Developer Snippet Playground

import { lazy, Suspense } from "react";

// Lazy-load heavy component — bundle is fetched on first render
const HeavyChart = lazy(() => import("./HeavyChart"));

export default function Dashboard() {
  return (
    <Suspense
      fallback={
        <div className="animate-pulse h-64 rounded-xl bg-gray-100 dark:bg-neutral-800" />
      }
    >
      <HeavyChart />
    </Suspense>
  );
}

import { useState, useTransition } from "react";

export function SearchResults({ items }: { items: string[] }) {
  const [query, setQuery] = useState("");
  const [filtered, setFiltered] = useState(items);
  const [isPending, startTransition] = useTransition();

  const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    const val = e.target.value;
    setQuery(val); // urgent — update input immediately
    startTransition(() => {
      // non-urgent — filter can be deferred
      setFiltered(items.filter((i) => i.toLowerCase().includes(val.toLowerCase())));
    });
  };

  return (
    <>
      <input value={query} onChange={handleChange} placeholder="Search..." />
      {isPending && <span className="text-xs text-gray-400">Updating…</span>}
      <ul>
        {filtered.map((item) => <li key={item}>{item}</li>)}
      </ul>
    </>
  );
}

import { useDeferredValue, useState, memo } from "react";

const ExpensiveList = memo(function ExpensiveList({ query }: { query: string }) {
  // Imagine this renders 10,000 items
  const results = heavyFilter(query);
  return <ul>{results.map((r) => <li key={r}>{r}</li>)}</ul>;
});

export default function Search() {
  const [query, setQuery] = useState("");
  const deferred = useDeferredValue(query); // lags behind — keeps old results visible

  const isStale = query !== deferred;

  return (
    <>
      <input value={query} onChange={(e) => setQuery(e.target.value)} />
      <div style={{ opacity: isStale ? 0.5 : 1, transition: "opacity 0.2s" }}>
        <ExpensiveList query={deferred} />
      </div>
    </>
  );
}

function heavyFilter(q: string): string[] {
  return Array.from({ length: 1000 }, (_, i) => `Item ${i}`)
    .filter((s) => s.toLowerCase().includes(q.toLowerCase()));
}

async function fetchWithRetry(
  url: string,
  options: RequestInit = {},
  retries = 3,
  timeoutMs = 5000
): Promise<Response> {
  for (let attempt = 0; attempt <= retries; attempt++) {
    const controller = new AbortController();
    const id = setTimeout(() => controller.abort(), timeoutMs);

    try {
      const res = await fetch(url, { ...options, signal: controller.signal });
      clearTimeout(id);
      if (!res.ok) throw new Error(`HTTP ${res.status}`);
      return res;
    } catch (err) {
      clearTimeout(id);
      if (attempt === retries) throw err;
      // Exponential back-off: 200ms, 400ms, 800ms …
      await new Promise((r) => setTimeout(r, 200 * 2 ** attempt));
    }
  }
  throw new Error("fetchWithRetry: exhausted all retries");
}

// Usage
const data = await fetchWithRetry("/api/data").then((r) => r.json());

import { useState, useEffect } from "react";

export function useSSEStream(url: string) {
  const [messages, setMessages] = useState<string[]>([]);
  const [error, setError] = useState<string | null>(null);

  useEffect(() => {
    const source = new EventSource(url);

    source.onmessage = (event) => {
      setMessages((prev) => [...prev, event.data]);
    };

    source.onerror = () => {
      setError("Stream connection lost");
      source.close();
    };

    return () => source.close(); // cleanup on unmount
  }, [url]);

  return { messages, error };
}

// Usage in component
function LiveFeed() {
  const { messages, error } = useSSEStream("/api/stream");
  return (
    <div>
      {error && <p className="text-red-500">{error}</p>}
      {messages.map((msg, i) => <p key={i}>{msg}</p>)}
    </div>
  );
}

import { useState } from "react";

type Todo = { id: number; text: string; done: boolean };

export function useTodos(initial: Todo[]) {
  const [todos, setTodos] = useState<Todo[]>(initial);

  async function toggleTodo(id: number) {
    // 1. Snapshot current state for rollback
    const prev = todos;

    // 2. Optimistically update UI
    setTodos((t) =>
      t.map((todo) => (todo.id === id ? { ...todo, done: !todo.done } : todo))
    );

    try {
      // 3. Confirm with server
      await fetch(`/api/todos/${id}/toggle`, { method: "PATCH" });
    } catch {
      // 4. Rollback on failure
      setTodos(prev);
      alert("Update failed — changes reverted");
    }
  }

  return { todos, toggleTodo };
}

// lib/rateLimit.ts
const rateLimitMap = new Map<string, { count: number; resetAt: number }>();

export function rateLimit(
  ip: string,
  limit = 60,
  windowMs = 60_000
): { allowed: boolean; remaining: number } {
  const now = Date.now();
  const entry = rateLimitMap.get(ip);

  if (!entry || now > entry.resetAt) {
    rateLimitMap.set(ip, { count: 1, resetAt: now + windowMs });
    return { allowed: true, remaining: limit - 1 };
  }

  if (entry.count >= limit) {
    return { allowed: false, remaining: 0 };
  }

  entry.count++;
  return { allowed: true, remaining: limit - entry.count };
}

// middleware.ts
import { NextRequest, NextResponse } from "next/server";
import { rateLimit } from "@/lib/rateLimit";

export function middleware(req: NextRequest) {
  const ip = req.headers.get("x-forwarded-for") ?? "127.0.0.1";
  const { allowed, remaining } = rateLimit(ip);

  if (!allowed) {
    return NextResponse.json({ error: "Too Many Requests" }, { status: 429 });
  }

  const res = NextResponse.next();
  res.headers.set("X-RateLimit-Remaining", String(remaining));
  return res;
}

export const config = { matcher: "/api/:path*" };

// Works in both browser and Node.js (JSDOM) environments
const ALLOWED_TAGS = new Set(["b", "i", "em", "strong", "a", "p", "br", "ul", "li"]);
const ALLOWED_ATTRS: Record<string, string[]> = { a: ["href", "rel", "target"] };

export function sanitizeHTML(dirty: string): string {
  const template = document.createElement("template");
  template.innerHTML = dirty;

  function clean(node: Element) {
    for (const child of [...node.childNodes]) {
      if (child.nodeType === Node.ELEMENT_NODE) {
        const el = child as Element;
        const tag = el.tagName.toLowerCase();

        if (!ALLOWED_TAGS.has(tag)) {
          el.replaceWith(...el.childNodes);
        } else {
          // Remove disallowed attributes
          for (const attr of [...el.attributes]) {
            const allowed = ALLOWED_ATTRS[tag] ?? [];
            if (!allowed.includes(attr.name)) el.removeAttribute(attr.name);
          }
          // Force safe link behaviour
          if (tag === "a") {
            el.setAttribute("rel", "noopener noreferrer");
            el.setAttribute("target", "_blank");
          }
          clean(el);
        }
      }
    }
  }

  clean(template.content as unknown as Element);
  return template.innerHTML;
}

let isRefreshing = false;
let queue: Array<(token: string) => void> = [];

async function refreshToken(): Promise<string> {
  const res = await fetch("/api/auth/refresh", { method: "POST", credentials: "include" });
  if (!res.ok) throw new Error("Session expired");
  const { accessToken } = await res.json();
  return accessToken;
}

export async function authFetch(url: string, init: RequestInit = {}): Promise<Response> {
  const token = localStorage.getItem("access_token");
  const headers = { ...init.headers, Authorization: `Bearer ${token}` };

  let res = await fetch(url, { ...init, headers });

  if (res.status !== 401) return res;

  if (!isRefreshing) {
    isRefreshing = true;
    try {
      const newToken = await refreshToken();
      localStorage.setItem("access_token", newToken);
      queue.forEach((cb) => cb(newToken));
    } finally {
      isRefreshing = false;
      queue = [];
    }
  }

  // Wait for ongoing refresh then retry
  const newToken = await new Promise<string>((resolve) => queue.push(resolve));
  return fetch(url, { ...init, headers: { ...headers, Authorization: `Bearer ${newToken}` } });
}

type DeepPartial<T> = T extends object
  ? { [P in keyof T]?: DeepPartial<T[P]> }
  : T;

// Example
type Config = {
  server: { host: string; port: number };
  db: { url: string; pool: { min: number; max: number } };
};

function patchConfig(base: Config, patch: DeepPartial<Config>): Config {
  return JSON.parse(JSON.stringify({ ...base, ...patch }));
}

const updated = patchConfig(
  { server: { host: "localhost", port: 3000 }, db: { url: "postgres://…", pool: { min: 1, max: 10 } } },
  { server: { port: 4000 } }  // only override port — all nested types are optional
);

// Without Prettify: hover shows "UserBase & WithTimestamps & WithRole"
// With Prettify: hover shows every single resolved property

type Prettify<T> = { [K in keyof T]: T[K] } & {};

type UserBase = { id: string; email: string };
type WithTimestamps = { createdAt: Date; updatedAt: Date };
type WithRole = { role: "admin" | "user" | "guest" };

// Before: hard to read in IDE
type UserRaw = UserBase & WithTimestamps & WithRole;

// After: all properties visible at a glance
type User = Prettify<UserBase & WithTimestamps & WithRole>;
// Hover tooltip: { id: string; email: string; createdAt: Date; updatedAt: Date; role: "admin" | "user" | "guest" }

function createUser(data: User): User {
  return data;
}

type Palette = Record<string, [number, number, number] | string>;

// ❌ With "as": no error, but loses specific type info
const palette1 = {
  red: [255, 0, 0],
  green: "#00ff00",
} as Palette;

// ✅ With "satisfies": TypeScript validates the shape AND
//    keeps each property's specific type
const palette = {
  red: [255, 0, 0],
  green: "#00ff00",
} satisfies Palette;

// palette.red is still typed as [number, number, number] — not the wider string | [number,number,number]
palette.red.at(0); // ✅ TypeScript knows .at() is valid on a tuple
palette.green.toUpperCase(); // ✅ TypeScript knows it's a string

// Practical use: config objects, theme tokens, route definitions
type Routes = Record<string, { path: string; auth: boolean }>;

const ROUTES = {
  home: { path: "/", auth: false },
  dashboard: { path: "/dashboard", auth: true },
} satisfies Routes;

// ROUTES.home.path is typed as "/" (literal), not "string"

import { useState, useEffect } from "react";

export function useDebounce<T>(value: T, delay = 300): T {
  const [debounced, setDebounced] = useState<T>(value);

  useEffect(() => {
    const timer = setTimeout(() => setDebounced(value), delay);
    return () => clearTimeout(timer); // cancel if value changes within delay
  }, [value, delay]);

  return debounced;
}

// Usage
function SearchBar() {
  const [query, setQuery] = useState("");
  const debouncedQuery = useDebounce(query, 400);

  useEffect(() => {
    if (debouncedQuery) {
      fetch(`/api/search?q=${debouncedQuery}`); // only fires 400ms after user stops typing
    }
  }, [debouncedQuery]);

  return <input value={query} onChange={(e) => setQuery(e.target.value)} />;
}

import { useState, useEffect, useCallback } from "react";

export function useLocalStorage<T>(key: string, initialValue: T) {
  const [storedValue, setStoredValue] = useState<T>(() => {
    if (typeof window === "undefined") return initialValue; // SSR safe
    try {
      const item = window.localStorage.getItem(key);
      return item ? (JSON.parse(item) as T) : initialValue;
    } catch {
      return initialValue;
    }
  });

  const setValue = useCallback(
    (value: T | ((prev: T) => T)) => {
      try {
        const next = value instanceof Function ? value(storedValue) : value;
        setStoredValue(next);
        window.localStorage.setItem(key, JSON.stringify(next));
      } catch (e) {
        console.error("useLocalStorage write error:", e);
      }
    },
    [key, storedValue]
  );

  // Sync across tabs
  useEffect(() => {
    const handler = (e: StorageEvent) => {
      if (e.key === key && e.newValue) {
        setStoredValue(JSON.parse(e.newValue) as T);
      }
    };
    window.addEventListener("storage", handler);
    return () => window.removeEventListener("storage", handler);
  }, [key]);

  return [storedValue, setValue] as const;
}

import { useEffect, useRef, useState } from "react";

export function useIntersectionObserver(
  options: IntersectionObserverInit = {}
): [React.RefObject<HTMLDivElement>, boolean] {
  const ref = useRef<HTMLDivElement>(null);
  const [isVisible, setIsVisible] = useState(false);

  useEffect(() => {
    const el = ref.current;
    if (!el) return;

    const observer = new IntersectionObserver(([entry]) => {
      setIsVisible(entry.isIntersecting);
    }, { threshold: 0.1, ...options });

    observer.observe(el);
    return () => observer.disconnect();
  }, [options]);

  return [ref, isVisible];
}

// Usage: fade-in on scroll
function AnimatedSection() {
  const [ref, isVisible] = useIntersectionObserver();

  return (
    <div
      ref={ref}
      className={`transition-all duration-700 ${
        isVisible ? "opacity-100 translate-y-0" : "opacity-0 translate-y-8"
      }`}
    >
      <h2>I fade in when scrolled into view!</h2>
    </div>
  );
}

import { useEffect, useState } from "react";

export function useMediaQuery(query: string): boolean {
  const [matches, setMatches] = useState(false);

  useEffect(() => {
    if (typeof window === "undefined") return;
    const mql = window.matchMedia(query);
    setMatches(mql.matches);

    const handler = (e: MediaQueryListEvent) => setMatches(e.matches);
    mql.addEventListener("change", handler);
    return () => mql.removeEventListener("change", handler);
  }, [query]);

  return matches;
}

// Usage
function ResponsiveNav() {
  const isMobile = useMediaQuery("(max-width: 768px)");
  const prefersReducedMotion = useMediaQuery("(prefers-reduced-motion: reduce)");

  return isMobile ? <MobileMenu /> : <DesktopNav />;
}

import { Request, Response, NextFunction, RequestHandler } from "express";

// Wrap any async route handler — no more try/catch in every route!
export const asyncHandler =
  (fn: (req: Request, res: Response, next: NextFunction) => Promise<unknown>): RequestHandler =>
  (req, res, next) => {
    Promise.resolve(fn(req, res, next)).catch(next);
  };

// Global error middleware (register last)
export function errorMiddleware(
  err: Error,
  _req: Request,
  res: Response,
  _next: NextFunction
) {
  console.error(err.stack);
  res.status(500).json({ error: err.message ?? "Internal Server Error" });
}

// Usage in routes
import express from "express";
import { asyncHandler, errorMiddleware } from "./middleware";

const app = express();

app.get("/users/:id", asyncHandler(async (req, res) => {
  const user = await db.findUser(req.params.id); // any thrown error is auto-forwarded
  res.json(user);
}));

app.use(errorMiddleware);

import { z } from "zod";

const envSchema = z.object({
  NODE_ENV: z.enum(["development", "test", "production"]).default("development"),
  DATABASE_URL: z.string().url("DATABASE_URL must be a valid URL"),
  PORT: z.coerce.number().int().positive().default(3000),
  JWT_SECRET: z.string().min(32, "JWT_SECRET must be at least 32 characters"),
  REDIS_URL: z.string().url().optional(),
});

// Parse and export — throws at startup if any required var is missing/invalid
export const env = envSchema.parse(process.env);

// Usage anywhere in codebase — fully typed!
import { env } from "@/lib/env";
const port = env.PORT; // type: number (not string!)

import { createClient } from "redis";

const redis = createClient({ url: process.env.REDIS_URL });
await redis.connect();

export async function cached<T>(
  key: string,
  ttlSeconds: number,
  fetcher: () => Promise<T>
): Promise<T> {
  // 1. Check cache
  const hit = await redis.get(key);
  if (hit) return JSON.parse(hit) as T;

  // 2. Cache miss — fetch from source
  const data = await fetcher();

  // 3. Populate cache (fire-and-forget)
  redis.setEx(key, ttlSeconds, JSON.stringify(data)).catch(console.error);

  return data;
}

// Usage
const user = await cached(
  `user:${userId}`,
  300, // 5 min TTL
  () => db.users.findUnique({ where: { id: userId } })
);

import { vi, describe, it, expect, beforeEach } from "vitest";
import { sendWelcomeEmail } from "./email";
import { createUser } from "./users";

// vi.mock is hoisted — runs before imports
vi.mock("./email", () => ({
  sendWelcomeEmail: vi.fn().mockResolvedValue({ messageId: "mock-id" }),
}));

describe("createUser", () => {
  beforeEach(() => vi.clearAllMocks());

  it("sends a welcome email on success", async () => {
    const user = await createUser({ email: "test@example.com", name: "Alice" });

    expect(sendWelcomeEmail).toHaveBeenCalledOnce();
    expect(sendWelcomeEmail).toHaveBeenCalledWith(
      expect.objectContaining({ email: "test@example.com" })
    );
    expect(user.id).toBeDefined();
  });

  it("does not send email if user creation fails", async () => {
    vi.spyOn(db, "insert").mockRejectedValueOnce(new Error("DB error"));
    await expect(createUser({ email: "bad@example.com", name: "Bob" })).rejects.toThrow();
    expect(sendWelcomeEmail).not.toHaveBeenCalled();
  });
});

import { render, RenderOptions } from "@testing-library/react";
import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
import { ThemeProvider } from "next-themes";
import React, { PropsWithChildren } from "react";

function createTestQueryClient() {
  return new QueryClient({
    defaultOptions: { queries: { retry: false }, mutations: { retry: false } },
  });
}

function AllProviders({ children }: PropsWithChildren) {
  const queryClient = createTestQueryClient();
  return (
    <QueryClientProvider client={queryClient}>
      <ThemeProvider attribute="class" defaultTheme="light">
        {children}
      </ThemeProvider>
    </QueryClientProvider>
  );
}

export function renderWithProviders(
  ui: React.ReactElement,
  options?: RenderOptions
) {
  return render(ui, { wrapper: AllProviders, ...options });
}

// Usage in tests — no boilerplate!
import { renderWithProviders } from "@/test-utils";
import { screen } from "@testing-library/react";
import { Dashboard } from "./Dashboard";

it("renders the dashboard heading", () => {
  renderWithProviders(<Dashboard />);
  expect(screen.getByRole("heading", { name: /dashboard/i })).toBeInTheDocument();
});

import { clsx, type ClassValue } from "clsx";
import { twMerge } from "tailwind-merge";

// Merge clsx conditionals + resolve Tailwind class conflicts
export function cn(...inputs: ClassValue[]): string {
  return twMerge(clsx(inputs));
}

// ─── Usage examples ───────────────────────────────

// Conditional classes
cn("p-4 rounded-xl", isActive && "bg-blue-500 text-white");
// → "p-4 rounded-xl bg-blue-500 text-white"  (when active)

// Conflict resolution (last class wins properly)
cn("p-4", "p-2");           // → "p-2"
cn("text-red-500", "text-blue-500"); // → "text-blue-500"

// Variant component pattern
interface ButtonProps extends React.ButtonHTMLAttributes<HTMLButtonElement> {
  variant?: "primary" | "ghost" | "danger";
  size?: "sm" | "md" | "lg";
}

export function Button({ variant = "primary", size = "md", className, ...props }: ButtonProps) {
  return (
    <button
      className={cn(
        "inline-flex items-center justify-center rounded-xl font-bold transition-all",
        {
          "bg-blue-600 text-white hover:bg-blue-700": variant === "primary",
          "bg-transparent hover:bg-gray-100 dark:hover:bg-white/10": variant === "ghost",
          "bg-red-600 text-white hover:bg-red-700": variant === "danger",
          "px-3 py-1.5 text-xs": size === "sm",
          "px-4 py-2 text-sm": size === "md",
          "px-6 py-3 text-base": size === "lg",
        },
        className
      )}
      {...props}
    />
  );
}

"use client";
import { motion } from "framer-motion";

const containerVariants = {
  hidden: { opacity: 0 },
  visible: {
    opacity: 1,
    transition: {
      staggerChildren: 0.08,   // delay between each child
      delayChildren: 0.1,      // initial delay before first child
    },
  },
};

const itemVariants = {
  hidden: { opacity: 0, y: 20, scale: 0.96 },
  visible: {
    opacity: 1,
    y: 0,
    scale: 1,
    transition: { type: "spring", stiffness: 300, damping: 24 },
  },
};

interface Card { id: number; title: string }

export function CardGrid({ cards }: { cards: Card[] }) {
  return (
    <motion.ul
      className="grid grid-cols-3 gap-6"
      variants={containerVariants}
      initial="hidden"
      animate="visible"
    >
      {cards.map((card) => (
        <motion.li
          key={card.id}
          variants={itemVariants}
          className="rounded-2xl border p-6 shadow-sm"
        >
          {card.title}
        </motion.li>
      ))}
    </motion.ul>
  );
}

/* globals.css — define tokens in @layer base */
@layer base {
  :root {
    /* Colors (HSL for easy opacity control) */
    --color-brand: 221 83% 53%;       /* blue-600 */
    --color-brand-hover: 221 83% 45%;
    --color-surface: 0 0% 100%;
    --color-surface-muted: 220 14% 96%;
    --color-text: 222 47% 11%;
    --color-text-muted: 215 16% 47%;

    /* Typography */
    --font-sans: "Inter Variable", system-ui, sans-serif;
    --font-mono: "JetBrains Mono", monospace;

    /* Spacing scale */
    --space-1: 0.25rem;
    --space-2: 0.5rem;
    --space-4: 1rem;
    --space-8: 2rem;

    /* Shadows */
    --shadow-sm: 0 1px 2px rgba(0,0,0,0.05);
    --shadow-md: 0 4px 12px rgba(0,0,0,0.08);
    --shadow-lg: 0 20px 40px rgba(0,0,0,0.12);

    /* Radii */
    --radius-sm: 0.5rem;
    --radius-md: 0.75rem;
    --radius-lg: 1rem;
    --radius-xl: 1.5rem;
  }

  .dark {
    --color-surface: 222 47% 7%;
    --color-surface-muted: 222 40% 11%;
    --color-text: 210 40% 98%;
    --color-text-muted: 215 20% 65%;
    --shadow-md: 0 4px 12px rgba(0,0,0,0.4);
  }
}

/* Usage in a component */
.card {
  background: hsl(var(--color-surface));
  color: hsl(var(--color-text));
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-md);
  padding: var(--space-8);
}