code-review

name	code-review
description	Guide for conducting code reviews. Use when reviewing pull requests, auditing code quality, identifying security issues, or providing code feedback.

Code Review Best Practices

This skill activates when reviewing code for quality, correctness, security, and maintainability.

When to Use This Skill

Activate when:

• Reviewing pull requests
• Conducting code audits
• Providing feedback on code quality
• Identifying security vulnerabilities
• Suggesting refactoring improvements
• Checking adherence to coding standards

Code Review Checklist

1. Correctness and Functionality

Does the code do what it's supposed to do?

• Logic is correct and handles all cases
• Edge cases are considered
• Error handling is appropriate
• No obvious bugs or logical errors
• Assertions and validations are present
• Return values are correct

Questions to ask:

• What happens if this receives null/nil?
• What if the list is empty?
• What if the number is negative/zero?
• Are there off-by-one errors?
• Are comparisons correct (>, >=, <, <=)?

2. Security

Is the code secure?

• No SQL injection vulnerabilities
• No XSS (Cross-Site Scripting) vulnerabilities
• No CSRF vulnerabilities (CSRF protection in place)
• User input is validated and sanitized
• Sensitive data is not logged
• Authentication and authorization are properly implemented
• No hardcoded secrets or credentials
• File uploads are validated (type, size, content)
• External URLs are validated
• Rate limiting is in place for APIs

Common security issues:

# BAD: SQL injection vulnerability
query = "SELECT * FROM users WHERE id = #{user_id}"

# GOOD: Use parameterized queries
query = from u in User, where: u.id == ^user_id

# BAD: XSS vulnerability
raw("<div>#{user_input}</div>")

# GOOD: Escape user input
<div><%= user_input %></div>

# BAD: Hardcoded secrets
api_key = "sk_live_123456789"

# GOOD: Use environment variables
api_key = System.get_env("API_KEY")

# BAD: Mass assignment vulnerability
User.changeset(%User{}, params)

# GOOD: Whitelist allowed fields
User.changeset(%User{}, params)
# Where changeset only casts allowed fields:
# cast(user, attrs, [:name, :email])

3. Performance

Is the code efficient?

• No N+1 query problems
• Appropriate data structures chosen
• Algorithms are efficient
• Database indexes are used
• Caching is implemented where appropriate
• Large datasets are paginated or streamed
• Unnecessary computations are avoided
• Resources are cleaned up properly

Common performance issues:

# BAD: N+1 query
posts = Repo.all(Post)
Enum.map(posts, fn post ->
  author = Repo.get(User, post.author_id)  # Query for each post!
  {post, author}
end)

# GOOD: Preload associations
posts = Post |> preload(:author) |> Repo.all()

# BAD: Loading entire dataset
users = Repo.all(User)  # Loads all millions of users
Enum.filter(users, & &1.active)

# GOOD: Query in database
users = User |> where(active: true) |> Repo.all()

# BAD: Inefficient data structure
list = [1, 2, 3, 4, 5]
if 3 in list do  # O(n) lookup in list
  # ...
end

# GOOD: Use set/map for lookups
set = MapSet.new([1, 2, 3, 4, 5])
if MapSet.member?(set, 3) do  # O(1) lookup
  # ...
end

4. Code Quality and Maintainability

Is the code readable and maintainable?

• Clear, descriptive variable and function names
• Functions are small and focused (single responsibility)
• No code duplication (DRY principle)
• Comments explain "why", not "what"
• Code follows project conventions and style guide
• Magic numbers are replaced with named constants
• Complexity is minimized
• Code is self-documenting

Code quality issues:

# BAD: Unclear names
def calc(x, y, z) do
  r = x * y / z
  r * 1.2
end

# GOOD: Clear names
def calculate_discounted_price(quantity, unit_price, discount_percentage) do
  subtotal = quantity * unit_price
  discount_amount = subtotal * (discount_percentage / 100)
  subtotal - discount_amount
end

# BAD: Long function with multiple responsibilities
def process_order(order) do
  # Validate order (responsibility 1)
  # Calculate totals (responsibility 2)
  # Update inventory (responsibility 3)
  # Send email (responsibility 4)
  # Log analytics (responsibility 5)
end

# GOOD: Single responsibility functions
def process_order(order) do
  with {:ok, order} <- validate_order(order),
       {:ok, order} <- calculate_totals(order),
       {:ok, order} <- update_inventory(order),
       :ok <- send_confirmation_email(order),
       :ok <- log_order_analytics(order) do
    {:ok, order}
  end
end

# BAD: Magic numbers
if user.age >= 13 do
  # ...
end

# GOOD: Named constants
@minimum_age_coppa 13

if user.age >= @minimum_age_coppa do
  # ...
end

5. Error Handling

Are errors handled properly?

• Errors don't crash the system unexpectedly
• Error messages are helpful
• Errors are logged appropriately
• Happy path and error paths are both tested
• No swallowed errors (empty catch blocks)
• Proper error types are used

Error handling patterns:

# BAD: Silent failure
try do
  dangerous_operation()
rescue
  _ -> nil  # Error is swallowed!
end

# GOOD: Handle errors explicitly
case dangerous_operation() do
  {:ok, result} -> result
  {:error, reason} ->
    Logger.error("Operation failed: #{inspect(reason)}")
    {:error, reason}
end

# BAD: Generic error message
{:error, "failed"}

# GOOD: Specific error
{:error, :invalid_email_format}
{:error, {:validation_failed, errors}}

# BAD: Let it crash when shouldn't
def parse_config(path) do
  File.read!(path)  # Crashes if file missing
  |> Jason.decode!()  # Crashes if invalid JSON
end

# GOOD: Handle expected errors
def parse_config(path) do
  with {:ok, content} <- File.read(path),
       {:ok, config} <- Jason.decode(content) do
    {:ok, config}
  else
    {:error, :enoent} -> {:error, :config_file_not_found}
    {:error, %Jason.DecodeError{}} -> {:error, :invalid_config_format}
  end
end

6. Testing

Is the code properly tested?

• New functionality has tests
• Edge cases are tested
• Error conditions are tested
• Tests are clear and focused
• Tests are deterministic (no flaky tests)
• Test names describe what they test
• Mocks are used appropriately
• Test coverage is adequate

Testing concerns:

# BAD: Unclear test name
test "test1" do
  # ...
end

# GOOD: Descriptive test name
test "create_user/1 returns error when email is invalid" do
  # ...
end

# BAD: Testing too much at once
test "user workflow" do
  # Creates user
  # Updates user
  # Deletes user
  # All in one test!
end

# GOOD: Focused tests
test "create_user/1 creates user with valid attributes" do
  # ...
end

test "update_user/2 updates user name" do
  # ...
end

test "delete_user/1 removes user from database" do
  # ...
end

# BAD: Non-deterministic test
test "async operation completes" do
  start_async_operation()
  Process.sleep(100)  # Race condition!
  assert operation_completed?()
end

# GOOD: Deterministic test
test "async operation completes" do
  start_async_operation()
  assert_receive {:completed, _result}, 1000
end

7. Documentation

Is the code documented?

• Public APIs have documentation
• Complex logic has explanatory comments
• README is updated if needed
• Changelog is updated for user-facing changes
• API documentation is accurate
• Examples are provided

8. Dependencies

Are dependencies handled properly?

• New dependencies are justified
• Dependencies are up-to-date and maintained
• Licenses are compatible with project
• Security vulnerabilities are checked
• Dependency versions are pinned or bounded

9. Restraint and Scope

Does every new symbol earn its place in the diff?

For each NEW symbol introduced by the diff (function, class, config, dependency), check it against the restraint ladder before approving:

• Does this need to exist at all, or is it speculative scaffolding? (YAGNI)
• Is it a duplicate of an existing helper, stdlib function, or already-installed dependency? (Reuse-First)
• Could it be expressed in a smaller form — fewer lines, no new abstraction?

Flag speculative abstractions (interfaces with one implementation, config for values that never change, factories for one product) and unrequested features as findings, not nits. See /core:restraint.

Review Process

Before Reviewing

1. Understand the context

   • Read the PR description
   • Understand the problem being solved
   • Check related issues

2. Build and test locally

   • Pull the branch
   • Run tests
   • Test the functionality manually

During Review

1. Start with the big picture

   • Is the approach sound?
   • Does it fit the architecture?
   • Is there a better way?

2. Review for correctness

   • Does it work as intended?
   • Are edge cases handled?
   • Is error handling appropriate?

3. Check security and performance

   • Are there security vulnerabilities?
   • Will it perform well at scale?

4. Review code quality

   • Is it readable and maintainable?
   • Does it follow conventions?
   • Is it well-tested?

Providing Feedback

Be constructive and specific:

# BAD: Vague criticism
"This function is bad."

# GOOD: Specific, actionable feedback
"This function has three responsibilities: validation, database update, and email sending. Consider splitting it into separate functions for better testability and maintainability:

```elixir
def update_user(user, attrs) do
  with {:ok, changeset} <- validate_user_update(user, attrs),
       {:ok, user} <- save_user(changeset),
       :ok <- send_update_notification(user) do
    {:ok, user}
  end
end

BAD: Demanding

"You must change this."

GOOD: Collaborative

"What do you think about extracting this into a separate function? It would make the code easier to test."

BAD: Nitpicking without context

"Use single quotes instead of double quotes."

GOOD: Explain reasoning

"Our style guide prefers single quotes for consistency (see CONTRIBUTING.md section 3.2)."

**Use labels to categorize feedback:**

- **[blocking]**: Must be fixed before merging
- **[suggestion]**: Optional improvement
- **[question]**: Asking for clarification
- **[nit]**: Very minor, cosmetic issue
- **[security]**: Security concern
- **[performance]**: Performance concern

**Example:**

```markdown
[blocking] This creates a SQL injection vulnerability. Use parameterized queries:

```elixir
# Instead of:
query = "SELECT * FROM users WHERE name = '#{name}'"

# Use:
from(u in User, where: u.name == ^name)

[suggestion] Consider extracting this logic into a separate function for reusability.

[question] Why are we using a map here instead of a struct?

[nit] Extra blank line here.

### After Review

1. **Respond to author's questions**
2. **Re-review after changes**
3. **Approve when satisfied**
4. **Celebrate good code**

## Language-Specific Considerations

### Elixir

- Pattern matching is used effectively
- Functions leverage pipe operator for readability
- Atoms aren't created dynamically from untrusted input
- `with` statements handle errors properly
- Changesets validate all input
- No direct database queries in controllers/LiveViews (use contexts)

### JavaScript/TypeScript

- Types are properly defined (TypeScript)
- Promises are handled with .catch() or try/catch
- == vs === is used correctly
- Arrays/objects aren't mutated unexpectedly
- this binding is correct
- Async operations are properly awaited

### Python

- Type hints are used
- List comprehensions aren't overly complex
- Exceptions are specific (not bare except:)
- Resources are closed (use with statements)
- Code follows PEP 8

### Rust

- Ownership and borrowing are correct
- Error handling uses Result/Option properly
- Unsafe blocks are justified and minimal
- Clone/copy is used appropriately
- Lifetimes are correctly specified

## Common Code Smells

### Complexity Smells

- **Long functions** - Function does too much
- **Long parameter list** - Too many parameters
- **Deep nesting** - Too many levels of indentation
- **Complex conditionals** - Hard to understand if statements

### Duplication Smells

- **Copy-paste code** - Same code in multiple places
- **Similar functions** - Functions that do almost the same thing
- **Magic numbers** - Repeated literal values

### Naming Smells

- **Unclear names** - Variables like x, tmp, data
- **Misleading names** - Name doesn't match behavior
- **Inconsistent names** - Same concept called different things

### Design Smells

- **God object** - Class/module doing everything
- **Feature envy** - Function using another object's data more than its own
- **Inappropriate intimacy** - Too much coupling between modules

## Anti-Patterns to Watch For

### Premature Optimization

```elixir
# BAD: Optimizing before measuring
def calculate(data) do
  # Complex, hard-to-read optimization
  # that saves 0.1ms
end

# GOOD: Start simple, optimize if needed
def calculate(data) do
  # Clear, simple code
  # Optimize later if profiling shows bottleneck
end

Premature Abstraction

# BAD: Abstract after one use
defmodule AbstractDataProcessorFactoryBuilder do
  # Complex abstraction for single use case
end

# GOOD: Wait for second use case
def process_user_data(data) do
  # Simple, direct implementation
  # Abstract when pattern emerges
end

Error Swallowing

# BAD: Hiding errors
try do
  risky_operation()
rescue
  _ -> :ok  # What went wrong?
end

# GOOD: Handle explicitly
case risky_operation() do
  {:ok, result} -> {:ok, result}
  {:error, reason} ->
    Logger.error("Operation failed: #{inspect(reason)}")
    {:error, reason}
end

Review Etiquette

DO:

• Be respectful and constructive
• Assume good intent
• Ask questions instead of making demands
• Praise good code
• Explain the "why" behind suggestions
• Offer to pair program on complex issues
• Respond promptly to author's replies

DON'T:

• Be sarcastic or condescending
• Bike-shed on minor style issues
• Block on personal preferences
• Review your own code without another reviewer
• Approve code you don't understand
• Nitpick excessively

Self-Review Checklist

Before submitting code for review:

• Code compiles and runs
• All tests pass
• Added tests for new functionality
• No commented-out code
• No debug print statements
• Documentation is updated
• Commit messages are clear
• No secrets or sensitive data
• Code follows project style guide
• Changes are focused (no unrelated changes)

Key Principles

• Correctness first: Code must work correctly
• Security matters: Always consider security implications
• Be specific: Provide actionable, concrete feedback
• Be respectful: Kind, constructive communication
• Focus on important issues: Don't bike-shed
• Explain reasoning: Help author learn, don't just dictate
• Approve good code: Don't let perfect be enemy of good
• Collaborate: You're on the same team
• Check restraint: Every new symbol earns its place on the ladder — see /core:restraint

References

• references/no-todos-scan.md — Scan PR diffs for new TODO/FIXME/XXX/HACK/KLUDGE/DEFERRED markers; treat as BLOCKER, not nit