Which characters are NOT allowed in MongoDB field names?

Do not use $ symbol or period (.) because these characters are not allowed for MongoDB field names. The field shouldn’t start with $.

Here is an example of the allowed characters −

> db.charactersAllowedDemo.insertOne({"Employee Name" : "John"});
   "acknowledged" : true,
   "insertedId" : ObjectId("5c7fefbc8d10a061296a3c6d")

Display all documents from a collection with the help of find() method. The query is as follows −

> db.charactersAllowedDemo.find().pretty();

The following is the output −

   "_id" : ObjectId("5c7fefbc8d10a061296a3c6d"),
   "Employee Name" : "John"