function multiply(a, b = 1) {
  return a * b;
}

console.log(multiply(5, 2));
// Expected output: 10

console.log(multiply(5));
// Expected output: 5

function fnName(param1 = defaultValue1, /* , */ paramN = defaultValueN) {
  // 
}

function multiply(a, b) {
  return a * b;
}

multiply(5, 2); // 10
multiply(5); // NaN !

function multiply(a, b) {
  b = typeof b !== "undefined" ? b : 1;
  return a * b;
}

multiply(5, 2); // 10
multiply(5); // 5

function multiply(a, b = 1) {
  return a * b;
}

multiply(5, 2); // 10
multiply(5); // 5
multiply(5, undefined); // 5

function f(x = 1, y) {
  return [x, y];
}

f(); // [1, undefined]
f(2); // [2, undefined]

function f(a = go()) {
  function go() {
    return ":P";
  }
}

f(); // ReferenceError: go  .

function f(a, b = () => console.log(a)) {
  var a = 1;
  b();
}

f(); // undefined
f(5); // 5

function test(num = 1) {
  console.log(typeof num);
}

test(); // 'number' (num 1 )
test(undefined); // 'number' (num  1 )

//  falsy values  
test(""); // 'string' (num '' )
test(null); // 'object' (num null )

function append(value, array = []) {
  array.push(value);
  return array;
}

append(1); // [1]
append(2); // [2], [1, 2] 

function callSomething(thing = something()) {
  return thing;
}

let numberOfTimesCalled = 0;
function something() {
  numberOfTimesCalled += 1;
  return numberOfTimesCalled;
}

callSomething(); // 1
callSomething(); // 2

function greet(name, greeting, message = `${greeting} ${name}`) {
  return [name, greeting, message];
}

greet("David", "Hi"); // ["David", "Hi", "Hi David"]
greet("David", "Hi", "Happy Birthday!"); // ["David", "Hi", "Happy Birthday!"]

function go() {
  return ":P";
}

function withDefaults(
  a,
  b = 5,
  c = b,
  d = go(),
  e = this,
  f = arguments,
  g = this.value,
) {
  return [a, b, c, d, e, f, g];
}

function withoutDefaults(a, b, c, d, e, f, g) {
  switch (arguments.length) {
    case 0:
    case 1:
      b = 5;
    case 2:
      c = b;
    case 3:
      d = go();
    case 4:
      e = this;
    case 5:
      f = arguments;
    case 6:
      g = this.value;
  }
  return [a, b, c, d, e, f, g];
}

withDefaults.call({ value: "=^_^=" });
// [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]

withoutDefaults.call({ value: "=^_^=" });
// [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]

function preFilledArray([x = 1, y = 2] = []) {
  return x + y;
}

preFilledArray(); // 3
preFilledArray([]); // 3
preFilledArray([2]); // 4
preFilledArray([2, 3]); // 5

//   .
function preFilledObject({ z = 3 } = {}) {
  return z;
}

preFilledObject(); // 3
preFilledObject({}); // 3
preFilledObject({ z: 2 }); // 2