MongoDB ifNull Array - Aggregation

$ifNull  (aggregation)

$ifNull is used to evaluate an expression and returns value. If the expression evaluates to a null value that time it returns to replacement expression value.

Syntax: 

{ $ifNull: [ “expression”, “replacement-expression-if-value-is-null”] }

For example,

db.customers.aggregate([{

       $project: {CustID: 1001,

       CustomerCode: { $ifNull: [ "$CustomerCode ", "Pending Approval" ] } }

 } ]);

In the above example, if customer code is null that time the expression returns to “Pending Approval” (as per you).

For more https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/

Thank you!