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self-explanatory-code-commenting.instructions.md

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description applyTo
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.
**

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

// Bad: States the obvious
let counter = 0;  // Initialize counter to zero
counter++;  // Increment counter by one

Redundant Comments

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

Outdated Comments

// 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

// 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

// 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

// Good: Explains what the regex matches
// Match email format: username@domain.extension
const emailPattern = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;

API Constraints or Gotchas

// 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

/**
 * 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

// 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

// 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

// Bad: Don't comment out code
// const oldFunction = () => { ... };
const newFunction = () => { ... };

Changelog Comments

// 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

// 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.