npm install mock-fs mock-require --save

var fs = require('fs');
var mock = require('mock-fs');

describe('fs', function() {
 beforeEach(function() {
  mock({
   './CNAME': 'harttle.com',
   './_config.yml': 'empty'
  });
 });
 afterEach(function() {
  mock.restore();
 });
 describe('#readFileSync()', function() {
  it('should read all content', function() {
   var str = fs.readFileSync('CNAME', 'utf8');
   expect(str).to.equal('harttle.com');
  });
 });
});

const mock = require('mock-require');

describe('parser', function() {
 beforeEach(function() {
  mock('/package.json', {
   "name": "sample-module",
   "version": "1.0.0",
   "view": "htmls/my-html.hbs",
   "router": "svr.js"
  });
 });
 afterEach(function() {
  mock.stopAll();
 });