How to access this.property of class in prototype method in nodejs

Here is what i am trying to do. I am writing an app in nodejs express and trying to divide different parts of apps in separate modules. I have some problem access this variable in middleware functions

Below is my routes module

"use strict";
function route(params){
  var app = params.app;
  var ApiHelper = require('../library/apiHelper').init(params);
  //get
  app.get('/', Api.index);
  app.get('/check', ApiHelper.validateRequest, Api.check);
}
module.exports = route;

My ApiHelper class

"use strict";
function ApiHelper(params){
  this.params = params;
}
module.exports.init = function (params){
  return new ApiHelper(params);
};
ApiHelper.prototype.validateRequest = function validateRequest(req, res, next){
  console.log(this.params);//this is undefined
};

I am unable to access this.params in validateRequest object of the ApiHelper