Testing private functions 05 August 2014

Last week I saw a blog post about testing private functions within CommonJS modules. The approach taken was checking enviroment variables and exporting private functions when in "test mode".

It reminded me of a conversation I had at the pub with Mark Everitt a few weeks back about testing private methods, and this post shows that way of doing this.

In the original article we have a stats module file

// file: stats.js
var sum = function (nums) { ... };
module.exports = {
  mean: function (nums) { ... },
  standardDeviation: function (nums) { ... },
};
if (process.env.NODE_ENV === 'test') {
  module.exports._private = { sum: sum };
}

This works, but I think it's ugly.

The reason I think this is ugly is that the bottom conditional doesn't belong in this file, logic to do with testing should be contained the the test files themselves, you'll also end up bloating your code base with these conditionals when you have a lot of modules with private functions.

What I would suggest having a stats module folder, with private functions within that.

// file: stats/index.js
var sum = require('./sum');
module.exports = {
  mean: function (nums) { ... },
  standardDeviation: function (nums) { ... },
};

// file: stats/sum.js
var sum = function (nums) { ... };
module.exports = sum;

There are a few benefits from this, mainly:

• The module is now a self contained folder, perfect for publishing to npm, a simple git filter-branch --subdirectory-filter makes it a seperate repo.
• There is still no access to the sum function from the stats module, but can require it when we're writing tests for it.

I should note that yes, the consumer of the module could require in the sum.js file (even if it meant traversing node_modules). But this is clearly not the correct thing to do, and arguably, with the method shown in the mentioned blog post, the consumer could just set theit $NODE_ENV to 'test' but again, this is not the correct thing to do, but they can do it.

If you can test it, you can reach it.

Mark also wrote some slides on this but never got to present them, they're availiable on speakerdeck