Custom error classes in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when throwing a custom error in Express?

Consider this Express middleware that throws a custom error. What will be the HTTP status code sent to the client?

Express

class NotFoundError extends Error {
  constructor(message) {
    super(message);
    this.status = 404;
  }
}

app.use((req, res, next) => {
  throw new NotFoundError('Resource not found');
});

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

A The client receives status 500 with message 'Internal Server Error'

B The client receives status 404 with message 'Resource not found'

C The client receives status 404 with message 'Internal Server Error'

D The client receives status 500 with message 'Resource not found'

💡 Hint

Look at the error's status property and how the error handler uses it.

The custom error sets status to 404. The error handler uses err.status or defaults to 500. So the response status is 404 with the error message.

📝 Syntax

intermediate

Which option correctly defines a custom error class in Express?

Choose the option that correctly defines a custom error class named ValidationError that sets a status property to 400.

A

function ValidationError(message) {
  this.status = 400;
  Error.call(this, message);
}
ValidationError.prototype = Object.create(Error.prototype);

B

class ValidationError extends Error {
  constructor(message) {
    this.status = 400;
    super(message);
  }
}

C

class ValidationError extends Error {
  constructor(message) {
    super(message);
    this.status = 400;
  }
}

D

class ValidationError {
  constructor(message) {
    super(message);
    this.status = 400;
  }
}

💡 Hint

Remember to call super(message) before using 'this' in a class constructor.

Option C correctly calls super(message) first, then sets this.status. Option C calls this.status before super, which is invalid. Option C uses function syntax but misses proper Error initialization. Option C lacks 'extends Error'.

🔧 Debug

advanced

Why does this custom error not send the correct status code?

This Express error handler always sends status 500 even when a custom error with a status property is thrown. Why?

Express

class AuthError extends Error {
  constructor(message) {
    super(message);
    this.status = 401;
  }
}

app.use((req, res, next) => {
  next(new AuthError('Unauthorized'));
});

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

A If err.status is undefined, res.status(undefined) defaults to 500, but here err.status is 401, so this is not the cause.

B The error handler does not check if err.status exists, so if err.status is undefined it sends status 0, causing fallback to 500.

C The error handler should use res.status(err.status || 500) to handle cases where err.status is undefined.

D The error handler is missing the 'next' parameter, so Express does not recognize it as an error handler.

💡 Hint

Express requires error handlers to have four parameters to work properly.

Express identifies error handlers by the presence of four parameters (err, req, res, next). If the handler misses 'next', it is treated as a normal middleware and won't catch errors properly, causing fallback to default 500.

❓ state_output

advanced

What is the value of err.name for this custom error instance?

Given this custom error class and instance, what is the value of err.name?

Express

class DatabaseError extends Error {
  constructor(message) {
    super(message);
    this.status = 503;
  }
}

const err = new DatabaseError('DB connection failed');
console.log(err.name);

A "DatabaseError"

B "Error"

C "ServiceUnavailableError"

D "CustomError"

💡 Hint

The name property of an Error instance defaults to the class name.

When extending Error, the name property is automatically set to the class name, here 'DatabaseError'.

🧠 Conceptual

expert

Why use custom error classes in Express applications?

Which of the following is the best reason to use custom error classes in Express?

A To group related errors and easily set HTTP status codes and messages for consistent error handling.

B To avoid using try-catch blocks anywhere in the code by throwing only custom errors.

C To automatically log errors to a database without additional code.

D To replace Express's built-in error handler with a custom one that never sends 500 status.

💡 Hint

Think about how custom errors help organize error handling and responses.

Custom error classes help categorize errors, attach status codes, and provide clear messages, making error handling consistent and maintainable.