reduceGrouping.js

// group object by property
const books = [
  {
    title: "Sebuah Seni Untuk Bersikap Bodo Amat",
    author: "Mark Manson"
  },
  {
    title: "Segala - galanya Ambyar",
    author: "Mark Manson"
  },
  {
    title: "Berani Bahagia",
    author: " Ichiro Kishimi dan Fumitake Koga"
  }
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, current) {
    let key = current[property];
    if (!acc[key]) {
      acc[key] = [];
    }
    acc[key].push(current);
    return acc;
  }, {});
}

let groupBook = groupBy(books, 'author')
console.log(groupBook)

// hasilnya adalah :
// { 
//  Mark Manson: [
//   { title: "Sebuah Seni Untuk Bersikap Bodo Amat", author: "Mark Manson"},
//   { title: "Segala - galanya Ambyar", author: "Mark Manson"}
//  ],
//  Ichiro Kishimi dan Fumitake Koga: [
//   { title: "Berani Bahagia", author: " Ichiro Kishimi dan Fumitake Koga"},
//  ]
// }