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JavaScript JavaScript Array Iteration Methods Combining Array Methods Nested Data and Additional Exploration

Why is the initial value set to []?

Why is the initial value of the reducer set to []?

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'}
    ]
  }
];

    // Result: ['The Iliad', 'The Brothers Karamazov', 'Tenth of December', 'Cloud Atlas', 'One Hundred Years of Solitude', 'Candide'];

const books  = users
 .map(user => user.favoriteBooks.map(book => book.title))
  .reduce((arr, titles) => [...arr, ...titles], []);

console.log(books);

1 Answer

Steven Parker
Steven Parker
229,004 Points

For this particular data, supplying the initial value doesn't affect the result.

But if the list happened to be empty, supplying the initial value results in an empty array instead of causing an error.