JavaScript Debugging Skill

Quick Reference Card

Console Methods

console.log('Basic log');
console.info('Info');
console.warn('Warning');
console.error('Error');

// Formatted output
console.table([{ a: 1 }, { a: 2 }]);
console.dir(object, { depth: null });

// Grouping
console.group('Group');
console.log('Inside group');
console.groupEnd();

// Timing
console.time('operation');
doSomething();
console.timeEnd('operation');

// Assertions
console.assert(condition, 'Failed message');

// Counting
console.count('label'); // label: 1, 2, 3...

Debugger

function processData(data) {
  debugger; // Pause here in DevTools
  const result = transform(data);
  return result;
}

DevTools Breakpoints

Node.js Debugging

# Start with inspector
node --inspect app.js
node --inspect-brk app.js  # Break at start

# Chrome: chrome://inspect
# VS Code: Use launch.json

VS Code launch.json

{
  "version": "0.2.0",
  "configurations": [{
    "type": "node",
    "request": "launch",
    "name": "Debug",
    "program": "${workspaceFolder}/src/index.js"
  }]
}

Troubleshooting Patterns

Async Debugging

async function debug() {
  console.log('1. Start');
  try {
    const result = await asyncOperation();
    console.log('2. Result:', result);
  } catch (error) {
    console.error('Error:', error);
    console.log('Stack:', error.stack);
  }
  console.log('3. End');
}

Event Debugging

// Log all events
element.addEventListener('click', (e) => {
  console.log('Event:', e.type);
  console.log('Target:', e.target);
  console.log('CurrentTarget:', e.currentTarget);
  console.log('Phase:', e.eventPhase);
});

// Monitor events (DevTools)
// monitorEvents(element, 'click')

Memory Debugging

// Check memory usage
if (performance.memory) {
  console.log('Heap:', performance.memory.usedJSHeapSize);
}

// Force garbage collection (DevTools)
// gc()

Network Debugging

// Log fetch requests
const originalFetch = window.fetch;
window.fetch = async (...args) => {
  console.log('Fetch:', args[0]);
  const response = await originalFetch(...args);
  console.log('Response:', response.status);
  return response;
};

Common Issues

Debug Checklist

1. Reproduce - Isolate the issue
2. Inspect - Check values and types
3. Trace - Follow execution flow
4. Compare - Working vs broken state
5. Fix - Apply smallest change
6. Verify - Test the fix

Production Patterns

Error Tracking

window.onerror = (msg, url, line, col, error) => {
  console.error('Error:', { msg, url, line, col });
  // Send to error tracking service
};

window.onunhandledrejection = (event) => {
  console.error('Unhandled rejection:', event.reason);
};

Performance Profiling

// Mark timing points
performance.mark('start');
doWork();
performance.mark('end');

performance.measure('work', 'start', 'end');
console.log(performance.getEntriesByName('work'));

Related

• Agent 08: Testing & Quality (detailed learning)
• Skill: testing: Test debugging
• Skill: performance: Performance profiling