Custom error classes in Node.js

Introduction

Custom error classes help you create specific error types. This makes it easier to find and fix problems in your code.

When you want to clearly identify different error causes in your app.

When you need to add extra information to errors for better debugging.

When you want to handle certain errors differently in your code.

When building libraries or APIs to give users clear error messages.

When you want to keep your error handling organized and readable.

Syntax

Node.js

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

Always extend the built-in Error class to keep standard error behavior.

Set the name property to your custom error class name for clear error identification.

Examples

This creates a custom error for validation problems.

Node.js

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

This error class adds a code property to give more details.

Node.js

class DatabaseError extends Error {
  constructor(message, code) {
    super(message);
    this.name = 'DatabaseError';
    this.code = code; // extra info
  }
}

Shows how to throw and catch a custom error.

Node.js

try {
  throw new ValidationError('Invalid input');
} catch (err) {
  if (err instanceof ValidationError) {
    console.log('Caught a validation error:', err.message);
  }
}

Sample Program

This program defines a custom error class and uses it to check if input is a number. It doubles the number if valid, or throws a custom error if not. The error is caught and handled gracefully.

Node.js

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

function checkNumber(num) {
  if (typeof num !== 'number') {
    throw new CustomError('Not a number!');
  }
  return num * 2;
}

try {
  console.log(checkNumber(5));
  console.log(checkNumber('hello'));
} catch (err) {
  if (err instanceof CustomError) {
    console.log(`Custom error caught: ${err.message}`);
  } else {
    console.log('Unknown error:', err);
  }
}

Important Notes

Always call super(message) in the constructor to set the error message.

Use instanceof to check for your custom error type when catching errors.

Custom errors improve code clarity and debugging experience.

Summary

Custom error classes let you create specific error types for your app.

Extend the built-in Error class and set a custom name.

Use them to handle errors clearly and add extra info if needed.

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