Chapter 121 — Night-Before Revision Sheet

One page. Read it slowly. Sleep early.

🟡 JavaScript

• Closure → inner function + lexical scope reference.
• Hoisting → declarations move up; let/const in TDZ.
• Scope → global → function → block; lexical lookup.
• this → call-site dependent; arrow inherits.
• Prototype → instance.__proto__ = Constructor.prototype.
• == vs === → always ===.
• Event bubbling/delegation → one listener on parent for many children.
• Array methods → map, filter, reduce, find, some, every, flatMap.
• Promise.all → parallel, fail-fast. allSettled → wait for all.
• Debounce → wait until silent. Throttle → at most once per N ms.

// Closure
function counter() {
  let n = 0;
  return () => ++n;
}

// Debounce
const debounce = (fn, ms) => {
  let t;
  return (...args) => { clearTimeout(t); t = setTimeout(() => fn(...args), ms); };
};

⚛️ React

• Components → functional + hooks.
• State → immutable updates; functional form for dependent updates.
• useEffect → cleanup runs before next effect / on unmount.
• useMemo for values, useCallback for functions.
• useRef → mutable, no re-render.
• React.memo + stable props = fewer re-renders.
• Reconciliation uses element type + key to diff.
• Lift state up, push it down only as far as needed.
• Context for low-frequency global data; split contexts to limit re-renders.
• Code splitting with React.lazy + Suspense.
• List virtualization with react-window for huge lists.

// Custom debounce hook
function useDebounce(v, delay = 300) {
  const [d, setD] = useState(v);
  useEffect(() => {
    const id = setTimeout(() => setD(v), delay);
    return () => clearTimeout(id);
  }, [v, delay]);
  return d;
}

🟢 Node.js

• Event loop phases: timers → I/O → check → close, microtasks between.
• Async/await is sugar over Promises; always returns a Promise.
• Centralized error middleware with AppError.
• JWT → short access (15m) + long refresh (httpOnly cookie, rotated).
• REST → noun URLs, verb operations, status codes.
• MVC + service layer → thin controllers, fat services.
• Rate limit auth routes (5 attempts / 15min).
• Pagination → cursor at scale, offset for simple cases.
• S3 pre-signed URLs for large uploads.

// Async wrapper
const asyncHandler = (fn) => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next);

// JWT middleware
function auth(req, res, next) {
  try {
    req.user = jwt.verify(
      req.headers.authorization?.split(" ")[1],
      process.env.JWT_SECRET
    );
    next();
  } catch {
    res.status(401).json({ error: "Invalid token" });
  }
}

🔵 NestJS

• Modules, controllers, providers, DI.
• Pipes validate inputs, guards authorize, interceptors transform, filters catch errors.
• DTOs + class-validator + whitelist: true.
• @Injectable() + constructor injection.
• Swagger auto-docs from decorators.

@Controller("users")
@UseGuards(JwtAuthGuard)
export class UserController {
  constructor(private readonly userService: UserService) {}

  @Get(":id")
  @Roles("admin")
  get(@Param("id", ParseUUIDPipe) id: string) {
    return this.userService.findById(id);
  }
}

🟣 MongoDB

• Use .explain("executionStats") to confirm IXSCAN.
• $match early in aggregation pipeline.
• ESR rule for compound indexes: Equality → Sort → Range.
• populate = extra query; $lookup = DB-side join.
• Embed small bounded sub-docs; reference when independent / unbounded.
• Cursor pagination with _id > lastId scales infinitely.
• TTL index = auto-expire docs.

// Pipeline
[
  { $match: { status: "paid" } },
  { $group: { _id: "$userId", total: { $sum: "$amount" } } },
  { $sort: { total: -1 } },
  { $limit: 10 },
]

🟠 System Design

• Stateless API → horizontally scale behind LB.
• Cache in layers (browser → CDN → Redis → DB).
• Cache-aside is the default pattern; invalidate on writes.
• Queue anything slow (email, image, webhook).
• Modular monolith > microservices for small teams.
• OWASP: validate input, hash passwords (bcrypt 10+), HTTPS, Helmet, allowlist CORS, rate-limit auth.
• Observability: structured logs, metrics, tracing (RED method).

💬 HR

• Self-intro: 90s, 4 beats (role / day-to-day / achievement / why TCS).
• STAR for behavioral.
• Numbers > adjectives.
• Use "I" for your contribution.
• Always ask questions back.

🌅 Morning of

• Eat well. Sleep > caffeine.
• Run through self-intro once.
• Skim this sheet. Stop revising after that.
• Test mic/camera (remote) or arrive 30 min early (in-person).

🎯 If You Have 60 Seconds

"Closures, hooks, event loop, JWT, REST, indexes, $match early, cache-aside, STAR, numbers > adjectives, ask questions."

That's your kernel.

You've got this. 🚀

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