JavaScript Fundamentals

const name = "Asha";       // cannot be reassigned
let count = 0;             // can be reassigned
count += 1;

// primitives: string, number, boolean, null, undefined, symbol, bigint
// everything else is an object (arrays, functions, dates...)
console.log(typeof name, typeof count); // string number

0 == "0";            // true  (coerces types)
0 === "0";           // false (strict, prefer this)

// falsy: false, 0, "", null, undefined, NaN. Everything else is truthy
if (!"") console.log("empty string is falsy");

function add(a, b) {
  return a + b;
}

const multiply = (a, b) => a * b;   // arrow function

const greet = (name = "friend") => `Hello, ${name}!`;

const nums = [1, 2, 3, 4];

const doubled = nums.map((n) => n * 2);         // [2, 4, 6, 8]
const evens = nums.filter((n) => n % 2 === 0);  // [2, 4]
const total = nums.reduce((sum, n) => sum + n, 0); // 10

const user = { name: "Asha", age: 21 };
const { name, age } = user;                     // destructuring
const copy = { ...user, city: "Pune" };         // spread

function makeCounter() {
  let count = 0;
  return () => ++count;   // remembers `count`
}

const next = makeCounter();
next(); // 1
next(); // 2