function show(a, b) {
  console.log(a, b, arguments.length);
}
show(1);
show(1, 2, 3);

1 undefined 1
1 2 3

function greet(name = "guest", greeting = `Hello, ${name}`) {
  return greeting;
}
console.log(greet());
console.log(greet("Asha"));
console.log(greet(undefined, "Hey"));   // undefined triggers the default
console.log(greet(null));               // null does not

Hello, guest
Hello, Asha
Hey
Hello, null

function append(item, list = []) {
  list.push(item);
  return list;
}
console.log(append(1), append(2));
let counter = 0;
const next = (id = ++counter) => id;
console.log(next(), next(), next(99), next());

[ 1 ] [ 2 ]
1 2 99 3

function oldStyle() {
  console.log(Array.isArray(arguments), Array.from(arguments));
}
function modern(first, ...rest) {
  console.log(first, rest, Array.isArray(rest));
}
oldStyle(1, 2);
modern(1, 2, 3);
modern(1);

false [ 1, 2 ]
1 [ 2, 3 ] true
1 [] true

function createUser({ name, role = "viewer", active = true } = {}) {
  return `${name} (${role}${active ? "" : ", inactive"})`;
}
console.log(createUser({ name: "Ravi" }));
console.log(createUser({ name: "Mei", role: "admin", active: false }));
console.log(createUser());

Ravi (viewer)
Mei (admin, inactive)
undefined (viewer)