var data = [[1,2,3], [4,5,6], [7,8,9]];
var flatData = data.reduce( (acc, value) => {
  return acc.concat(value);
}, []);

console.log(flatData); //[1, 2, 3, 4, 5, 6, 7, 8, 9]

var input = [
  {
    title: "Batman Begins",
    year: 2005,
    cast: [
      "Christian Bale",
      "Michael Caine",
      "Liam Neeson",
      "Katie Holmes",
      "Gary Oldman",
      "Cillian Murphy"
    ]
  },
  {
    title: "The Dark Knight",
    year: 2008,
    cast: [
      "Christian Bale",
      "Heath Ledger",
      "Aaron Eckhart",
      "Michael Caine",
      "Maggie Gyllenhal",
      "Gary Oldman",
      "Morgan Freeman"
    ]
  },
  {
    title: "The Dark Knight Rises",
    year: 2012,
    cast: [
      "Christian Bale",
      "Gary Oldman",
      "Tom Hardy",
      "Joseph Gordon-Levitt",
      "Anne Hathaway",
      "Marion Cotillard",
      "Morgan Freeman",
      "Michael Caine"
    ]
  }
];

var flatMapInput = input.reduce((acc, value)=>{
  value.cast.forEach((star)=>{
    if(acc.indexOf(star) === -1){
      acc.push(star);
    };
  });
  
  return acc;
}, []);


//["Christian Bale", "Michael Caine", "Liam Neeson", "Katie Holmes", "Gary Oldman", "Cillian Murphy", "Heath Ledger", "Aaron Eckhart", "Maggie Gyllenhal", "Morgan Freeman", "Tom Hardy", "Joseph Gordon-Levitt", "Anne Hathaway", "Marion Cotillard"]

var countDown = [1,2,3,4,"5"];

var str = countDown.reduceRight((acc, value)=>{
  return acc + value;
}, "");

console.log(str); //"54321"