Express JS How to push object to an array of child object

You can do it using $push.

I think this code may help you !

router.patch('/:slug/:floorSlug/add-location', (req, res) => {
  const slug = req.params.slug;
  const { floorSlug } = req.body;
  const querySlug = '^' + slug + '$';
  const locations = req.body.locations;
  Map.findOneAndUpdate(
    {
      $and: [
        { "slug": { '$regex': querySlug, $options: 'i' } },
        { 'floors.floorSlug': floorSlug }]
    },
    {
      $push: { 'floors.$.locations': locations }
    }
  );
});