Hash Table #1

Hash Table #1.js

const groupBy = (array, prop) => {
  const obj = {};

  for (let el of array) {
    // el[prop] in obj проверяеть еще и prototype
    // Object.prototype.hasOwnProperty.call(obj, 'key') вот самый safe вариант
    // if Array.isArray(obj[key]) как вариант
    if (!(obj.hasOwnProperty(el[prop]))) {
      obj[el[prop]] = [];
    }
    obj[el[prop]].push(el);
  }

  return obj;
};

console.log(
  groupBy(
    [
      { id: 1, universe: "marvel", name: "Hulk" },
      { id: 2, universe: "marvel", name: "Iron Man" },
      { id: 3, universe: "dc", name: "Aqua Man" },
      { id: 4, universe: "dc", name: "Bat Man" },
      { id: 5, universe: "marvel", name: "Hulk" }
    ],
    "universe"
  )
);

// OUTPUT
// => {
//     marvel: [
//       { id: 1, universe: "marvel", name: "Spider Man"},
//       { id: 2, universe: "marvel", name: "Iron Man"},
//       { id: 5, universe: "marvel", name: "Hulk"},
//     ],
//     dc: [
//       { id: 3, universe: "dc", name: "Aqua Man"},
//       { id: 4, universe: "dc", name: "Bat Man"},
//     ]
// }