[j0hnik]

var friends = [ 
{ name: "Anna", books: ["Bible", "Harry Potter"], age: 21 }, 
{ name: "Bob", books: ["War and peace", "Romeo and Juliet"], age: 26 },
{ name: "Alice", books: ["The Lord of the Rings", "The Shining"], age: 18 }
]

var allbooks = friends.reduce(function(prev, curr) {
	console.log(prev);
	console.log(...prev); // опа!
  return [...prev, ...curr.books];
});