what does mongoose .save() return code example

Example: mongoose save method

var Tank = mongoose.model('Tank', yourSchema);

var small = new Tank({ size: 'small' });
small.save(function (err) {
  if (err) return handleError(err);
  // saved!
});

// or

Tank.create({ size: 'small' }, function (err, small) {
  if (err) return handleError(err);
  // saved!
});

// or, for inserting large batches of documents
Tank.insertMany([{ size: 'small' }], function(err) {

});