const shop = "BOSS Pacific";   // cannot be reassigned
let count = 0;                 // can change
// never use var in new code

typeof 42          // "number"
typeof "x"         // "string"
typeof null        // "object" (a famous historical bug)
0.1 + 0.2 === 0.3  // false — floating point; use integers (cents) for money

Always use strict equality: === and !==.

const status = stock === 0 ? "Out of stock" : stock < 5 ? "Low" : "In stock";
for (const item of cart) { total += item.price * item.qty; }
const name = user?.profile?.name ?? "Guest";   // optional chaining, nullish coalescing