truthy_falsy.js

let Dean = {
  name: 'Dean Winchester',
  age: 34,
  car: 'Chevy Imala (Baby)',
  enemy_demons: [ 'Abadon', 'Yellow Eyes', 'Crowley' ],
  enemy_angels: [ 'Zacharia', 'Michael', 'Lucifer' ],
  family: {
    mother: 'Mary',
    father: 'John',
  }
}

let supernatural = (cr1, cr2) => {
  let main_char = cr1;
  let second_char = cr2;

  console.log(main_char, second_char); // {...} undefined
  
  // IF FALSY (undefined, null)
  if (!second_char) {
    second_char = {
      ...main_char, 
      name: 'Sam Winchester', 
      age: 31,      
    };
    console.log(main_char, second_char);
  }
}
supernatural(Dean);

// LINE 26's OUTPUT:
/*
  {
    name: 'Dean Winchester',
    age: 34,
    car: 'Chevy Imala (Baby)',
    enemy_demons: [ 'Abadon', 'Yellow Eyes', 'Crowley' ],
    enemy_angels: [ 'Zacharia', 'Michael', 'Lucifer' ],
    family: { mother: 'Mary', father: 'John' }
  } 
  {
    name: 'Sam Winchester',
    age: 31,
    car: 'Chevy Imala (Baby)',
    enemy_demons: [ 'Abadon', 'Yellow Eyes', 'Crowley' ],
    enemy_angels: [ 'Zacharia', 'Michael', 'Lucifer' ],
    family: { mother: 'Mary', father: 'John' }
  }
*/