// way 1
mergedPairs.sort((a, b) => 1000 * (b.length - a.length) + 1 * (a <= b ? -1 : 1));
// way 2
mergedPairs.sort((a, b) => ((b.length - a.length) || (a <= b ? -1 : 1)));
// way 3
mergedPairs.sort(function(a, b) {
  if(b.length > a.length) return b.length - a.length;
  else if(b.length < a.length) return -1
  else {
    var values = {'1': 2, '2': 1, '=': 0}
    if (values[b[0]] > values[a[0]]) return values[b[0]] - values[a[0]];
    else if (values[b[0]] < values[a[0]]) return -1;
    else{
       return a.localeCompare(b);
    }
  }
});