self-explanatory-code-commenting — GitHub Copilot rule

---
description: 'Guidelines for GitHub Copilot to write comments to achieve self-explanatory code with less comments. Examples are in JavaScript but it should work on any language that has comments.'
applyTo: '**'
---

# Self-explanatory Code Commenting Instructions

## Core Principle
**Write code that speaks for itself. Comment only when necessary to explain WHY, not WHAT.**
We do not need comments most of the time.

## Commenting Guidelines

### ❌ AVOID These Comment Types

**Obvious Comments**
```javascript
// Bad: States the obvious
let counter = 0;  // Initialize counter to zero
counter++;  // Increment counter by one
```

**Redundant Comments**
```javascript
// Bad: Comment repeats the code
function getUserName() {
    return user.name;  // Return the user's name
}
```

**Outdated Comments**
```javascript
// Bad: Comment doesn't match the code
// Calculate tax at 5% rate
const tax = price * 0.08;  // Actually 8%
```

### ✅ WRITE These Comment Types

**Complex Business Logic**
```javascript
// Good: Explains WHY this specific calculation
// Apply progressive tax brackets: 10% up to 10k, 20% above
const tax = calculateProgressiveTax(income, [0.10, 0.20], [10000]);
```

**Non-obvious Algorithms**
```javascript
// Good: Explains the algorithm choice
// Using Floyd-Warshall for all-pairs shortest paths
// because we need distances between all nodes
for (let k = 0; k < vertices; k++) {
    for (let i = 0; i < vertices; i++) {
        for (let j = 0; j < vertices; j++) {
            // ... implementation
        }
    }
}
```

**Regex Patterns**
```javascript
// Good: Explains what the regex matches
// Match email format: [email protected]
const emailPattern = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
```

**API Constraints or Gotchas**
```javascript
// Good: Explains external constraint
// GitHub API rate limit: 5000 requests/hour for authenticated users
await rateLimiter.wait();
const response = await fetch(githubApiUrl);
```

## Decision Framework

Before writing a comment, ask:
1. **Is the code self-explanatory?** → No comment needed
2. **Would a better variable/function name eliminate the need?** → Refactor instead
3. **Does this explain WHY, not WHAT?** → Good comment
4. **Will this help future maintainers?** → Good comment

## Special Cases for Comments

### Public APIs
```javascript
/**
 * Calculate compound interest using the standard formula.
 * 
 * @param {number} principal - Initial amount invested
 * @param {number} rate - Annual interest rate (as decimal, e.g., 0.05 for 5%)
 * @param {number} time - Time period in years
 * @param {number} compoundFrequency - How many times per year interest compounds (default: 1)
 * @returns {number} Final amount after compound interest
 */
function calculateCompoundInterest(principal, rate, time, compoundFrequency = 1) {
    // ... implementation
}
```

### Configuration and Constants
```javascript
// Good: Explains the source or reasoning
const MAX_RETRIES = 3;  // Based on network reliability studies
const API_TIMEOUT = 5000;  // AWS Lambda timeout is 15s, leaving buffer
```

### Annotations
```javascript
// TODO: Replace with proper user authentication after security review
// FIXME: Memory leak in production - investigate connection pooling
// HACK: Workaround for bug in library v2.1.0 - remove after upgrade
// NOTE: This implementation assumes UTC timezone for all calculations
// WARNING: This function modifies the original array instead of creating a copy
// PERF: Consider caching this result if called frequently in hot path
// SECURITY: Validate input to prevent SQL injection before using in query
// BUG: Edge case failure when array is empty - needs investigation
// REFACTOR: Extract this logic into separate utility function for reusability
// DEPRECATED: Use newApiFunction() instead - this will be removed in v3.0
```

## Anti-Patterns to Avoid

### Dead Code Comments
```javascript
// Bad: Don't comment out code
// const oldFunction = () => { ... };
const newFunction = () => { ... };
```

### Changelog Comments
```javascript
// Bad: Don't maintain history in comments
// Modified by John on 2023-01-15
// Fixed bug reported by Sarah on 2023-02-03
function processData() {
    // ... implementation
}
```

### Divider Comments
```javascript
// Bad: Don't use decorative comments
//=====================================
// UTILITY FUNCTIONS
//=====================================
```

## Quality Checklist

Before committing, ensure your comments:
- [ ] Explain WHY, not WHAT
- [ ] Are grammatically correct and clear
- [ ] Will remain accurate as code evolves
- [ ] Add genuine value to code understanding
- [ ] Are placed appropriately (above the code they describe)
- [ ] Use proper spelling and professional language

## Summary

Remember: **The best comment is the one you don't need to write because the code is self-documenting.**