无法向 js 对象添加属性

Question

I am returning a Mongoose document and wish to add some meta data to it before I send it off. I am however unable to add any properties and I am not sure why. I have checked that it is extensible with Object.isExtensible(doc) and it is.

Item.findById(req.params.id).exec(function(err, doc) {
   doc.blah = 'hello';
   console.log(doc); // No trace of 'blah'. I can change/delete existing props however
})

What could be issue?

Answer 1

Ah.. My object is a Mongoose document which doesn't allow adding properties. The solution is to either convert the returned document to a plain object or to call lean() in the query.

Item.findById(req.params.id).exec(function(err, doc) {
  var obj = doc.toObject();
  ...
});

Item.findById(req.params.id).lean().exec(function(err, doc) {      
  ...
});