Debugging Strategies

When to Use This Skill

• Tracking down elusive bugs
• Investigating performance issues
• Understanding unfamiliar codebases
• Debugging production issues
• Analyzing crash dumps and stack traces
• Profiling application performance
• Investigating memory leaks
• Debugging distributed systems

Core Principles

1. The Scientific Method

2. Debugging Mindset

• "It can't be X" - Yes it can
• "I didn't change Y" - Check anyway
• "It works on my machine" - Find out why

• Reproduce consistently
• Isolate the problem
• Keep detailed notes
• Question everything
• Take breaks when stuck

3. Rubber Duck Debugging

Systematic Debugging Process

Phase 1: Reproduce

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## Reproduction Checklist

1. **Can you reproduce it?**
   - Always? Sometimes? Randomly?
   - Specific conditions needed?
   - Can others reproduce it?

2. **Create minimal reproduction**
   - Simplify to smallest example
   - Remove unrelated code
   - Isolate the problem

3. **Document steps**
   - Write down exact steps
   - Note environment details
   - Capture error messages
`### Phase 2: Gather Information`
## Information Collection

1. **Error Messages**
   - Full stack trace
   - Error codes
   - Console/log output

2. **Environment**
   - OS version
   - Language/runtime version
   - Dependencies versions
   - Environment variables

3. **Recent Changes**
   - Git history
   - Deployment timeline
   - Configuration changes

4. **Scope**
   - Affects all users or specific ones?
   - All browsers or specific ones?
   - Production only or also dev?
`### Phase 3: Form Hypothesis`
## Hypothesis Formation

Based on gathered info, ask:

1. **What changed?**
   - Recent code changes
   - Dependency updates
   - Infrastructure changes

2. **What's different?**
   - Working vs broken environment
   - Working vs broken user
   - Before vs after

3. **Where could this fail?**
   - Input validation
   - Business logic
   - Data layer
   - External services
`### Phase 4: Test & Verify`
## Testing Strategies

1. **Binary Search**
   - Comment out half the code
   - Narrow down problematic section
   - Repeat until found

2. **Add Logging**
   - Strategic console.log/print
   - Track variable values
   - Trace execution flow

3. **Isolate Components**
   - Test each piece separately
   - Mock dependencies
   - Remove complexity

4. **Compare Working vs Broken**
   - Diff configurations
   - Diff environments
   - Diff data

Debugging Tools

JavaScript/TypeScript Debugging

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// Chrome DevTools Debugger
function processOrder(order: Order) {
  debugger; // Execution pauses here

  const total = calculateTotal(order);
  console.log("Total:", total);

  // Conditional breakpoint
  if (order.items.length > 10) {
    debugger; // Only breaks if condition true
  }

  return total;
}

// Console debugging techniques
console.log("Value:", value); // Basic
console.table(arrayOfObjects); // Table format
console.time("operation");
/* code */ console.timeEnd("operation"); // Timing
console.trace(); // Stack trace
console.assert(value > 0, "Value must be positive"); // Assertion

// Performance profiling
performance.mark("start-operation");
// ... operation code
performance.mark("end-operation");
performance.measure("operation", "start-operation", "end-operation");
console.log(performance.getEntriesByType("measure"));
`**VS Code Debugger Configuration:**`
// .vscode/launch.json
{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "node",
      "request": "launch",
      "name": "Debug Program",
      "program": "${workspaceFolder}/src/index.ts",
      "preLaunchTask": "tsc: build - tsconfig.json",
      "outFiles": ["${workspaceFolder}/dist/**/*.js"],
      "skipFiles": ["<node_internals>/**"]
    },
    {
      "type": "node",
      "request": "launch",
      "name": "Debug Tests",
      "program": "${workspaceFolder}/node_modules/jest/bin/jest",
      "args": ["--runInBand", "--no-cache"],
      "console": "integratedTerminal"
    }
  ]
}
`### Python Debugging`
# Built-in debugger (pdb)
import pdb

def calculate_total(items):
    total = 0
    pdb.set_trace()  # Debugger starts here

    for item in items:
        total += item.price * item.quantity

    return total

# Breakpoint (Python 3.7+)
def process_order(order):
    breakpoint()  # More convenient than pdb.set_trace()
    # ... code

# Post-mortem debugging
try:
    risky_operation()
except Exception:
    import pdb
    pdb.post_mortem()  # Debug at exception point

# IPython debugging (ipdb)
from ipdb import set_trace
set_trace()  # Better interface than pdb

# Logging for debugging
import logging
logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger(__name__)

def fetch_user(user_id):
    logger.debug(f'Fetching user: {user_id}')
    user = db.query(User).get(user_id)
    logger.debug(f'Found user: {user}')
    return user

# Profile performance
import cProfile
import pstats

cProfile.run('slow_function()', 'profile_stats')
stats = pstats.Stats('profile_stats')
stats.sort_stats('cumulative')
stats.print_stats(10)  # Top 10 slowest
`### Go Debugging`
// Delve debugger
// Install: go install github.com/go-delve/delve/cmd/dlv@latest
// Run: dlv debug main.go

import (
    "fmt"
    "runtime"
    "runtime/debug"
)

// Print stack trace
func debugStack() {
    debug.PrintStack()
}

// Panic recovery with debugging
func processRequest() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Panic:", r)
            debug.PrintStack()
        }
    }()

    // ... code that might panic
}

// Memory profiling
import _ "net/http/pprof"
// Visit http://localhost:6060/debug/pprof/

// CPU profiling
import (
    "os"
    "runtime/pprof"
)

f, _ := os.Create("cpu.prof")
pprof.StartCPUProfile(f)
defer pprof.StopCPUProfile()
// ... code to profile

Advanced Debugging Techniques

Technique 1: Binary Search Debugging

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# Git bisect for finding regression
git bisect start
git bisect bad                    # Current commit is bad
git bisect good v1.0.0            # v1.0.0 was good

# Git checks out middle commit
# Test it, then:
git bisect good   # if it works
git bisect bad    # if it's broken

# Continue until bug found
git bisect reset  # when done

Technique 2: Differential Debugging

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## What's Different?

| Aspect       | Working     | Broken         |
| ------------ | ----------- | -------------- |
| Environment  | Development | Production     |
| Node version | 18.16.0     | 18.15.0        |
| Data         | Empty DB    | 1M records     |
| User         | Admin       | Regular user   |
| Browser      | Chrome      | Safari         |
| Time         | During day  | After midnight |

Hypothesis: Time-based issue? Check timezone handling.
`### Technique 3: Trace Debugging`
// Function call tracing
function trace(
  target: any,
  propertyKey: string,
  descriptor: PropertyDescriptor,
) {
  const originalMethod = descriptor.value;

  descriptor.value = function (...args: any[]) {
    console.log(`Calling ${propertyKey} with args:`, args);
    const result = originalMethod.apply(this, args);
    console.log(`${propertyKey} returned:`, result);
    return result;
  };

  return descriptor;
}

class OrderService {
  @trace
  calculateTotal(items: Item[]): number {
    return items.reduce((sum, item) => sum + item.price, 0);
  }
}
`### Technique 4: Memory Leak Detection`
// Chrome DevTools Memory Profiler
// 1. Take heap snapshot
// 2. Perform action
// 3. Take another snapshot
// 4. Compare snapshots

// Node.js memory debugging
if (process.memoryUsage().heapUsed > 500 * 1024 * 1024) {
  console.warn("High memory usage:", process.memoryUsage());

  // Generate heap dump
  require("v8").writeHeapSnapshot();
}

// Find memory leaks in tests
let beforeMemory: number;

beforeEach(() => {
  beforeMemory = process.memoryUsage().heapUsed;
});

afterEach(() => {
  const afterMemory = process.memoryUsage().heapUsed;
  const diff = afterMemory - beforeMemory;

  if (diff > 10 * 1024 * 1024) {
    // 10MB threshold
    console.warn(`Possible memory leak: ${diff / 1024 / 1024}MB`);
  }
});

Debugging Patterns by Issue Type

Pattern 1: Intermittent Bugs

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## Strategies for Flaky Bugs

1. **Add extensive logging**
   - Log timing information
   - Log all state transitions
   - Log external interactions

2. **Look for race conditions**
   - Concurrent access to shared state
   - Async operations completing out of order
   - Missing synchronization

3. **Check timing dependencies**
   - setTimeout/setInterval
   - Promise resolution order
   - Animation frame timing

4. **Stress test**
   - Run many times
   - Vary timing
   - Simulate load
`### Pattern 2: Performance Issues`
## Performance Debugging

1. **Profile first**
   - Don't optimize blindly
   - Measure before and after
   - Find bottlenecks

2. **Common culprits**
   - N+1 queries
   - Unnecessary re-renders
   - Large data processing
   - Synchronous I/O

3. **Tools**
   - Browser DevTools Performance tab
   - Lighthouse
   - Python: cProfile, line_profiler
   - Node: clinic.js, 0x
`### Pattern 3: Production Bugs`
## Production Debugging

1. **Gather evidence**
   - Error tracking (Sentry, Bugsnag)
   - Application logs
   - User reports
   - Metrics/monitoring

2. **Reproduce locally**
   - Use production data (anonymized)
   - Match environment
   - Follow exact steps

3. **Safe investigation**
   - Don't change production
   - Use feature flags
   - Add monitoring/logging
   - Test fixes in staging

Best Practices

1. Reproduce First: Can't fix what you can't reproduce
2. Isolate the Problem: Remove complexity until minimal case
3. Read Error Messages: They're usually helpful
4. Check Recent Changes: Most bugs are recent
5. Use Version Control: Git bisect, blame, history
6. Take Breaks: Fresh eyes see better
7. Document Findings: Help future you
8. Fix Root Cause: Not just symptoms

Common Debugging Mistakes

• Making Multiple Changes: Change one thing at a time
• Not Reading Error Messages: Read the full stack trace
• Assuming It's Complex: Often it's simple
• Debug Logging in Prod: Remove before shipping
• Not Using Debugger: console.log isn't always best
• Giving Up Too Soon: Persistence pays off
• Not Testing the Fix: Verify it actually works

Quick Debugging Checklist

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## When Stuck, Check:

- [ ] Spelling errors (typos in variable names)
- [ ] Case sensitivity (fileName vs filename)
- [ ] Null/undefined values
- [ ] Array index off-by-one
- [ ] Async timing (race conditions)
- [ ] Scope issues (closure, hoisting)
- [ ] Type mismatches
- [ ] Missing dependencies
- [ ] Environment variables
- [ ] File paths (absolute vs relative)
- [ ] Cache issues (clear cache)
- [ ] Stale data (refresh database)

Resources

• references/debugging-tools-guide.md: Comprehensive tool documentation
• references/performance-profiling.md: Performance debugging guide
• references/production-debugging.md: Debugging live systems
• assets/debugging-checklist.md: Quick reference checklist
• assets/common-bugs.md: Common bug patterns
• scripts/debug-helper.ts: Debugging utility functions