One of the most important reasons for the popularity of MongoDB is that it is a JSON-friendly database. This means that documents are stored and retrieved from MongoDB as JavaScript objects. Internally, this JSON data gets converted to BSON format while getting persisted to the system. So, this gives extreme flexibility, where we can use the same data format from client to server and eventually to the database.

A typical document (record) in a MongoDB collection (table) might look like the following code:

$ mongo 
> db.contacts.find({email: '[email protected]'}).pretty() 
{ 
   "email" : "[email protected]", 
   "phone" : "123-456-7890", 
   "gravatar" : "751e957d48e31841ff15d8fa0f1b0acf", 
   "_id" : ObjectId("52fad824392f58ac2452c992"), 
   "name" : { 
      "first" : "Jason", 
      "last" : "Krol" 
   }, 
   "__v" : 0 
} 

After examining the preceding output, we can see a key called _id. It is a MongoDB ID which must be encoded as a binary JSON objectID(BSON). If failed to encode, MongoDB won't be able to retrieve or update an object.