const itemGroups = [
  [1, [10, 11, 12]],
  [2, [13, 14, 15]]
]

const itemGroups = [
  [1, [10, 11, 12]],
  [2, [13, 14, 15]]
]

function moveItem(oldIndex, newIndex) {
  const groupIds = Object.keys(itemGroups)
  const [id] = groupIds.splice(oldIndex, 1)

  groupIds.splice(newIndex, 0, id)

  const ordered = groupIds.reduce((acc, id) => {
    acc.set(id, itemGroups[id])

    return acc
  }, new Map())

  return Object.fromEntries(ordered)
}

function moveItem(oldIndex, newIndex) {
  const groupIds = Object.keys(itemGroups)
  const [id] = groupIds.splice(oldIndex, 1)

  groupIds.splice(newIndex, 0, id)

  const ordered = groupIds.reduce((acc, id) => {
    acc.set(id, itemGroups[id])

    return acc
  }, new Map())

  return Object.fromEntries(ordered)
}

The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value. 

The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value.