const arr = [
  {
    id: 10,
    title: "Test 10",
    item: "Item 1"
  },
  {
    id: 10,
    title: "Test 10",
    item: "Item 2"
  },
  {
    id: 11,
    title: "Test 11",
    item: "Item 1"
  },
  {
    id: 11,
    title: "Test 11",
    item: "Item 2"
  },
  {
    id: 11,
    title: "Test 11",
    item: "Item 3"
  },
  {
    id: 11,
    title: "Test 11",
    item: "Item 4"
  },
]

function convertArr(arr) {
  return arr
  .filter((e, i, a) =>

    !a.slice(0, i).some(other => other.id === e.id)
  )
   .map(({id, title}) => ({
     id,
     title,
     items:
        arr
          .filter(el => el.id === id)
          .map(({item}) => ({item}))
    }))
}

console.log(convertArr(arr))