reduce function

authors.js

const posts = [
  {
    id: '1',
    name: 'Post 1',
    author: {
      id: '1',
      name: 'Author 1'
    },
  },
  {
    id: '2',
    name: 'Post 2',
    author: {
      id: '1',
      name: 'Author 1'
    },
  },
  {
    id: '3',
    name: 'Post 3',
    author: {
      id: '2',
      name: 'Author 2'
    }
  },
  {
    id: '4',
    name: 'Post 4',
    author: {
      id: '2',
      name: 'Author 2'
    }
  },
  {
    author: {
      id: '3',
      name: 'Author 3'
    }
  }
];

const authors = posts.reduce((mem, input) => {
  mem[input.author.id] = {
    author: {
      id: input.author.id,
      name: input.author.name
    },
    ...(input.id
      ? {
          posts: [
            ...(mem[input.author.id] ? mem[input.author.id].posts : []),
            {
              id: input.id,
              name: input.name
            }
          ]
        }
      : {})
  };
  return mem;
}, {});

console.log(authors);