code-explanation

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.

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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

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What to Comment (and What Not To)

Comment This

// 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

// Increment counter by 1
count++

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

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Docstring Template

/**
 * 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.
 */

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PR Description: Explaining Your Decisions

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

## 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]