whole_func_string: function add(snapshot) {
      return Promise
        .resolve(mapFn(snapshot))
        .then(function(val) {
          data[snapshot.key()] = val;
        });
    }
func_code_string: function add(snapshot) {
      return Promise
        .resolve(mapFn(snapshot))
        .then(function(val) {
          data[snapshot.key()] = val;
        });
    }
func_documentation_string: add a snapshot to data
  
  @param {DataSnapshot} snapshot
  @return {Promise}