function add(snapshot) {
    return Promise
      .resolve(mapFn(snapshot))
      .then(function(val) {
        data[snapshot.key()] = val;
      });
  }
function add(snapshot) {
    return Promise
      .resolve(mapFn(snapshot))
      .then(function(val) {
        data[snapshot.key()] = val;
      });
  }
function add ( snapshot ) { return Promise . resolve ( mapFn ( snapshot ) ) . then ( function ( val ) { data [ snapshot . key ( ) ] = val ; } ) ; }
add a snapshot to data

@param {DataSnapshot} snapshot
@return {Promise}
add a snapshot to data