1let a = 10, b = 3;
2 
3console.log(a + b);   // 13  (addition)
4console.log(a - b);   // 7   (subtraction)
5console.log(a * b);   // 30  (multiplication)
6console.log(a / b);   // 3.333...  (division)
7console.log(a % b);   // 1   (modulus - remainder)
8console.log(a ** b);  // 1000 (exponentiation - 10³)
9 
10// Increment / Decrement
11let x = 5;
12console.log(x++);  // 5 (returns, THEN increments)
13console.log(x);    // 6
14console.log(++x);  // 7 (increments, THEN returns)
15console.log(x--);  // 7 (returns, THEN decrements)
16console.log(x);    // 6

1let x = 10;
2 
3x += 5;   // x = x + 5  → 15
4x -= 3;   // x = x - 3  → 12
5x *= 2;   // x = x * 2  → 24
6x /= 4;   // x = x / 4  → 6
7x %= 4;   // x = x % 4  → 2
8x **= 3;  // x = x ** 3 → 8
9 
10console.log(x); // 8

1console.log(5 > 3);    // true
2console.log(5 < 3);    // false
3console.log(5 >= 5);   // true
4console.log(5 <= 4);   // false
5 
6// Equality
7console.log(5 == "5");   // true  (loose - coerces types)
8console.log(5 === "5");  // false (strict - no coercion)
9console.log(5 != "5");   // false (loose not-equal)
10console.log(5 !== "5");  // true  (strict not-equal)

1// AND (&&) - both must be true
2console.log(true && true);   // true
3console.log(true && false);  // false
4 
5// OR (||) - at least one must be true
6console.log(false || true);  // true
7console.log(false || false); // false
8 
9// NOT (!) - inverts the value
10console.log(!true);   // false
11console.log(!false);  // true
12console.log(!0);      // true  (0 is falsy)
13console.log(!"");     // true  ("" is falsy)
14 
15// Double NOT (!!) converts to boolean
16console.log(!!"hello"); // true
17console.log(!!0);        // false

1// && returns the first falsy value, or the last value
2console.log("hello" && 42);     // 42 (both truthy, returns last)
3console.log(0 && "hello");      // 0 (first is falsy, returns it)
4console.log("a" && "b" && "c"); // "c"
5 
6// || returns the first truthy value, or the last value
7console.log("" || "default");       // "default"
8console.log(null || undefined || 0); // 0 (all falsy, returns last)
9 
10// Practical: default values
11const username = userInput || "Guest";
12 
13// ?? (Nullish coalescing) - only checks null/undefined
14const count = 0;
15console.log(count || 10);  // 10 (0 is falsy)
16console.log(count ?? 10);  // 0  (0 is NOT null/undefined)

1const age = 20;
2const status = age >= 18 ? "Adult" : "Minor";
3console.log(status); // "Adult"
4 
5// Nested (avoid for readability)
6const grade = score >= 90 ? "A" : score >= 80 ? "B" : score >= 70 ? "C" : "F";

1// Concatenation with +
2const first = "Hello";
3const second = "World";
4console.log(first + " " + second); // "Hello World"
5 
6// Template literal (preferred)
7console.log(`${first} ${second}`); // "Hello World"
8 
9// += for appending
10let message = "Hello";
11message += " World";
12console.log(message); // "Hello World"

1const user = {
2name: "Alice",
3address: { city: "NYC" }
4};
5 
6console.log(user.address.city);      // "NYC"
7console.log(user.phone?.number);     // undefined (no error!)
8// Without ?.: user.phone.number → TypeError!
9 
10// Works with methods too
11console.log(user.greet?.());         // undefined
12console.log([1,2,3]?.[5]);           // undefined

1// Spread (...) - expands arrays/objects
2const nums = [1, 2, 3];
3const more = [...nums, 4, 5]; // [1, 2, 3, 4, 5]
4 
5const obj = { a: 1, b: 2 };
6const extended = { ...obj, c: 3 }; // { a: 1, b: 2, c: 3 }
7 
8// Rest (...) - collects remaining arguments
9function sum(...numbers) {
10return numbers.reduce((total, n) => total + n, 0);
11}
12console.log(sum(1, 2, 3, 4)); // 10

1function calculatePrice(price, discount = 0, taxRate = 0.08) {
2const discountAmount = price * (discount / 100);
3const subtotal = price - discountAmount;
4const tax = subtotal * taxRate;
5const total = subtotal + tax;
6 
7return {
8  original: price,
9  discount: discountAmount,
10  tax: tax.toFixed(2),
11  total: total.toFixed(2),
12  saved: discountAmount > 0 ? `You saved $${discountAmount.toFixed(2)}!` : null,
13};
14}
15 
16const result = calculatePrice(100, 20);
17console.log(result);
18// { original: 100, discount: 20, tax: "6.40", total: "86.40", saved: "You saved $20.00!" }