wraps async functions and automatically catches errors to pass to next().

B It converts async functions to synchronous ones.

C It logs errors but does not forward them.

D It disables error handling.

The wrapper catches errors from async functions and forwards them to Express error middleware.

How do you define an Express error handling middleware?

A A middleware with two parameters: (req, res).

B A middleware with four parameters: (err, req, res, next).

C A middleware with three parameters: (req, res, next).

D A middleware with one parameter: (err).

Error handling middleware in Express must have four parameters to catch errors.

Which of these is NOT a good practice for async error handling in Express?

A Using a centralized error handler middleware.

B Wrapping async routes to catch errors automatically.

C Ignoring errors inside async routes.

D Calling next(err) when catching errors.

Ignoring errors can cause unhandled rejections and unstable server behavior.

Explain how to handle errors in async Express routes and why it's important.

Think about how Express handles errors and what happens with promises.

Describe how to create a reusable wrapper function for async error handling in Express routes.

Focus on wrapping async functions to catch errors automatically.