Make common to all function in nodejs

I create requests from nodejs to server. For different functions I have same auth, so I want to make 1 auth function and use it when we need. But in strict mode this is undefined. In not strict mode getUserAuth is undefined. Where I'm wrong.

 'use strict';
    var request = require('request');

    module.exports.getUserAuth = function () {
      // do some stuff.
      return stuff;
    };

    module.exports.clientCheck = function (req, res) {

      var auth = this.getUserAuth();

      // make some request with auth;
    };

via YoroDiallo