Custom error classes in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom error class in Node.js?

A custom error class is a user-defined class that extends the built-in Error class to create specific error types with custom messages or properties.

beginner

Why should you extend the built-in Error class when creating custom errors?

Extending the built-in Error class ensures your custom error behaves like a standard error, including stack traces and compatibility with error handling mechanisms.

beginner

How do you set a custom error message in a custom error class?

You pass the message to the super() call inside the constructor of your custom error class, which sets the message property on the error instance.

intermediate

What is the purpose of setting the name property in a custom error class?

Setting the name property helps identify the error type when logging or handling errors, making it easier to distinguish between different custom errors.

beginner

Show a simple example of a custom error class in Node.js.

class ValidationError extends Error {
  constructor(message) {
    super(message);
    this.name = 'ValidationError';
  }
}

// Usage:
throw new ValidationError('Invalid input');

What must a custom error class in Node.js extend to behave like a standard error?

A Error

B Object

C Function

D Array

Custom error classes should extend the built-in Error class to inherit standard error behavior.

Which method is used to pass the error message in a custom error class constructor?

A super(message)

B this.message = message

C Error(message)

D setMessage(message)

The super(message) call passes the message to the base Error class constructor.

Why is setting the 'name' property in a custom error class useful?

A To change the error message

B To improve performance

C To fix syntax errors

D To identify the error type

The 'name' property helps identify the error type when handling or logging errors.

What will happen if you throw a custom error without extending Error?

A It behaves exactly like a standard error

B It won't have a stack trace

C It will crash Node.js immediately

D It will automatically extend Error

Without extending Error, the custom error won't have a proper stack trace or behave like a standard error.

Which of these is a correct way to throw a custom error?

A throw Error('ValidationError')

B throw ValidationError('Invalid input')

C throw new ValidationError('Invalid input')

D throw new Error('ValidationError')

You must create an instance of the custom error class using 'new' before throwing it.

Explain how to create and use a custom error class in Node.js.

Think about inheritance and how errors carry messages and names.

Why is it important to extend the built-in Error class when making custom errors?

Consider what makes errors useful when debugging and handling.

Practice

1. What is the main reason to create a custom error class in Node.js?

easy

A. To define specific error types for clearer error handling

B. To make the program run faster

C. To avoid using try-catch blocks

D. To replace the built-in Error class completely

Solution

1. Step 1: Understand the purpose of custom errors

   Custom error classes help identify and handle specific error cases clearly in your code.

2. Step 2: Compare with other options

   Custom errors do not speed up code, avoid try-catch, or replace Error class but extend it.

3. Final Answer:

   To define specific error types for clearer error handling -> Option A

4. Quick Check:

   Custom error purpose = Specific error types [OK]

Hint: Custom errors clarify error types, not speed or structure [OK]

Common Mistakes:

• Thinking custom errors improve performance
• Believing custom errors remove need for try-catch
• Assuming custom errors replace built-in Error

2. Which of the following is the correct way to define a custom error class named MyError in Node.js?

easy

A. class MyError extends Object { constructor(message) { super(message); this.name = 'MyError'; } }

B. function MyError() { this.message = 'Error'; this.name = 'MyError'; }

C. class MyError { constructor(message) { this.message = message; } }

D. class MyError extends Error { constructor(message) { super(message); this.name = 'MyError'; } }

Solution

1. Step 1: Check class inheritance

   Custom errors must extend the built-in Error class to behave like errors.

2. Step 2: Verify constructor and name setting

   The constructor calls super(message) and sets this.name to the class name for clarity.

3. Final Answer:

   class MyError extends Error { constructor(message) { super(message); this.name = 'MyError'; } } -> Option D

4. Quick Check:

   Extend Error and set name = MyError [OK]

Hint: Extend Error and set this.name in constructor [OK]

Common Mistakes:

• Not extending Error class
• Forgetting to call super(message)
• Not setting the error name property

3. What will be the output of this code snippet?

class NotFoundError extends Error {
  constructor(message) {
    super(message);
    this.name = 'NotFoundError';
  }
}

try {
  throw new NotFoundError('Item not found');
} catch (e) {
  console.log(e.name + ': ' + e.message);
}

medium

A. Error: Item not found

B. NotFoundError: Item not found

C. TypeError: Item not found

D. undefined: Item not found

Solution

1. Step 1: Understand custom error name

   The custom error sets this.name = 'NotFoundError', so e.name is 'NotFoundError'.

2. Step 2: Check the output format

   The console.log prints e.name + ': ' + e.message, which becomes 'NotFoundError: Item not found'.

3. Final Answer:

   NotFoundError: Item not found -> Option B

4. Quick Check:

   Custom error name shows in output [OK]

Hint: Custom error name property appears in error output [OK]

Common Mistakes:

• Assuming default 'Error' name instead of custom
• Confusing error type with TypeError
• Missing the custom name property effect

4. Identify the error in this custom error class definition:

class ValidationError extends Error {
  constructor(msg) {
    this.message = msg;
    this.name = 'ValidationError';
  }
}

medium

A. Missing call to super() in constructor

B. Incorrect class name

C. Should extend Object instead of Error

D. No error, code is correct

Solution

1. Step 1: Check constructor for super call

   When extending Error, constructor must call super() before using this.

2. Step 2: Identify missing super call

   The code sets this.message and this.name without calling super(msg), causing a runtime error.

3. Final Answer:

   Missing call to super() in constructor -> Option A

4. Quick Check:

   Always call super() first in subclass constructor [OK]

Hint: Always call super() before using this in constructor [OK]

Common Mistakes:

• Forgetting super() call causes ReferenceError
• Setting this.message before super()
• Extending wrong base class

5. You want to create a custom error class AuthError that includes a statusCode property for HTTP status codes. Which implementation correctly adds this property and preserves the error behavior?

hard

A. class AuthError extends Error {
constructor(statusCode) {
super();
this.name = 'AuthError';
this.statusCode = statusCode;
}
}

B. class AuthError extends Error {
constructor(message, statusCode) {
this.message = message;
this.statusCode = statusCode;
this.name = 'AuthError';
}
}

C. class AuthError extends Error {
constructor(message, statusCode) {
super(message);
this.name = 'AuthError';
this.statusCode = statusCode;
}
}

D. class AuthError {
constructor(message, statusCode) {
this.message = message;
this.statusCode = statusCode;
this.name = 'AuthError';
}
}

Solution

1. Step 1: Check proper Error extension and constructor

   The class must extend Error and call super(message) to set the error message correctly.

2. Step 2: Verify additional property and name

   Setting this.statusCode after super() adds the extra info; setting this.name clarifies error type.

3. Final Answer:

   class AuthError extends Error { constructor(message, statusCode) { super(message); this.name = 'AuthError'; this.statusCode = statusCode; } } -> Option C

4. Quick Check:

   Extend Error, call super(message), add extra properties [OK]

Hint: Call super(message) first, then add extra properties [OK]

Common Mistakes:

• Not calling super(message) in constructor
• Not extending Error class
• Missing message parameter in super call