reorder.js

function reorder (state, data) {
  const { from, to } = data

  const rowsCopy = state.rows.slice()

  const fromRowCopy = rowsCopy[from.row].slice()
  const item = fromRowCopy[from.index]
  fromRowCopy.splice(from.index, 1) // remove from row
  rowsCopy[from.row] = fromRowCopy // replace 'from' row

  const toRowCopy = rowsCopy[to.row].slice()
  toRowCopy.splice(to.index, 0, item) // add to row
  rowsCopy[to.row] = toRowCopy // replace 'to' row

  return { rows: rowsCopy }
}

test('move column between rows', (t) => {
  t.plan(1)

  const state = Object.freeze({
    rows: [
      [ {a: 'a'}, {b: 'b'}, {c: 'c'} ], // move b from here
      [ {d: 'd'}, {e: 'e'} ]
    ]
  })
  const expected = {
    rows: [
      [ {a: 'a'}, {c: 'c'} ],
      [ {d: 'd'}, {b: 'b'}, {e: 'e'} ] // to here
    ]
  }
  const payload = {
    from: { row: 0, index: 1 },
    to: { row: 1, index: 1 }
  }

  const result = reorder(state, payload)
  t.deepEqual(result, expected)
})

test('move column within same row', (t) => {
  t.plan(1)

  const state = Object.freeze({
    rows: [
      [ {a: 'a'}, {b: 'b'}, {c: 'c'} ],
      [ {d: 'd'}, {e: 'e'} ]
    ]
  })
  const expected = {
    rows: [
      [ {a: 'a'}, {c: 'c'}, {b: 'b'} ],
      [ {d: 'd'}, {e: 'e'} ]
    ]
  }
  const payload = {
    from: { row: 0, index: 1 },
    to: { row: 0, index: 2 }
  }

  const result = reorder(state, payload)
  t.deepEqual(result, expected)
})