db.mongo.insert({name:'mongo'});
> var t=db.mongo.findOne();
> t
{ "_id" : ObjectId("5141b98723616e67f947f356"), "name" : "mongo" }
> var d=t   
> d
{ "_id" : ObjectId("5141b98723616e67f947f356"), "name" : "mongo" }
> d.password=123
123
> d
{
	"_id" : ObjectId("5141b98723616e67f947f356"),
	"name" : "mongo",
	"password" : 123
}
> t
{
	"_id" : ObjectId("5141b98723616e67f947f356"),
	"name" : "mongo",
	"password" : 123
}

Question 1

var d = t

Is it a reference assignment? If so, how about not using a reference assignment?

delete d.password
true
> d
{ "_id" : ObjectId("5141b98723616e67f947f356"), "name" : "mongo" }
> d.password=124
124
> d
{
	"_id" : ObjectId("5141b98723616e67f947f356"),
	"name" : "mongo",
	"password" : 124,
	"password" : 124
}

Question 2: I will execute it.

delete d.password

Returns true to indicate that it has been deleted, why should I execute it again

d.password=124

Two additions are performed

> d
{
	"_id" : ObjectId("5141b98723616e67f947f356"),
	"name" : "mongo",
	"password" : 124,
	"password" : 124
}

At first contact is not very clear, hope to know the reason