Code Explanation

Core Principle

Comments explain WHY. Code explains WHAT.

If you need a comment to explain what the code does, the code should be cleaner. If you need a comment to explain why it's done this way — write that comment.

The Four Levels of Code Explanation

| Level | Where | Explains | |---|---|---| | Inline comment | Next to specific line | Non-obvious logic or gotcha | | Function docstring | Top of function | What it does, parameters, returns, side effects | | Module header | Top of file | What this module is responsible for | | Architecture note | README / ADR | Why this approach was chosen over alternatives |

What to Comment (and What Not To)

Comment This

typescript
// Using exponential backoff here because the payment API 
// has a 429 rate limit that triggers on burst traffic
await retryWithBackoff(() => paymentAPI.charge(amount))

// Intentionally not validating here — validation happens 
// upstream in the middleware layer (see auth.middleware.ts)
const user = req.user

// This looks backwards but the API returns results 
// in reverse chronological order — newest = index 0
const latest = results[0]

Don't Comment This

typescript
// Increment counter by 1
count++

// Check if user is authenticated
if (user.isAuthenticated) {

Docstring Template

typescript
/**
 * Calculates the discount amount for a given order.
 * 
 * @param order - The order to calculate discount for
 * @param promoCode - Optional promo code to apply
 * @returns The discount amount in cents, or 0 if no discount applies
 * @throws InvalidPromoCodeError if the promo code format is invalid
 * 
 * Note: Does NOT check if the promo code is active — 
 * call validatePromoCode() first.
 */

PR Description: Explaining Your Decisions

When your PR makes non-obvious decisions, explain them:

markdown
## Technical Decisions

**Why I chose X over Y:**
[reasoning — constraints, trade-offs, what you considered]

**Known limitations:**
[what this doesn't handle and why that's acceptable for now]

**What I'd do differently with more time:**
[honest reflection — shows growth mindset]

Architecture Decision Records (ADR)

For significant technical decisions, write a short ADR:

markdown
# ADR: [decision title]

**Date:** [date]  
**Status:** Accepted

## Context
[What situation led to this decision?]

## Decision
[What did we decide to do?]

## Reasoning
[Why this over the alternatives?]

## Trade-offs
[What do we give up with this choice?]

## Consequences
[What becomes easier? What becomes harder?]

Store in /docs/decisions/ or similar — these are gold for future developers.