promissinha.js

const fn = ( f, key ) => ( e ) => ( e ) ? f( e ) : f( key )

const getKey = ( database, table, userUID, child ) => 
    database.ref( table )
            .child( userUID )
            .child( child )
            .push()

const create = ( userUID, customerObj ) => {
    return new Promise( ( resolve, reject ) =>
      getKey( database, 'user_data', userUID )
        .key
        .set( customerObj )
        .then( fn(resolve, ref.key) )
        .catch( fn(reject) ) )
}