/debug [arguments]

**Hypothesis:** Database query timeout
**Test:** Add query timing logs
**Result:** Query completes in 50ms
**Conclusion:** Not the database

**Hypothesis:** Network latency
**Test:** Check network tab, add timing
**Result:** API call takes 5 seconds
**Conclusion:** Found the issue

// Strategic console.log placement
function processOrder(order) {
  console.log('processOrder START:', { orderId: order.id })

  const items = order.items
  console.log('items:', items.length)

  const validated = validate(items)
  console.log('validation result:', validated)

  if (!validated.success) {
    console.log('validation failed:', validated.errors)
    throw new Error('Invalid order')
  }

  const total = calculateTotal(items)
  console.log('total calculated:', total)

  console.log('processOrder END')
  return total
}

// Browser
function buggyFunction(input) {
  debugger;  // Execution pauses here
  const result = transform(input)
  debugger;  // And here
  return result
}

// Node.js
node --inspect app.js
# Then open chrome://inspect in Chrome

// 100 lines of code, bug somewhere

// Comment out lines 50-100
// Bug still happens? It's in lines 1-50

// Comment out lines 25-50
// Bug disappears? It's in lines 25-50

// Comment out lines 37-50
// Bug still happens? It's in lines 25-37

// Continue until isolated to specific lines

# Find which commit broke it
git bisect start
git bisect bad HEAD
git bisect good v1.0.0
# Git checks out middle commit
npm test
git bisect good/bad
# Repeat until found

// Complex case with bug
processUserOrderWithDiscountsAndShipping(user, cart, promo, address)

// Simplify inputs one at a time
processUserOrderWithDiscountsAndShipping(user, [], null, null)
// Still breaks? Not discount or address

processUserOrderWithDiscountsAndShipping(null, [], null, null)
// Works now? It's the user object

// What about the user object causes it?

// Assumption: API returns array
const users = await api.getUsers()
users.forEach(...)  // Crashes

// Check assumption
console.log(typeof users)  // "undefined"
console.log(users)          // undefined

// Assumption was wrong!

// Take heap snapshot
// Do operation that leaks
// Take another heap snapshot
// Compare - what increased?

Error: Cannot read property 'map' of undefined
    at processUsers (app.js:42:15)
    at handleRequest (app.js:23:3)
    at Server.<anonymous> (server.js:12:5)

// Bug
function process(user) {
  return user.name.toUpperCase()  // Crashes if user is null
}

// Investigation
console.log('user:', user)  // undefined - why?
// Trace back to where user comes from

// Bug
for (let i = 0; i <= array.length; i++) {  // <= instead of <
  process(array[i])  // Crashes on last iteration
}

// Investigation
console.log('i:', i, 'length:', array.length)
// Notice i === array.length causes array[i] === undefined

// Bug
let data
fetchData().then(result => {
  data = result
})
console.log(data)  // undefined - async not complete

// Investigation
console.log('1. Before fetch')
fetchData().then(result => {
  console.log('3. Got result')
  data = result
})
console.log('2. After fetch call')
// Output: 1, 2, 3 - async completes later

// Bug
function addItem(cart, item) {
  cart.items.push(item)  // Mutates input!
  return cart
}

const originalCart = { items: [] }
const newCart = addItem(originalCart, item)
// originalCart was modified - unexpected!

// Investigation
console.log('before:', originalCart)
const newCart = addItem(originalCart, item)
console.log('after:', originalCart)  // Changed!

// Bug
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100)
}
// Prints: 3, 3, 3 (expected 0, 1, 2)

// Investigation
// var is function-scoped, i is shared
// By time timeout fires, loop is done, i === 3

// Fix: Use let (block-scoped) or capture i

## Investigation: [Issue description]

### Symptoms
[What's happening that's wrong?]

### Evidence
- Error message: [exact text]
- When it happens: [conditions]
- Frequency: [always/sometimes/rarely]
- Affected users: [all/some/specific group]

### Reproduction Steps
1. [Step 1]
2. [Step 2]
3. [Observe error]

### Investigation Timeline

**Hypothesis 1:** [What I thought might be wrong]
- Tested by: [What I did to test]
- Result: [What I found]
- Conclusion: [Ruled out / Confirmed]

**Hypothesis 2:** [Next theory]
- Tested by: [What I did]
- Result: [What I found]
- Conclusion: [Ruled out / Confirmed]

### Root Cause
[What's actually causing the issue]

**Evidence:**
- [Log showing the problem]
- [Stack trace pointing to source]
- [Data showing the pattern]

### Impact
- Severity: [Critical/High/Medium/Low]
- Scope: [How many users/scenarios affected]
- Workaround: [Any temporary solutions]

### Next Steps
- [ ] [What should be done to fix]
- [ ] [Any additional investigation needed]
- [ ] [Related issues to check]

# Search for text in files
grep -r "error" logs/

# Follow log file
tail -f logs/app.log

# Search with context
grep -B 5 -A 5 "ERROR" logs/app.log

# Check disk space
df -h

# Check memory
free -m

# Check running processes
ps aux | grep node

-- PostgreSQL
EXPLAIN ANALYZE SELECT * FROM users WHERE email = 'test@example.com';

-- Show slow queries
SELECT * FROM pg_stat_statements ORDER BY total_time DESC LIMIT 10;

-- Check table size
SELECT pg_size_pretty(pg_total_relation_size('users'));

-- Check indexes
\d users

BAD: "Maybe if I change this... nope, try this... nope, try this..."
GOOD: "Hypothesis: X causes Y. Test: Change X. Result: Y still happens.
       Conclusion: X is not the cause."

BAD: "The API must be returning valid data"
GOOD: "Let me log the API response to see what it actually returns"

BAD: "The page is blank. Fixed by adding a null check."
GOOD: "The page is blank because user is null. User is null because
       authentication token expired. Root cause: token not being refreshed."

BAD: "Let me add console.log to production to see..."
GOOD: "Let me reproduce locally and debug there, or use proper logging"

BAD: "It crashed once, let me guess why"
GOOD: "Let me find reliable way to reproduce it first"