Skip to main content

JavaScript: Perfectly Encapsulated May Mean Perfectly Untestable

It's no secret that JavaScript is like Lisp in that you can accomplish amazing things using a huge number of small, nested functions. In fact, you can write things like:
(function () {
function pickNose() {

function fart() {

In this code, an anonymous function is defined and immediately called. pickNose() and fart() are two internal functions that are used by the outer function, but they are not available to the outside world.

It's amazing what you can get done using nested closures like this, but there's a cost. How do you write tests for pickNose() and fart()? Certainly, you can write a test for the outer function as a whole, but there's no way to test those inner functions in a standalone way without doing some refactoring. In a certain sense, the code is like a script in that you can test the thing as a whole, but you can't test the parts in a standalone way.

What's the solution? I'm sure there are many. You could have the outer function take a parameter such that when the correct value is passed, the code could flip over to testing mode and test itself. Another approach is to use Douglas Crockford's module pattern. The outer function could return an object that has references to the inner functions. That way you can call them externally and test them. However, that may not be an option if you are really paranoid about the outside world getting references to those functions.

In my opinion, getting overly paranoid about people calling your inner functions isn't very fruitful. JavaScript doesn't have much to help you keep modules away from each other. As soon as you have hostile JavaScript on the page, it's sort of game over. All of the JavaScript operates with the same permissions--it's not like you can sandbox a particular module.

Furthermore, it's difficult if not impossible to prevent people from just grabbing your JavaScript source code and hacking it to do whatever they want (especially if they can rely on the help of an external server). To state the obvious, JavaScript doesn't have very good internal security boundaries, so your server must always be distrustful of the JavaScript that it's talking to. Of course, that's the way the web has worked for as long as I've been coding.


Shailen Tuli said…
How do you test the internal functions? You don't. How about giving the outer function a name and defining/calling the inner functions anonymously? You would loose the colorful names, admittedly, but you could make up for that by naming the outer function imaginatively. Then you can write tests for the outer function. You wouldn't be unit-testing in that case, but you always have preferred a more holistic/integrated approach to testing anyway, right?
jjinux said…
If the inner function has some complicated, purely functional logic that simply takes a value and returns a value, there is real value in unit testing it. If the outer function involves lots of DOM manipulation, it may not be possible to fully test the logic of the internal function because it's too hard to test it via its affect on the DOM. As much as I like integration testing, unit testing has real value too.

Popular posts from this blog

Ubuntu 20.04 on a 2015 15" MacBook Pro

I decided to give Ubuntu 20.04 a try on my 2015 15" MacBook Pro. I didn't actually install it; I just live booted from a USB thumb drive which was enough to try out everything I wanted. In summary, it's not perfect, and issues with my camera would prevent me from switching, but given the right hardware, I think it's a really viable option. The first thing I wanted to try was what would happen if I plugged in a non-HiDPI screen given that my laptop has a HiDPI screen. Without sub-pixel scaling, whatever scale rate I picked for one screen would apply to the other. However, once I turned on sub-pixel scaling, I was able to pick different scale rates for the internal and external displays. That looked ok. I tried plugging in and unplugging multiple times, and it didn't crash. I doubt it'd work with my Thunderbolt display at work, but it worked fine for my HDMI displays at home. I even plugged it into my TV, and it stuck to the 100% scaling I picked for the othe

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python . Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening. The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for B

Creating Windows 10 Boot Media for a Lenovo Thinkpad T410 Using Only a Mac and a Linux Machine

TL;DR: Giovanni and I struggled trying to get Windows 10 installed on the Lenovo Thinkpad T410. We struggled a lot trying to create the installation media because we only had a Mac and a Linux machine to work with. Everytime we tried to boot the USB thumb drive, it just showed us a blinking cursor. At the end, we finally realized that Windows 10 wasn't supported on this laptop :-/ I've heard that it took Thomas Edison 100 tries to figure out the right material to use as a lightbulb filament. Well, I'm no Thomas Edison, but I thought it might be noteworthy to document our attempts at getting it to boot off a USB thumb drive: Download the ISO. Attempt 1: Use Etcher. Etcher says it doesn't work for Windows. Attempt 2: Use Boot Camp Assistant. It doesn't have that feature anymore. Attempt 3: Use Disk Utility on a Mac. Erase a USB thumb drive: Format: ExFAT Scheme: GUID Partition Map Mount the ISO. Copy everything from