Explain the SET Modifier in MongoDB?

If the value of a field does not yet exist, the "$set" sets the value. This can be useful for updating schemas or adding user-defined keys.

Example :

> db.users.findOne()
{
   "_id" : ObjectId("4b253b067525f35f94b60a31"),
   "name" : "alice",
   "age" : 23,
   "sex" : "female",
   "location" : "India"
}

To add a field to this, we use “$set” :

> db.users.updateOne({"_id" : 
ObjectId("4b253b067525f35f94b60a31")},
... {"$set" : {"favorite book" : "Start with Why"}})

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