function getTags(entity) {
    return BPromise.resolve(items) // fetch database logic
      .then(function(tags) {
        console.log(tags, 'tags==================') // coming properly
        entity.tags = tags; // but assiagnment not working
        return entity;
      });
}

data.map(item => {
   getTags(item).then(updatedItem => {
     console.log(updatedItem.tags) // prints ['tag1', 'tag2'] which is item.tags
  })
})