Async error handling in routes in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an async route handler in Express.

Express

app.get('/data', async (req, res) => { const data = await fetchData(); res.send(data); });

app.get('/async', [1] (req, res) => { const result = await getResult(); res.json(result); });

Drag options to blanks, or click blank then click option'

Afunction

Bawait

Casync

Dcallback

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add 'async' before the function causes syntax errors when using await.

Using 'await' outside an async function.

The route handler must be declared with 'async' to use await inside it.

2fill in blank

medium

Complete the code to catch errors in an async Express route using try-catch.

Express

app.get('/user', async (req, res) => {
  try {
    const user = await getUser(req.params.id);
    res.json(user);
  } catch ([1]) {
    res.status(500).send('Error fetching user');
  }
});

Drag options to blanks, or click blank then click option'

Aerror

Be

Cerr

Dexception

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the catch parameter empty causes syntax errors.

Using a variable name not declared in the catch block.

The catch block needs a variable to hold the error; 'e' is a common choice.

3fill in blank

hard

Fix the error in this async route by adding the missing error handler call.

Express

app.get('/items', async (req, res, next) => {
  const items = await fetchItems();
  res.json(items);
  [1]
});

Drag options to blanks, or click blank then click option'

Areturn

Bthrow new Error()

Cnext(error)

Dnext()

Attempts:

3 left

💡 Hint

Common Mistakes

Not calling next() causes the request to hang.

Throwing errors without try-catch in async routes causes unhandled promise rejections.

Calling next() without arguments passes control to the next middleware, useful for error handling.

4fill in blank

hard

Fill both blanks to create an async route that forwards errors to Express error middleware.

Express

app.get('/profile', async (req, res, [1]) => {
  try {
    const profile = await getProfile(req.user.id);
    res.json(profile);
  } catch ([2]) {
    next(error);
  }
});

Drag options to blanks, or click blank then click option'

Anext

Berr

Cerror

Dcallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter names for next or error causes runtime errors.

Not calling next(error) inside catch blocks prevents error middleware from running.

The third parameter is 'next' to forward errors; the catch block error variable is 'error' to pass to next().

5fill in blank

hard

Fill all three blanks to create a reusable async error wrapper for Express routes.

Express

const asyncHandler = (fn) => (req, res, [1]) => {
  Promise.resolve(fn(req, res, next)).catch([2]);
};

app.get('/orders', asyncHandler(async (req, res) => {
  const orders = await getOrders();
  res.json(orders);
}));

Drag options to blanks, or click blank then click option'

Anext

Berror

Derr

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names for next or error.

Not returning the Promise causes unhandled errors.

The wrapper uses 'next' as the third parameter and calls .catch(next) to forward errors.

Practice

1. What is the main reason to use try/catch blocks inside async route handlers in Express?

easy

A. To automatically retry failed requests

B. To catch errors from asynchronous code and pass them to Express error handlers

C. To make the route handler run faster

D. To prevent the server from starting

Solution

1. Step 1: Understand async errors in Express routes

   Async functions can throw errors that are not caught automatically by Express, so we need to catch them manually.

2. Step 2: Use try/catch to handle errors

   Wrapping async code in try/catch allows catching errors and passing them to Express with next(error).

3. Final Answer:

   To catch errors from asynchronous code and pass them to Express error handlers -> Option B

4. Quick Check:

   try/catch in async routes = catch async errors [OK]

Hint: Use try/catch to catch async errors and call next(error) [OK]

Common Mistakes:

• Ignoring errors in async functions
• Not calling next(error) inside catch
• Assuming Express catches async errors automatically

2. Which of the following is the correct syntax to pass an error to the next middleware in an async Express route?

easy

A. res.send(error)

B. throw error

C. return error

D. next(error)

Solution

1. Step 1: Identify how Express handles errors

   Express expects errors to be passed to the next middleware using next(error).

2. Step 2: Confirm correct syntax in async routes

   Inside a catch block, calling next(error) forwards the error to the error handler middleware.

3. Final Answer:

   next(error) -> Option D

4. Quick Check:

   Pass errors with next(error) [OK]

Hint: Use next(error) to forward errors in Express [OK]

Common Mistakes:

• Using throw instead of next(error) inside async catch
• Returning error instead of passing to next
• Sending error directly with res.send inside catch

3. Consider this Express route handler:

app.get('/data', async (req, res, next) => {
  try {
    const result = await fetchData();
    res.json(result);
  } catch (error) {
    next(error);
  }
});

What happens if fetchData() throws an error?

medium

A. The error is caught and passed to the next error handler middleware

B. The server crashes immediately

C. The request hangs without response

D. The error is ignored and response is sent anyway

Solution

1. Step 1: Analyze the try/catch block

   If fetchData() throws, the catch block runs and calls next(error).

2. Step 2: Understand Express error flow

   Calling next(error) sends the error to Express error handler middleware, preventing crashes or hanging.

3. Final Answer:

   The error is caught and passed to the next error handler middleware -> Option A

4. Quick Check:

   Errors in try/catch + next(error) = handled error [OK]

Hint: Errors in async try/catch go to next(error) middleware [OK]

Common Mistakes:

• Assuming server crashes on async error
• Thinking response is sent despite error
• Not using next(error) to handle errors

4. Identify the error in this async Express route handler:

app.get('/user', async (req, res, next) => {
  const user = await getUser();
  res.json(user);
});

What is the problem with this code regarding error handling?

medium

A. It sends response twice

B. It incorrectly uses next() inside the route

C. It lacks a try/catch block to handle errors from the async function

D. It uses synchronous code inside async function

Solution

1. Step 1: Check for error handling in async code

   The async function getUser() might throw errors, but no try/catch is used.

2. Step 2: Understand Express error propagation

   Without try/catch and calling next(error), errors will not be handled properly and may crash the server.

3. Final Answer:

   It lacks a try/catch block to handle errors from the async function -> Option C

4. Quick Check:

   Async routes need try/catch for error handling [OK]

Hint: Always wrap async code in try/catch to handle errors [OK]

Common Mistakes:

• Omitting try/catch in async route handlers
• Expecting Express to catch async errors automatically
• Not calling next(error) inside catch

5. You want to create a centralized error handler middleware in Express that catches all errors passed by next(error). Which of the following is the correct way to define this middleware?

hard

A. app.use((err, req, res, next) => { res.status(500).json({ message: err.message }); });

B. app.use((req, res, next) => { res.status(500).send('Error'); });

C. app.get('/error', (err, req, res) => { res.send(err); });

D. app.use((err, req, res) => { res.json({ error: err }); });

Solution

1. Step 1: Recall Express error handler signature

   Error handlers must have four parameters: (err, req, res, next).

2. Step 2: Check each option for correct signature and response

   app.use((err, req, res, next) => { res.status(500).json({ message: err.message }); }); uses all four parameters and sends JSON with error message and status 500, which is correct.

3. Final Answer:

   app.use((err, req, res, next) => { res.status(500).json({ message: err.message }); }); -> Option A

4. Quick Check:

   Error handler middleware = 4 params + send error response [OK]

Hint: Error middleware needs 4 params: err, req, res, next [OK]

Common Mistakes:

• Missing the 'next' parameter in error middleware
• Using app.get instead of app.use for error handler
• Not setting status code in error response