console.log("a" && "b", 0 && "b", "a" || "b", "" || "b");
console.log(null || undefined || 0);
const user = { name: "Kai" };
user && console.log("logged in as", user.name);   // guard idiom

b 0 a b
0
logged in as Kai

const order = { customer: { address: null }, items: [], total() { return 42; } };
console.log(order.customer?.address?.city);
console.log(order.shipping?.method ?? "standard");
console.log(order.items?.[0], order.total?.(), order.refund?.());
console.log(order.customer.address?.city.toUpperCase());  // stops at null

undefined
standard
undefined 42 undefined
undefined

const opts = { retries: 0, name: "", cache: null };
opts.retries ||= 3;   // assigns if falsy
opts.name &&= opts.name.trim();  // assigns if truthy
opts.cache ??= new Map();        // assigns if null/undefined
opts.timeout ??= 1000;
console.log(opts);

{ retries: 3, name: '', cache: Map(0) {}, timeout: 1000 }

const a = [1, 2], b = [3, 4];
console.log([...a, ...b], Math.max(...b));
const base = { x: 1, y: 2 };
console.log({ ...base, y: 20, z: 3 });
function sum(...nums) { return nums.reduce((s, n) => s + n, 0); }
const [first, ...others] = [10, 20, 30];
console.log(sum(1, 2, 3), first, others);

[ 1, 2, 3, 4 ] 4
{ x: 1, y: 20, z: 3 }
6 10 [ 20, 30 ]

let i = 5;
console.log(i++, i, ++i);
console.log(2 ** 3 ** 2, (2 ** 3) ** 2);
console.log("x" in { x: undefined }, void 0, (1, 2, 3));
console.log(5 & 3, 5 | 3, 5 ^ 3, ~5, 1 << 4, -16 >> 2, -16 >>> 28);

5 6 7
512 64
true undefined 3
1 7 6 -6 16 -4 15