solution.js

function groupBy(collection, it) {
  let object = {}
  const key = it instanceof Function ? it : (item) => item[it]

  collection.forEach((item) => {
    const value = key(item)
    if (object[value] === undefined) {
      object[value] = []
    }
    object[value].push(item)
  })

  return object
}

console.log(groupBy([6.1, 4.2, 6.3], Math.floor)) // { 6: [6.1, 6.3], 4: [4.2] }
console.log(groupBy(['one', 'two', 'three'], 'length')) // { 3: ['one', 'two'], 5: ['three'] }
console.log(groupBy([{ age: 23 }, { age: 24 }], 'age')) // { 23: [{age: 23}], 24: [{age: 24}] }

console.log(
  groupBy([1397639141184, 1363223700000], (timestamp) =>
    new Date(timestamp).getFullYear()
  )
)
// { 2013: [1363223700000], 2014: [1397639141184] }

console.log(
  groupBy(
    [
      { title: 'JavaScript: The Good Parts', rating: 8 },
      { title: 'Aprendiendo Git', rating: 10 },
      { title: 'Clean Code', rating: 9 },
    ],
    'rating'
  )
)
// { 8: [{ title: 'JavaScript: The Good Parts', rating: 8 }],
//   9: [{ title: 'Clean Code', rating: 9 }],
//   10: [{ title: 'Aprendiendo Git', rating: 10 }] }