code: function sortCollectionByKey(col, isDESC) {
    col.sort(function(a, b) {
      return getKeyLen(a) - getKeyLen(b);
    });
  
    if (isDESC) col.reverse();
  
    return col;
  }
docs: sort a collection by its elements' key(length)
  
  e.g.
  sortCollectionByKey([ {abc: 1}, {ab: 1}, {abcd: 1}, {a: 1} ])
  > [ {a: 1}, {ab: 1}, {abc: 1}, {abcd: 1} ]
  
  @param  {Array}   col
  @param  {Boolean} isDESC
  @return {Array}   (a sorted col)