checking date overlaps

1.js

{
  start: new Date("January 2, 2003"),
  end: new Date("March 12, 2006")
}

2.js

var sortedRanges = dateRanges.sort((previous, current) => {
  
  // get the start date from previous and current
  var previousTime = previous.start.getTime();
  var currentTime = current.start.getTime();

  // if the previous is earlier than the current
  if (previousTime < currentTime) {
    return -1;
  }

  // if the previous time is the same as the current time
  if (previousTime === currentTime) {
    return 0;
  }

  // if the previous time is later than the current time
  return 1;
});

3.js

// start at index of 1 so there will 
// always be a previous range to use
var length = dateRanges.length - 1;
for (var i = 1; i < length; i++){
  var previous = dateRanges[i-1];
  var current = dateRanges[i];
  
  // check for overlap, here
}

4.js

sorted.reduce((val, current, idx, arr) => {
  if (idx === 0) { return; }
  var previous = arr[idx-1];
});

5.js

var previousEnd = previous.end.getTime();
var currentStart = current.start.getTime();

var overlap = (previousEnd >= currentStart)

6.js

var result = sorted.reduce((result, current, idx, arr) => {
  
  // get the previous range
  if (idx === 0) { return result; }
  var previous = arr[idx-1];
  
  // check for any overlap
  var previousEnd = previous.end.getTime();
  var currentStart = current.start.getTime();
  var overlap = (previousEnd >= currentStart);
  
  // store the result
  if (overlap) {
    // yes, there is overlap
    result.overlap = true;
    // store the specific ranges that overlap
    result.ranges.push({
      previous: previous,
      current: current
    })
  }
 
  return result;
 
   // seed the reduce  
}, {overlap: false, ranges: []});

2.js could be simplified to a single line of code:

return previous.start.getTime() - current.start.getTime();