Synchronous error handling in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an Express error handler middleware.

Express

app.use(function(err, req, res, [1]) {
  res.status(500).send('Something broke!');
});

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

Anext

Berror

Cresponse

Drequest

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the next parameter causes Express not to recognize the middleware as an error handler.

The error handler middleware in Express must have four parameters: err, req, res, and next. The next parameter is required even if not used.

2fill in blank

medium

Complete the code to throw a synchronous error inside a route handler.

Express

app.get('/', (req, res) => {
  throw new [1]('Oops!');
});

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

AException

BError

CString

DObject

Attempts:

3 left

💡 Hint

Common Mistakes

Throwing a string instead of an Error object prevents Express from handling the error properly.

To throw an error in JavaScript, use the Error constructor. This creates a proper error object that Express can catch.

3fill in blank

hard

Fix the error in the middleware to correctly pass the error to the next handler.

Express

app.use((req, res, next) => {
  const err = new Error('Not found');
  [1](err);
});

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

Anext

Bthrow

Cres.send

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using throw inside async middleware causes unhandled exceptions.

Calling res.send does not forward the error.

To pass an error to the next middleware in Express, call next(err). Throwing the error or returning it does not work here.

4fill in blank

hard

Fill both blanks to create an error handler that logs the error and sends a 500 status.

Express

app.use(function(err, req, res, [1]) {
  console.[2](err.message);
  res.status(500).send('Server error');
});

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

Anext

Blog

Cerror

Dwarn

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.log instead of console.error for errors.

Omitting the next parameter.

The error handler middleware must have next as the fourth parameter. To log errors, use console.error for clear error messages.

5fill in blank

hard

Fill all three blanks to create a route that catches synchronous errors and passes them to the error handler.

Express

app.get('/data', (req, res, [1]) => {
  try {
    throw new [2]('Failed to load data');
  } catch (err) {
    [3](err);
  }
});

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

Anext

BError

Dres

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to call next(err) inside the catch block.

Using res instead of next to forward errors.

The third parameter in route handlers is next. Use Error to create an error object. Call next(err) inside catch to forward the error.

Practice

1. In Express, how do you handle synchronous errors inside a route handler?

easy

A. Use setTimeout to catch errors asynchronously

B. Use async/await without try...catch

C. Ignore errors and let the server crash

D. Use a try...catch block and call next(error) inside catch

Solution

1. Step 1: Understand synchronous error handling

   Synchronous errors happen immediately and can be caught with try...catch blocks.

2. Step 2: Pass error to Express error handler

   Inside catch, call next(error) to forward the error to Express's error middleware.

3. Final Answer:

   Use a try...catch block and call next(error) inside catch -> Option D

4. Quick Check:

   try...catch + next(error) = A [OK]

Hint: Catch sync errors with try...catch and call next(error) [OK]

Common Mistakes:

• Not using try...catch for synchronous errors
• Calling next() without an error object
• Using async/await without try...catch for sync errors

2. Which of the following is the correct syntax to pass a synchronous error to the next middleware in Express?

easy

A. next(new Error('Something went wrong'))

B. throw new Error('Something went wrong')

C. res.send(new Error('Something went wrong'))

D. console.error('Something went wrong')

Solution

1. Step 1: Identify how to forward errors in Express

   Express expects errors to be passed to next() as an argument to trigger error middleware.

2. Step 2: Check syntax correctness

   Calling next(new Error('...')) correctly passes the error object to Express.

3. Final Answer:

   next(new Error('Something went wrong')) -> Option A

4. Quick Check:

   next(error) syntax = B [OK]

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

Common Mistakes:

• Throwing error without try...catch in sync code
• Sending error object in response instead of next()
• Logging error without forwarding it

3. What will be the output when this Express route is called?

app.get('/test', (req, res, next) => {
  try {
    throw new Error('Fail');
  } catch (err) {
    next(err);
  }
});

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

medium

A. The server crashes with uncaught exception

B. The client receives a 200 status with empty body

C. The client receives a 500 status with message 'Fail'

D. The client receives a 404 Not Found error

Solution

1. Step 1: Analyze error throwing and catching

   The route throws an error inside try, caught immediately, then passed to next(err).

2. Step 2: Understand error middleware response

   The error middleware sends a 500 status with the error message 'Fail' to the client.

3. Final Answer:

   The client receives a 500 status with message 'Fail' -> Option C

4. Quick Check:

   next(err) triggers error middleware = A [OK]

Hint: Errors passed to next() trigger error middleware response [OK]

Common Mistakes:

• Assuming error crashes server without middleware
• Expecting 200 status despite error
• Confusing error middleware with normal middleware

4. Identify the error in this Express route for synchronous error handling:

app.get('/error', (req, res, next) => {
  throw new Error('Oops');
  next();
});

medium

A. No error, code works fine

B. Error is thrown but not caught or passed to next()

C. Error middleware is missing

D. next() is called after throw, which is unreachable

Solution

1. Step 1: Check error throwing without try...catch

   The error is thrown synchronously but not caught, so Express cannot handle it properly.

2. Step 2: Understand consequence of uncaught error

   Without try...catch and calling next(err), the server may crash or not handle error gracefully.

3. Final Answer:

   Error is thrown but not caught or passed to next() -> Option B

4. Quick Check:

   Throw without catch + next(err) = D [OK]

Hint: Always catch sync errors and call next(error) [OK]

Common Mistakes:

• Throwing error without try...catch
• Calling next() after throw (unreachable code)
• Assuming Express auto-catches thrown errors

5. You want to handle synchronous errors in multiple routes without repeating try...catch blocks. Which Express feature helps you achieve this cleanly?

hard

A. Create a wrapper function that catches errors and calls next(err)

B. Use setTimeout to delay error throwing

C. Call res.end() immediately to avoid errors

D. Use global variables to store errors

Solution

1. Step 1: Recognize repetitive try...catch problem

   Wrapping every route in try...catch is repetitive and cluttered.

2. Step 2: Use a wrapper function to centralize error catching

   A wrapper function can run the route handler and catch sync errors, forwarding them with next(err).

3. Final Answer:

   Create a wrapper function that catches errors and calls next(err) -> Option A

4. Quick Check:

   Wrapper function for sync error handling = C [OK]

Hint: Use wrapper functions to catch and forward sync errors [OK]

Common Mistakes:

• Trying to delay errors with setTimeout
• Ignoring errors by ending response early
• Using global variables for error handling