const MongoClient = require('mongodb').MongoClient;
const url = 'mongodb://localhost:27017';
var a = "2018";
var b = "December";
var c = "12";
var d = "28";
MongoClient.connect(url,{ useNewUrlParser: true },(err, client) => {
    if (err){
  return console.log('Unable to login to Mongo DB');
}
console.log('Connected to Mongo DB');
const db = client.db('test');
db.collection('updateTest').updateOne({_id:a},{$addToSet:{"b.c":{
    "temp":d
}}},{upsert:true},(err,result) =>{
    if(err){
        return console.log(err);
    }
    console.log(JSON.stringify(result));
});
client.close();
});

{$addToSet:{`${b}.${c}`:

var e = `${b}.${c}`;

{$addToSet:{e: