Parse Xero reports from their API into a nicer format

parseXeroReport.js

/*
As it stands, Xero reports from their API are not very nice to use.  This function will return them in a flattened format.

This takes the Report data (so, `parseXeroReport(response.Reports)` where response is the raw response from the API), and parses it.
There's two formats it could take - one where it returns simply the value, and one where it returns an array of values.

  {
    "Date": "30 Sep 2018",
    "Sales": "12345.67",
    "Total Income": "12345.67",
    ...
  }

or

  {
    "Date": [
      "30 Sep 2018",
      "30 Sep 2017"
    ],
    "Some Account Name": [
      "234567.89",
      "123456.78"
    ],
    "Total Bank": [
      "234567.89",
      "123456.78"
    ]
    ...
  }

You can then use things like:
  const profitAndLoss = parseXeroReport(ProfitAndLossReport)
  profitAndLoss["Gross Profit"] = "..."
*/

function parseXeroReport (report) {
  // Internal Functions
  const arrays = el => Array.isArray(el)

  const turnIntoObject = (final, el) => {
    if (final.hasOwnProperty(el[0])) { // don't overwrite existing properties
      el[0] += ' (copy)'
    }
    
    if (el.length === 2) {
      final[el[0]] = el[1]
    } else if (el.length > 2) { // returns 'title': [value1, value2, ...]
      final[el[0]] = [
        ...el.slice(1, el.length)
      ]
    } else { // fail gracefully. set this to [] rather than '' if you are mapping the responses
      final[el[0]] = ''
    }
    return final
  }

  const getHeaderValues = (header, cell) => {
    cell.Value === ''
     ? header.push('Date') // the first one is normally blank, and is where the title goes.
     : header.push(cell.Value)
    return header
  }
  
  // Main Function
  return [].concat(...report[0].Rows.map(row => {
    if (row.RowType === 'Header') {
      return [ // return it inside an array, for the reducer below
        row.Cells.reduce(getHeaderValues, [])
      ]
    } else if (row.hasOwnProperty('Rows')) {
      return row.Rows.map(subRow => {
        return subRow.Cells.map(cell => {
          return cell.Value
        })
      })
    }
  }))
  .filter(arrays) // make sure all properties are arrays, so we can reduce them
  .reduce(turnIntoObject, {})
}