Debugging & Error Handling

Node.js Fundamentals

Course 1 · Chapter 7 · Debugging & Error Handling

🐛 Debugging & Error Handling

Errors are inevitable in programming. This chapter teaches you to identify, handle, and debug errors effectively. We'll cover error types, try/catch patterns, logging strategies, debugging tools, and best practices for robust error handling.

📋 Error Types

JavaScript has built-in error types:

Error

Base error class. Generic errors.

TypeError

Wrong type (e.g., calling non-function).

ReferenceError

Undefined variable or function.

SyntaxError

Invalid syntax (caught at parse time).

RangeError

Value out of valid range.

Custom Errors

Create your own error classes.

📝 Console Debugging

// Basic logging
console.log('Info message');
console.warn('Warning');
console.error('Error message');

// Debug levels
console.debug('Debug info');
console.info('Info');

// Structured output
console.table(data);
console.dir(obj);

// Timing
console.time('myTimer');
// ... code ...
console.timeEnd('myTimer');

// Assertions
console.assert(value > 0, 'Value must be positive');

🎯 Try/Catch Blocks

try {
  // Code that might throw
  const result = riskyOperation();
} catch (error) {
  // Handle specific errors
  if (error instanceof TypeError) {
    console.error('Type error:', error.message);
  } else {
    console.error('Unknown error:', error);
  }
} finally {
  // Cleanup (always runs)
  cleanup();
}

⚙️ Custom Error Classes

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

throw new ValidationError('Invalid email', 'email');

📊 Logging Strategies

Log Levels

// DEBUG: Detailed debugging info
console.debug('User ID: 123');

// INFO: General information
console.log('Server started');

// WARN: Warning (recoverable)
console.warn('High memory usage');

// ERROR: Error (needs attention)
console.error('Database connection failed');

🔄 Async Error Handling

Promise Errors

async function loadData() {
  try {
    const data = await fetchData();
    return data;
  } catch (error) {
    console.error('Failed to load:', error.message);
    throw error;
  }
}

// OR with .catch()
fetchData()
  .catch(error => {
    console.error('Error:', error);
  });

📤 Error Propagation

Handle or Propagate?

Don't Swallow Errors

try {
  riskyOp();
} catch (e) {
  // Empty! Bad!
}

Log or Re-throw

try {
  riskyOp();
} catch (e) {
  console.error(e);
  throw e;
}

🔍 Stack Traces

A stack trace shows the call chain when an error occurs:

TypeError: Cannot read property 'name' of undefined
    at getUser (/app/users.js:5:10)
    at getUserName (/app/app.js:12:5)
    at main (/app/app.js:20:3)

// Read from bottom to top (main → getUserName → getUser)
// Error happened in getUser at line 5, column 10

🛠️ Debugging Tools

Node.js Debugger

$ node inspect app.js
$ node --inspect app.js

Then open Chrome DevTools:
chrome://inspect

💻 Coding Challenges

Challenge 1: Custom Error Classes

Create custom error classes (ValidationError, DatabaseError) and use them in functions.

Goal: Learn to create and throw meaningful errors.

→ Solution

Challenge 2: Async Error Handling

Create async functions with proper try/catch and error propagation patterns.

Goal: Master async error handling in promises and async/await.

→ Solution

Challenge 3: Logging System

Build a simple logger with levels (debug, info, warn, error) and timestamps.

Goal: Understand structured logging for production apps.

→ Solution

💡 Error Handling Best Practices

1. Never swallow errors silently: Always log or re-throw.

2. Be specific: Create custom error classes for different scenarios.

3. Provide context: Include useful information in error messages.

4. Use proper status codes: 400 for bad requests, 500 for server errors.

5. Log at startup: Output config and status on app start.

🎯 What's Next

You've learned debugging and error handling! The Node.js Fundamentals course is nearly complete. We'll wrap up with Best Practices & Wrap-Up in the final chapter (node1-8).