Code Comment Guidelines

Core Philosophy

The best comment is the one you didn't need to write.

Self-documenting code reduces maintenance burden and prevents comment drift. Studies show clear naming and structure can reduce onboarding time by up to 30%.

Writing Style Guidelines

Tone: Be direct, practical, and clear. Write in a natural and relaxed tone. Be approachable and down-to-earth with some personality, but light on the slang and excessive casual terms.

Avoid:

Hierarchy of Documentation

1. Make code self-documenting (naming, structure, types)
2. Use type systems to document contracts
3. Add comments only for WHY, never for WHAT

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Refactoring: Preserve Existing Comments

This skill's guidance applies to writing new code. When refactoring existing code, preserve comments.

Existing comments represent institutional knowledge. Someone wrote them for a reason. During refactoring:

Never Remove

• Comments explaining WHY something exists
• Comments warning about gotchas or edge cases
• Comments referencing external context (tickets, specs, RFCs)
• Comments documenting non-obvious business logic

Update When Necessary

• If refactoring changes behavior the comment describes, update the comment
• If refactoring makes a workaround obsolete, update or remove with the workaround
• Add to existing comments if refactoring introduces new context

Only Remove When

• The comment is demonstrably incorrect (doesn't match code behavior)
• The comment documents code you're deleting entirely
• The refactoring eliminates the "why" (e.g., removing a workaround makes its explanation obsolete)

// BAD: Stripping context during refactoring
// Before: // Retry 3x - payment gateway has transient failures (JIRA-892)
// After:  (comment removed, code unchanged)

// GOOD: Preserving context during refactoring
// Before: // Retry 3x - payment gateway has transient failures (JIRA-892)
// After:  // Retry 3x - payment gateway has transient failures (JIRA-892)

// GOOD: Updating comment when refactoring changes behavior
// Before: // Retry 3x - payment gateway has transient failures
// After:  // Retry with exponential backoff - payment gateway has transient failures

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When NOT to Write Comments

Never Comment the Obvious

// ❌ BAD: Restates code
const name = user.name; // Get the user's name
items.forEach(item => process(item)); // Loop through items

// ✅ GOOD: Self-documenting
const userName = user.name;
items.forEach(processItem);

Never Duplicate Type Information

// ❌ BAD: Types already document this
/** @param {string} email - The email string to validate */
function validateEmail(email: string): boolean {}

// ✅ GOOD: Types speak for themselves
function validateEmail(email: string): boolean {}

Never Leave Stale Comments

// ❌ BAD: Comment doesn't match code
// Returns user's full name
const getEmail = () => user.email;

// ✅ GOOD: Remove or fix
const getEmail = () => user.email;

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When TO Write Comments

1. Explain WHY, Not WHAT

// ✅ Explains reasoning
// Use exponential backoff - service rate-limits after 3 rapid failures
const backoffMs = Math.pow(2, attempts) * 1000;

// ✅ Documents constraint
// Must run before useEffect to prevent hydration mismatch
useLayoutEffect(() => initTheme(), []);

2. Warn About Gotchas and Edge Cases

// ✅ Critical warning
// IMPORTANT: Assumes UTC - local timezone causes date drift
const dayStart = new Date(date.setHours(0, 0, 0, 0));

// ✅ Non-obvious behavior
// Returns null for deleted users (not undefined) - check explicitly
const user = await getUser(id);

3. Reference External Context

// ✅ Links to ticket
// Workaround for Safari flexbox bug (JIRA-1234)
display: '-webkit-flex';

// ✅ References specification
// Per RFC 7231 §6.5.4, return 404 for missing resources
return res.status(404);

4. Document Performance Decisions

// ✅ Explains optimization with data
// Map for O(1) lookup - benchmarked 3x faster than array.find() at n>100
const userMap = new Map(users.map(u => [u.id, u]));

5. Complex Business Logic

// ✅ Documents business rule
// Discount applies only to orders >$100 AND first-time customers
if (orderTotal > 100 && customer.orderCount === 0) {

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Comment Formatting Standards

Single-line Comments

// Sentence case, no period for fragments
// Full sentences get periods.

JSDoc/TSDoc for Public APIs

Only when behavior isn't obvious from signature:

/**
Validates email format and checks domain blacklist.
  @throws {ValidationError} If format invalid or domain blacklisted
  @example
    validateEmail('user@example.com'); // OK
    validateEmail('spam@blocked.com'); // throws
*/
function validateEmail(email: string): void {}

TODO Format

// ✅ GOOD: Actionable with ticket
// TODO(JIRA-567): Replace with batch API when available Q1 2025

// ❌ BAD: No context
// TODO: fix this later

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Refactor Before Commenting

Instead of commenting...	Refactor to...
// Get active users	const activeUsers = users.filter(u => u.isActive)
// Check if admin	const isAdmin = user.role === 'admin'
// 86400000 ms = 1 day	const ONE_DAY_MS = 24 * 60 * 60 * 1000
// Handle error case	Extract to handleAuthError(err) function
// Calculate total with tax	const totalWithTax = calculateTotalWithTax(items)

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Audit Checklist

When reviewing code comments:

1. Necessity: For new code, can it be self-documenting? For existing code, is this comment still accurate? If accurate, keep it.
2. Accuracy: Does comment match current code behavior?
3. Value: Does it explain WHY, not WHAT?
4. Freshness: Is it still relevant?
5. Actionability: If TODO, does it have a ticket reference?

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Language-Specific Patterns

TypeScript/JavaScript

• Prefer TypeScript types over JSDoc type annotations
• Use @deprecated JSDoc tag for deprecated APIs
• Document thrown errors in JSDoc when not obvious

Go

• Follow effective Go: first sentence is function name + verb
• Document exported functions, unexported can be brief
• Use // Deprecated: comment prefix

Python

• Use docstrings for modules, classes, functions
• Follow Google or NumPy docstring format consistently
• Type hints reduce need for parameter documentation