How to use $EXISTS to check the field is null or not in mongodb

$exists is an element query operator that can be use to check the existense of any field in a document.

Syntax: { field: { $exists: } }

If { $exists: true }, it will matches the documents which contain the field.

If { $exists: false }, it will matches the documents which does not contain the field.

For example:
Employee Collection

 {    
   "_id" : 1,    
   "name" : {    
   "first" : "Abhi",    
   "last" : "Dev"    
   },    
   "department" : "finance",    
   "joineddate" : "2000-04-10"     
  },    
  {    
   "_id" : 2,    
   "name" : {    
   "first" : "Alok",    
   "last" : "Agrawal"    
   },     
   "joineddate" : "2006-07-07"     
  },    
  {    
   "_id" : 3,    
   "name" : {    
   "first" : "Dhruv",    
   "last" : "Sharma"    
   },    
   "department" : "finance",    
   "joineddate" : "2010-09-07"     
  },   
  {    
   "_id" : 4,    
   "name" : {    
   "first" : "Ravi",    
   "last" : "Mann"    
   },    
   "department" : "information",    
   "joineddate" : "2008-04-07"     
  }  
 }    

In above collection, document {id : 2} does not have department field. So, lets say if we need to search the documents which does not have department field, the query would be:

 db.employee.find({department: {$exists : false}})  

Output of Above Query:

 {    
   "_id" : 2,    
   "name" : {    
   "first" : "Alok",    
   "last" : "Agrawal"    
   },     
   "joineddate" : "2006-07-07"     
 }