I came across a small problem with unit testing functions which take a callback. If you write your test like this:

"should callback with error on connection failed": function () {
  thing.doSomething(function(result) {
    Y.Assert.areEqual(result, "error");

That test will pass even when the callback is never called. Your code could silently swallow errors, but your test would always pass. The assert never gets called.

If you write the test like this instead:

"should callback with error on connection failed": function () {
  var result;
  thing.doSomething(function(res) {
    result = res;
  Y.Assert.areEqual(result, "error");

This will mean that your assert will always get called. You may run into problems if the doSomething function calls something asynchronous – but that’s where dependency injection and mocking can help you.

Dependency injection is a handy technique for writing testable code. If you’ve used Spring in the Java world, you’re probably familiar with it. Put simply, it means that code should not be responsible for instantiating the things it depends on – these dependencies should be passed in by the calling code. That means you can pass in mock versions for testing.

In node.js land, we use require to bring in external modules. For example:

var http = require('http');
exports.twitterData = function(callback) {
  http.createClient(... blah...);

This module is hard to test without needing internet access, which may not be available if your tests are running on a build server behind a corporate firewall. Besides, you don’t want your tests calling twitter all the time.

What you want to be able to do is replace http with a mock version, so that you can verify that your module makes all the right calls without actually needing access to the internet. There are a couple of ways to do this, like messing about with require.paths to bring in mock modules. Here’s the way we decided to do it.

First we write our modules in this style:

module.exports = function(http) {
  var http = http | require('http');
  // private functions and variables go here...

  //return the public functions
  return {
    twitterData: function(callback) {

In normal use, we’d require our twitter module like this:

var twitter = require('twitter')();

In our tests, we’d require it like this:

var mockHttp = { createClient: function() { assert(something); } };
var twitter = require('twitter')(mockHttp);
//do some tests.

It seems to be working fine for us so far, and having that first line of your module tell you explicitly what the dependencies are is quite handy as well. If you want some real-world examples of this, take a look at the log4js-node source.

Learning JavaScript – this should be your first stop for JS references.

One of our happy team of code monkeys wrote up his heroic efforts on getting node.js to build on Solaris x86.