Master effective code review practices to provide constructive feedback, catch bugs early, and foster knowledge sharing while maintaining team morale. Use when reviewing pull requests, establishing review standards, or mentoring developers.
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Transform code reviews from gatekeeping to knowledge sharing through constructive feedback, systematic analysis, and collaborative improvement.
Goals of Code Review:
Not the Goals:
Good Feedback is:
❌ Bad: "This is wrong."
✅ Good: "This could cause a race condition when multiple users
access simultaneously. Consider using a mutex here."
❌ Bad: "Why didn't you use X pattern?"
✅ Good: "Have you considered the Repository pattern? It would
make this easier to test. Here's an example: [link]"
❌ Bad: "Rename this variable."
✅ Good: "[nit] Consider `userCount` instead of `uc` for
clarity. Not blocking if you prefer to keep it."
What to Review:
What Not to Review Manually:
Before diving into code, understand:
1. Read PR description and linked issue
2. Check PR size (>400 lines? Ask to split)
3. Review CI/CD status (tests passing?)
4. Understand the business requirement
5. Note any relevant architectural decisions
1. **Architecture & Design**
- Does the solution fit the problem?
- Are there simpler approaches?
- Is it consistent with existing patterns?
- Will it scale?
2. **File Organization**
- Are new files in the right places?
- Is code grouped logically?
- Are there duplicate files?
3. **Testing Strategy**
- Are there tests?
- Do tests cover edge cases?
- Are tests readable?
For each file:
1. **Logic & Correctness**
- Edge cases handled?
- Off-by-one errors?
- Null/undefined checks?
- Race conditions?
2. **Security**
- Input validation?
- SQL injection risks?
- XSS vulnerabilities?
- Sensitive data exposure?
3. **Performance**
- N+1 queries?
- Unnecessary loops?
- Memory leaks?
- Blocking operations?
4. **Maintainability**
- Clear variable names?
- Functions doing one thing?
- Complex code commented?
- Magic numbers extracted?
1. Summarize key concerns
2. Highlight what you liked
3. Make clear decision:
- ✅ Approve
- 💬 Comment (minor suggestions)
- 🔄 Request Changes (must address)
4. Offer to pair if complex
## Security Checklist
- [ ] User input validated and sanitized
- [ ] SQL queries use parameterization
- [ ] Authentication/authorization checked
- [ ] Secrets not hardcoded
- [ ] Error messages don't leak info
## Performance Checklist
- [ ] No N+1 queries
- [ ] Database queries indexed
- [ ] Large lists paginated
- [ ] Expensive operations cached
- [ ] No blocking I/O in hot paths
## Testing Checklist
- [ ] Happy path tested
- [ ] Edge cases covered
- [ ] Error cases tested
- [ ] Test names are descriptive
- [ ] Tests are deterministic
Instead of stating problems, ask questions to encourage thinking:
❌ "This will fail if the list is empty."
✅ "What happens if `items` is an empty array?"
❌ "You need error handling here."
✅ "How should this behave if the API call fails?"
❌ "This is inefficient."
✅ "I see this loops through all users. Have we considered
the performance impact with 100k users?"
## Use Collaborative Language
❌ "You must change this to use async/await"
✅ "Suggestion: async/await might make this more readable:
```typescript
async function fetchUser(id: string) {
const user = await db.query('SELECT * FROM users WHERE id = ?', id);
return user;
}
```
What do you think?"
❌ "Extract this into a function"
✅ "This logic appears in 3 places. Would it make sense to
extract it into a shared utility function?"
Use labels to indicate priority:
🔴 [blocking] - Must fix before merge
🟡 [important] - Should fix, discuss if disagree
🟢 [nit] - Nice to have, not blocking
💡 [suggestion] - Alternative approach to consider
📚 [learning] - Educational comment, no action needed
🎉 [praise] - Good work, keep it up!
Example:
"🔴 [blocking] This SQL query is vulnerable to injection.
Please use parameterized queries."
"🟢 [nit] Consider renaming `data` to `userData` for clarity."
"🎉 [praise] Excellent test coverage! This will catch edge cases."
# Check for Python-specific issues
# ❌ Mutable default arguments
def add_item(item, items=[]): # Bug! Shared across calls
items.append(item)
return items
# ✅ Use None as default
def add_item(item, items=None):
if items is None:
items = []
items.append(item)
return items
# ❌ Catching too broad
try:
result = risky_operation()
except: # Catches everything, even KeyboardInterrupt!
pass
# ✅ Catch specific exceptions
try:
result = risky_operation()
except ValueError as e:
logger.error(f"Invalid value: {e}")
raise
# ❌ Using mutable class attributes
class User:
permissions = [] # Shared across all instances!
# ✅ Initialize in __init__
class User:
def __init__(self):
self.permissions = []
// Check for TypeScript-specific issues
// ❌ Using any defeats type safety
function processData(data: any) { // Avoid any
return data.value;
}
// ✅ Use proper types
interface DataPayload {
value: string;
}
function processData(data: DataPayload) {
return data.value;
}
// ❌ Not handling async errors
async function fetchUser(id: string) {
const response = await fetch(`/api/users/${id}`);
return response.json(); // What if network fails?
}
// ✅ Handle errors properly
async function fetchUser(id: string): Promise<User> {
try {
const response = await fetch(`/api/users/${id}`);
if (!response.ok) {
throw new Error(`HTTP ${response.status}`);
}
return await response.json();
} catch (error) {
console.error('Failed to fetch user:', error);
throw error;
}
}
// ❌ Mutation of props
function UserProfile({ user }: Props) {
user.lastViewed = new Date(); // Mutating prop!
return <div>{user.name}</div>;
}
// ✅ Don't mutate props
function UserProfile({ user, onView }: Props) {
useEffect(() => {
onView(user.id); // Notify parent to update
}, [user.id]);
return <div>{user.name}</div>;
}
When reviewing significant changes:
1. **Design Document First**
- For large features, request design doc before code
- Review design with team before implementation
- Agree on approach to avoid rework
2. **Review in Stages**
- First PR: Core abstractions and interfaces
- Second PR: Implementation
- Third PR: Integration and tests
- Easier to review, faster to iterate
3. **Consider Alternatives**
- "Have we considered using [pattern/library]?"
- "What's the tradeoff vs. the simpler approach?"
- "How will this evolve as requirements change?"
// ❌ Poor test: Implementation detail testing
test('increments counter variable', () => {
const component = render(<Counter />);
const button = component.getByRole('button');
fireEvent.click(button);
expect(component.state.counter).toBe(1); // Testing internal state
});
// ✅ Good test: Behavior testing
test('displays incremented count when clicked', () => {
render(<Counter />);
const button = screen.getByRole('button', { name: /increment/i });
fireEvent.click(button);
expect(screen.getByText('Count: 1')).toBeInTheDocument();
});
// Review questions for tests:
// - Do tests describe behavior, not implementation?
// - Are test names clear and descriptive?
// - Do tests cover edge cases?
// - Are tests independent (no shared state)?
// - Can tests run in any order?
## Security Review Checklist
### Authentication & Authorization
- [ ] Is authentication required where needed?
- [ ] Are authorization checks before every action?
- [ ] Is JWT validation proper (signature, expiry)?
- [ ] Are API keys/secrets properly secured?
### Input Validation
- [ ] All user inputs validated?
- [ ] File uploads restricted (size, type)?
- [ ] SQL queries parameterized?
- [ ] XSS protection (escape output)?
### Data Protection
- [ ] Passwords hashed (bcrypt/argon2)?
- [ ] Sensitive data encrypted at rest?
- [ ] HTTPS enforced for sensitive data?
- [ ] PII handled according to regulations?
### Common Vulnerabilities
- [ ] No eval() or similar dynamic execution?
- [ ] No hardcoded secrets?
- [ ] CSRF protection for state-changing operations?
- [ ] Rate limiting on public endpoints?
Traditional: Praise + Criticism + Praise (feels fake)
Better: Context + Specific Issue + Helpful Solution
Example:
"I noticed the payment processing logic is inline in the
controller. This makes it harder to test and reuse.
[Specific Issue]
The calculateTotal() function mixes tax calculation,
discount logic, and database queries, making it difficult
to unit test and reason about.
[Helpful Solution]
Could we extract this into a PaymentService class? That
would make it testable and reusable. I can pair with you
on this if helpful."
When author disagrees with your feedback:
1. **Seek to Understand**
"Help me understand your approach. What led you to
choose this pattern?"
2. **Acknowledge Valid Points**
"That's a good point about X. I hadn't considered that."
3. **Provide Data**
"I'm concerned about performance. Can we add a benchmark
to validate the approach?"
4. **Escalate if Needed**
"Let's get [architect/senior dev] to weigh in on this."
5. **Know When to Let Go**
If it's working and not a critical issue, approve it.
Perfection is the enemy of progress.
## Summary
[Brief overview of what was reviewed]
## Strengths
- [What was done well]
- [Good patterns or approaches]
## Required Changes
🔴 [Blocking issue 1]
🔴 [Blocking issue 2]
## Suggestions
💡 [Improvement 1]
💡 [Improvement 2]
## Questions
❓ [Clarification needed on X]
❓ [Alternative approach consideration]
## Verdict
✅ Approve after addressing required changes