Dotjs Extension Read through

Reading time ~9 minutes


As a developer we are constantly told by our peers (at least I am) that opensource projects are made of unicorns. You get to see the code behind projects that you are interested in and even make contributions when you find a problem. Sadly when asked how many projects they have contributed to many people default to 0 contributions. It may be argued that contributing to a new project is a leap that is not easy to take, it should then be a given that people at least read and review their favorite open source projects. Again when asked about this many people are content to use a open source library or application assuming someone else has checked it out.

In the same line of thought while watching this talk given by Richard Hamming, I felt inspired to get a long delayed project going. Calling it a project may be overkill but the idea is fairly simple. As Hamming suggests reading and reviewing papers weekly I have wanted to read and review opensource projects. On its own the idea is simple but has a number of benefits for me, I get to learn about a new cool project that I can always refer back to for inspiration , I get to learn about how a language is used (depends on the language the project uses) and finally I get learn about new ways to solve problems that other developers have fixed.

While it was tempting to build a entire system to make this process easy I figure I should get going and build a system once I have some data to work around. As a side note one of the few devs whose work I enjoy following, does regular series of code reviews where he does deep dives into various code bases (mostly game engines). If you don’t already know who I am talking about then you should go have a look at fabien sanglard’s blog.


Selected a project from githubs trending projects for productivity also since it was a chrome extension it would provide a good brush up on chrome dev techniques.Further the plugin seems to tackle a problem that i tried to solve recently (loading custom code easily into a domain) in a very clean manner.


Project is located on github @

Down the rabbit hole

So cloning the project results in the following file listing

-rw-r--r-- 1 rc1140 users 3.9K Apr 24 20:20 Rakefile                                            
-rw-r--r-- 1 rc1140 users 2.8K Apr 24 20:20 README.markdown                                     
-rw-r--r-- 1 rc1140 users 1.1K Apr 24 20:20 LICENSE                                             
-rw-r--r-- 1 rc1140 users  218 Apr 24 20:20 HACKING                                             
drwxr-xr-x 2 rc1140 users 4.0K Apr 24 20:20 ext                                                 
-rw-r--r-- 1 rc1140 users  498 Apr 24 20:20 com.github.dotjs.plist                              
drwxr-xr-x 2 rc1140 users 4.0K Apr 24 20:20 bin                                                 

A few things seem to stand out , there is a rake file which probably means that ruby is involved. Author has also taken the time to include a hacking file so modifications are expected to occur or have already happened in the past to warrant this.

Next there is a plist file which is mac/ios file used in apps for those operating systems , this either means this plugin is aimed @ mac/iOS users or the file got forgotten behind ?

The rest of the project seems to be split into the extension and a binary folder. This should prove interesting as it may explain how the extension accesses the file system (which is usually prohibited by chrome)


Since the project includes a readme starting with this should provide some directions.

Readme gives a nice intro into how the extension works and why you would want to use it. It also gives a example script very early in the file so that you can try stuff out.

It seems the core repo I am looking at is only meant for OSX which would explain the plist file I saw earlier. Will continue looking into this repo and switch to a windows repo when I need to actually test it.

Its also nice that the readme includes direct links to the ports.


As expected the code in the rakefile is ruby and is surprisingly easy to read. The file contains a number of tasks that are split up neatly as listed below

task :all => [ :prompt, :daemon, :create_dir, :agent, :chrome, :done ]  

The task above executes all the tasks in the rake file and provides a nice index to the list of tasks in the file.

The author has split up the tasks quite neatly and each task encapsulates each bit of functionality very nicely.

Not sure if I should should analyze every single task. The following tasks a grouped under a namespace :install , this means that all the tasks listed below will be executed when running the install task


Simply prompts the user to confirm what the rake file overall task will execute.


This is a neat little task which checks if the entire process completed successfully buy curling the local end point. I assume there is probably a http library that rake/ruby could use but by using curl it simplifies things a fair amount since the input string would never be changed. Potential errors could be that curl is not installed


This is the task that sets up the script to run as a system service and is what the plist file is used for.

What is slightly interesting is that instead of using the system cp command to copy the plist file to the required dir it uses the ruby file api to read and write to the required location.


This task copies the binary that the extension requires to the default setup location. The system agent will execute this binary when it starts up. I assume that the read and write method of copying the file was not used because it was a binary file.


Heh , as expected this creates the .js directory from which the extension takes its name.


Simply prints the message that the user should install the extension and link to the extension that should be installed.

There are a bunch of tasks grouped under the :uninstall namespace. Wont be covering this since it just does the reverse of the tasks I just looked at.


This is the core service that is setup by the rakefile. Running the command

file djsd

Tells us that the file is not a binary but rather a ruby script which means I can now inspect this using a text editor.

Opening the file and going from top to bottom tells us the following. The script does some basic argument checking. It checks for the help param which prints the usage commands for the script. It also checks for the version flag which just prints the version and exits.

Once the flag checking is complete a class called dotjs is created. The class is fairly basic and see to be more a interceptor/handler class. The class builds a body for a html request , as well as origin detection.

Further details on how this class is used will be noted when it is encountered in the code again (it should be encountered otherwise it can be removed)

After the class is setup a ssl cert is created and a basic set of options are passed to the web brick http server. Whats interesting is that the cert exists after the last line of code , I am not sure what the END does but I will provide details later.

Finally when starting the server we now notice the dotjs class being passed as the handler for the index request.

Reading back a little bit we notice that all that the class does is serve a js file called default.js. The server also handles terminations cleanly.

While the script is fairly clean and short i really think that it could have been implemented a bit cleaner by loading the ssl cert from an external file instead of script the script. Although this self encapsulation is reasonably useful.

I will also need to do more testing to determine why the author chose to use ssl when hosting the server since the cert is distributed with the extension changing the cert is easy enough.

This ends the files in the bin folder , as usual its pretty clean and efficient.

Extension folder

The ext folder contains the core files required required to build a chrome extension. This folder as with the rest of the repo is clean and straight to the point as seen below.

-rw-r--r-- 1 rc1140 users  380 Apr 24 20:20 manifest.json
-rw-r--r-- 1 rc1140 users  91K Apr 24 20:20 jquery.js   
-rw-r--r-- 1 rc1140 users 2.3K Apr 24 20:20 icon48.png  
-rw-r--r-- 1 rc1140 users 6.6K Apr 24 20:20 icon128.png 
-rw-r--r-- 1 rc1140 users  253 Apr 24 20:20 dotjs.js    

The core of this folder is the manifest.json file which is used to tie a chrome extension together. Aside from that we have a couple of icons which chrome requires for an extension. The last file is the main js file that the extension injects into every page. Once this file is injected it then makes a ajax request to the local service for the js file associated with the current site. Once this file is loaded it is then eval’d so that it runs in the correct domain.

This is a pretty neat way to dynamically load the js files without needing to setup any mappings between a js file and a website and get it executed on the correct page.

Random Notes

There are a few other things that I noticed about the project which were semi interesting to me. There were a number of outstanding pull requests which seem reasonably easy to merge, I think it would be useful to either decline them or push them forward to clean up the outstanding list (some of the pull requests are really old which really makes one wonder why they weren’t accepted). This has resulted in a huge number of forks (nothing wrong with that) but it would have been cool to have some of the feature in the core repo or least have the pull requests purged. This same issue is visible in the issues section (too many open issues), granted a bunch of them are related to the pull requests. I should mention that this is a fairly common issue with project on github (since they are the only people that make this data easily accessible) and I am just as guilty of having a bunch of hanging issues that i have never gotten to working on.

TODO : As a side note it would be interesting to pull data from github showing the average number of issues/pull requests open for overall projects listed on github.

Lastly it seems that the p3lim repo is the most updated version of all the forks but has changed the core to work a little differently where there is no local web server involved , instead the files are all hosted in the extension.


I have to say that after reading through the code and seeing how it pulls everything together in a simple package I think it would make a nice base for future projects where you need simple coms between a desktop and the browser.