code: function someCollections(db, collections, next) {
  
    var last = ~~collections.length, counter = 0;
    if (last === 0) { // empty set
      return next(null);
    }
  
    collections.forEach(function(collection) {
  
      db.collection(collection, function(err, collection) {
  
        logger('select collection ' + collection.collectionName);
        if (err) {
          return last === ++counter ? next(err) : error(err);
        }
        collection.drop(function(err) {
  
          if (err) {
            error(err); // log if missing
          }
          return last === ++counter ? next(null) : null;
        });
      });
    });
  }
docs: drop data from some collections
  
  @function someCollections
  @param {Object} db - database
  @param {Array} collections - selected collections
  @param {Function} next - callback