Example

const movies = [
  ['The Day the Earth Stood Still', 'Superman', 'Ghostbusters'],
  ['Finding Dory'],
  ['Jaws', 'On the Waterfront']
]

const flatMovies = movies.reduce((arr, innerMovies) => [ ...arr, ...innerMovies], []);

console.log(flatMovies);   // Result: ['The Day the Earth Stood Still', 'Superman', 'Ghostbusters', 'Finding Dory', 'Jaws', 'On the Waterfront']

This time, the initial value of reduce’s accumulator is an empty array [].

Spread operator ... allows an iterable such as an array expression or string to be expanded in places.

Example

Reduce the current array of user objects into an array strings containing favorite books.

const users = [
  {
    name: 'Samir',
    age: 27,
    favoriteBooks:[
      {title: 'The Iliad'},
      {title: 'The Brothers Karamazov'}
    ]
  },
  {
    name: 'Angela',
    age: 33,
    favoriteBooks:[
      {title: 'Tenth of December'},
      {title: 'Cloud Atlas'},
      {title: 'One Hundred Years of Solitude'}
    ]
  },
  {
    name: 'Beatrice',
    age: 42,
    favoriteBooks:[
      {title: 'Candide'}
    ]
  }
];

const books = users
   .map(user => user.favoriteBooks.map( userBooks => userBooks.title ))
   .reduce((acc, cur) => [...acc, ...cur], []);   

console.log(books);  // Result: ['The Iliad', 'The Brothers Karamazov', 'Tenth of December', 'Cloud Atlas', 'One Hundred Years of Solitude', 'Candide'];

Example

Using the reduce method, extract all the customer hobbies into their own array. Store the hobbies in the hobbies array. You can use the comments below for reference.

const customers = [
  {
    name: "Tyrone",
    personal: {
      age: 33,
      hobbies: ["Bicycling", "Camping"]
    }
  },
  {
    name: "Elizabeth",
    personal: {
      age: 25,
      hobbies: ["Guitar", "Reading", "Gardening"]
    }
  },
  {
    name: "Penny",
    personal: {
      age: 36,
      hobbies: ["Comics", "Chess", "Legos"]
    }
  }
];
let hobbies;

hobbies = customers
   .map( customer => customer.personal.hobbies)
  .reduce((acc, cur) => [...acc, ...cur], []);

console.log(hobbies)  // hobbies should be: ["Bicycling", "Camping", "Guitar", "Reading", "Gardening", "Comics", "Chess", "Legos"]