Find the number of object present in array of mongoDB

CountTheStudentMongodb

const Allstudentsaag =  await Attendance.aggregate(
    [ { $match : { "subject" : "chem"  } } ,
    { $project : { _id: 0,email:1, professorclass : 1  ,
       numberofstudent: { $cond: { if: { $isArray: "$professorclass" }, 
       then: { $size: "$professorclass" }, else: "NA"} }}},]);
 //In attendance collection Find the subject=chem ,only show email and professorclass field
 if professorclass is an array then show the size of the array if it's empty so N/A. 
 Show the size in a fieldname called numberofstudent