Why did you choose

[version1.length, version2.length].sort()[1]; 

over the classical :

Math.max(version1.length, version2.length);

?

function solution(list){
  // takes care of undefined or empty lists
  if (!list || list === []) return "";
  // init run
  var s = [], curr = [list[0]], i = 1, l = list.length;
  for (; i<=l; i++) {
    // checks if current value is consecutive to the last one in our current run
    if (list[i] === curr[curr.length -1] + 1) {curr.push(list[i]); continue;}
    // otherwise if current run is less than 3 consecutives values
    // we push all of them in our solution
    if (curr.length < 3) {s = s.concat(curr);} 
    // otherwise we push range value "14-18"
    else {s.push(curr[0] + "-" + curr[curr.length -1]);}
    // starts new current run
    curr = [list[i]];  
  }
  return s.join(",");
}

if you have the array then you have the comma separated list (arr.join(","))