How do you turn a Mongoose document into a plain object?

I have a document from a mongoose find that I want to extend before JSON encoding and sending out as a response. If I try adding properties to the doc it is ignored. The properties don't appear in Object.getOwnPropertyNames(doc) making a normal extend not possible. The strange thing is that JSON.parse(JSON.encode(doc)) works and returns an object with all of the correct properties. Is there a better way to do this?

Mongoose Models inherit from Documents, which have a toObject() method. I believe what you're looking for should be the result of doc.toObject().

http://mongoosejs.com/docs/api.html#document_Document-toObject

Another way to do this is to tell Mongoose that all you need is a plain JavaScript version of the returned doc by using lean() in the query chain. That way Mongoose skips the step of creating the full model instance and you directly get a doc you can modify:

MyModel.findOne().lean().exec(function(err, doc) {
    doc.addedProperty = 'foobar';
    res.json(doc);
});

the fast way if the property is not in the model :

document.set( key,value, { strict: false });

JohnnyHK suggestion:

In some cases as @JohnnyHK suggested, you would want to get the Object as a Plain Javascript. as described in this Mongoose Documentation there is another alternative to query the data directly as object:

const docs = await Model.find().lean();

Conditionally return Plain Object:

In addition if someone might want to conditionally turn to an object,it is also possible as an option argument, see find() docs at the third parameter:

const toObject = true;
const docs = await Model.find({},null,{lean:toObject});

its available on the functions: find(), findOne(), findById(), findOneAndUpdate(), and findByIdAndUpdate().

NOTE:

it is also worth mentioning that the _id attribute isn't a string object as if you would do JSON.parse(JSON.stringify(object)) but a ObjectId from mongoose types, so when comparing it to strings cast it to string before: String(object._id) === otherStringId