in the following code, external variables cannot be fetched within $where, for example, req, prompts that req is not defined. Why is the solution?
router.get("/get",(req,res)=>{
    comments.find({$where:function(){
      return this._id===req.query.id
    }},{email:0}).exec((err,docs)=>{
      if(err){
        res.json({
          sts:0,
          data:err
        })
        return
      }
      res.json({
        sts:1,
        data:docs
      })
    })
})