text: class UserService {
  
    constructor() {}
  
    // Make Singleton myself
    static getInstance() {
      if (!UserService.instance) {
        UserService.instance = new UserService();
      }
      return UserService.instance;
    }
  
    /**
     * 
     */
    getAllUsers(db,callback) {
      if(db){
       db.collection('User').find().toArray((err, result) => {
          if (err) return callback(err)
          callback(result);
        })
      }
      
    }
    addNewUser(db,body,callback) {
      if(db){
        db.collection('User').insertOne(body, (err, result) => {
          if (err) return callback(err)
          callback(result);
        })
       
      }
    }
    updateUser(db,ObjectId,id,body,callback) {
      if(db){
          db.collection('User')
          .update({_id: ObjectId(id)},body, {upsert:true, w: 1}, (err, result) => {
            if (err) return callback(err)
            callback(result);
          })
       
      }
    }
    deleteUser(db,ObjectId,id,callback) {
      if(db){
        db.collection('User').findOneAndDelete({_id: ObjectId(id)}, (err, result) => {
          if (err) return callback(err)
          console.log(result);
          callback(result)
        })
      }
    }
  }
  
  module.exports = UserService.getInstance();