JavaScript Cheatsheet

// ── Basics ──────────────────────────────
const x = 42;               // can't reassign (default)
let y = "hi";               // can reassign
`template ${x} literal`     // interpolation
x === y                     // always use ===, never ==
typeof x                    // "number"

// ── Functions ───────────────────────────
function add(a, b = 0) { return a + b; }
const add2 = (a, b) => a + b;          // arrow
const sq = n => n * n;                 // single param

// ── Destructuring & spread ──────────────
const { name, age = 18 } = person;
const [first, ...rest] = items;
const merged = { ...defaults, ...options };
const copy = [...arr];

// ── Control flow ────────────────────────
if (x > 10) {} else if (x > 5) {} else {}
for (const item of items) {}           // values
for (const key in obj) {}              // keys
items.forEach((item, i) => {});
const label = x > 10 ? "big" : "small";
value ?? fallback                      // null/undefined only
obj?.deep?.maybe                       // optional chaining

// ── Arrays ──────────────────────────────
arr.map(x => x * 2)
arr.filter(x => x > 0)
arr.reduce((acc, x) => acc + x, 0)
arr.find(x => x.id === 7); arr.some(f); arr.every(f)
arr.includes(3); arr.indexOf(3); arr.slice(1, 3)
arr.push(x); arr.pop(); arr.sort((a, b) => a - b)
[...new Set(arr)]                      // dedupe

// ── Objects ─────────────────────────────
Object.keys(o); Object.values(o); Object.entries(o)
JSON.stringify(o); JSON.parse(s)

// ── Async ───────────────────────────────
const res = await fetch(url);
const data = await res.json();
try { await risky(); } catch (e) { handle(e); }
await Promise.all([a(), b()]);         // concurrent
setTimeout(() => {}, 1000);

// ── Classes ─────────────────────────────
class Dog {
  #secret = "hidden";                  // private field
  constructor(name) { this.name = name; }
  bark() { return `${this.name} says woof`; }
  static create(n) { return new Dog(n); }
}

// ── DOM ─────────────────────────────────
document.querySelector(".btn")
document.querySelectorAll("li")
el.addEventListener("click", e => {})
el.textContent = "hi"; el.classList.toggle("on")