What is going wrong with the class defination in expressjs?

I have added a response class in expressjs as follows

module.exports = function(req,res) {
  this.req = req;
  this.res = res;
  console.log(this.res);

  this.ok = function() {
    this.res.status(200).end();
  }

  this.err = function(err) {
    console.log(this.res);
    this.res.status(400).json({err:err.message});
  }
};

And i am calling it using

route.post("/",function(req,res){
  var authCtx = new authHelper(req,res);
  var respCtx = new response(req,res);
  authCtx.checkForm()
    .then(respCtx.ok)
    .catch(respCtx.err);
});

The problem is when i create an object of class the first console.log prints the complete response fine without any errors but in second case when error is invoked the this.res becomes empty. Where am i going wrong with this ?

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