Async middleware wrapper in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this async middleware wrapper?

Consider this Express async middleware wrapper function. What will be the response if the wrapped async function throws an error?

Express

const asyncWrapper = fn => (req, res, next) => {
  Promise.resolve(fn(req, res, next)).catch(next);
};

app.get('/test', asyncWrapper(async (req, res) => {
  throw new Error('Oops');
}));

A The server crashes with an unhandled promise rejection error.

B The request hangs indefinitely with no response.

C The client receives a 200 OK response with an empty body.

D The error is passed to Express error handling middleware via next(), resulting in a 500 response.

💡 Hint

Think about how Promise.catch(next) works in Express middleware.

The asyncWrapper catches any error thrown in the async function and passes it to next(), which triggers Express's error handling middleware. This prevents the server from crashing and sends a 500 error response.

📝 Syntax

intermediate

Which option correctly defines an async middleware wrapper in Express?

Select the option that correctly implements an async middleware wrapper function for Express.

A const asyncWrapper = fn => async (req, res, next) => { await fn(req, res, next).catch(next); };

B const asyncWrapper = fn => (req, res, next) => { Promise.resolve(fn(req, res, next)).catch(next); };

C const asyncWrapper = fn => (req, res, next) => { try { fn(req, res, next); } catch (e) { next(e); } };

D const asyncWrapper = fn => (req, res, next) => { fn(req, res, next).catch(next); };

💡 Hint

Remember that async functions return promises and errors must be caught asynchronously.

Option B correctly wraps the async function call in Promise.resolve and catches errors to pass to next(). Option B misses Promise.resolve and may not catch non-promise errors. Option B misuses await with catch. Option B uses try/catch but does not handle async errors properly.

🔧 Debug

advanced

Why does this async middleware wrapper fail to catch errors?

Identify the problem in this async middleware wrapper code that causes errors thrown in the async function not to be caught.

Express

const asyncWrapper = fn => (req, res, next) => {
  try {
    fn(req, res, next);
  } catch (err) {
    next(err);
  }
};

A The try/catch block does not catch errors from the async function because it returns a promise and errors happen asynchronously.

B The next function is not called correctly inside the catch block.

C The wrapper should use setTimeout to catch errors asynchronously.

D The async function is not awaited, causing the wrapper to miss synchronous errors.

💡 Hint

Think about how errors in promises behave with try/catch.

Errors thrown inside async functions are wrapped in promises and happen asynchronously. The try/catch block only catches synchronous errors, so it misses async errors. The wrapper must catch promise rejections instead.

❓ state_output

advanced

What is the value of 'errorHandled' after this middleware runs?

Given this async middleware wrapper and route, what will be the value of the variable 'errorHandled' after a request triggers an error?

Express

let errorHandled = false;

const asyncWrapper = fn => (req, res, next) => {
  Promise.resolve(fn(req, res, next)).catch(err => {
    errorHandled = true;
    next(err);
  });
};

app.get('/fail', asyncWrapper(async (req, res) => {
  throw new Error('Fail');
}));

// Simulate a request to '/fail' and error handling middleware sets errorHandled to true

A true

B false

C undefined

D null

💡 Hint

The catch block sets errorHandled to true when an error occurs.

When the async function throws, the catch block runs, setting errorHandled to true before calling next with the error.

🧠 Conceptual

expert

Which statement best explains why async middleware wrappers are important in Express?

Choose the best explanation for why wrapping async middleware functions with an error-catching wrapper is necessary in Express applications.

A Async middleware wrappers improve performance by running middleware functions in parallel.

B Wrapping async middleware automatically converts them into synchronous functions, simplifying error handling.

C Express does not natively handle rejected promises in async middleware, so errors must be caught and passed to next() to avoid unhandled promise rejections and server crashes.

D Async middleware wrappers are only needed when using Express with TypeScript.

💡 Hint

Consider how Express handles errors from promises by default.

Express does not catch rejected promises from async middleware automatically. Without a wrapper, unhandled promise rejections can crash the server or cause silent failures. Wrapping async middleware ensures errors are forwarded to Express error handlers.