Codewars challenge to find the difference between numbers in an array, after deduplicating

cw_sumOfIntervals.js

// Description below

function sumIntervals(intervals){
  const sortedArr = intervals.map((e) => e).sort((a,b) => a[0] - b[0]);
  //console.log(sortedArr)
  const refArr = sortedArr.reduce(function (acc,curr,index,array) {
    let anterior = array[index - 1];
    if(anterior !== undefined) {
      if(curr[0] < acc[acc.length - 1] && curr[1] > acc[acc.length - 1]) {
        let newArr = [acc[acc.length -1],curr[1]];
        acc.push(...newArr);
      } else if (curr[0] > acc[acc.length -1]) {
        acc.push(...curr)
      }
    } else {
      acc.push(...curr)
    }
    return acc
  })
  //console.log(refArr)
  return refArr.reduce((a,b) => b - a)
}

/*
Write a function called sumIntervals/sum_intervals() that accepts an array of intervals, and returns the sum of all the interval lengths. Overlapping intervals should only be counted once.
Intervals

Intervals are represented by a pair of integers in the form of an array. The first value of the interval will always be less than the second value. Interval example: [1, 5] is an interval from 1 to 5. The length of this interval is 4.
Overlapping Intervals

List containing overlapping intervals:

[
   [1,4],
   [7, 10],
   [3, 5]
]

The sum of the lengths of these intervals is 7. Since [1, 4] and [3, 5] overlap, we can treat the interval as [1, 5], which has a length of 4.
*/