How is the _id created in mongoose generated?

  mongodb, question

The official mongoose v3.8.7 manual reads about Schema_id attribute, found that this attribute was generated when new was a model, and there was no communication with MongoDB at this time!
I wonder how mongoose generates the value of this _id. Can it guarantee uniqueness?

In addition, officials mentioned that this feature can be turned off, but

var schema = new Schema({ name: String }, { _id: false });
 var Page = mongoose.model('Page', schema);
 var p = new Page({ name: 'mongodb.org' });
 console.log(p);  // { name: 'mongodb.org' }
 
 // MongoDB will create the _id when inserted
 p.save(function (err) {
 if (err) return handleError(err);
 Page.findById(p, function (err, doc) {
 if (err) return handleError(err);
 console.log(doc);  // { name: 'mongodb.org', _id: '50341373e894ad16347efe12' }
 })
 })

The actual test found that save will not succeed at all and will prompt:

[Error: document must have an _id before saving]

How to close _id and save it?

Each document must have an _id and cannot be repeated.
You turn off this feature and you have to create an _ID yourself.
So, of course, you can’t save it.