Showing how you can use higher order functions to remove the need for mocks

no-mocks.js

// the way you'd normally think to do it. This requires mocking out writeFileSync to test
var fs = require('fs');

function writeXmlToFile(fileName, data) {
   // do stuff with data
   // make data into xml
   var xml = toXml(data);
   fs.writeFileSync(fileName, {encoding: 'utf-8'}, xml);
}

// with higher order functions, a mock isn't necessary to test

// 'writeXmlToFile' doesn't make sense anymore
function writeXmlData(writeFn, data) {
  var xml = toXml(data);
  writeFn(data); // external interface just got super simple
}

// usage:
var fs = require('fs');
// underscore, lodash, ramda, or whatever for your flavor of partial application
var writeFn = _.partial(fs.writeFileSync, fileName, {encoding: 'utf-8'});

writeXmlData(writeFn, data);

// testing:
var spy = jasmine.createSpy('writeFile');
writeXmlData(spy, data);
expect(spy).toHaveBeenCalledWith('<xml... ');