otsukare Thoughts after a day of work

Debug JavaScript On The Fly With DevTools

Identifying the origin of the issue

On webcompat, we try to diagnose bugs reported about sites, we do not have the original sources. If we are lucky, web developers have included a way to reach the original JavaScript source files (through sourcemap). But modern websites have often a series of optimization layers (with multiple minifications).

To check during the investigation, that we thought was right we need to play with the code. With real CSS files (as opposed to CSS in JavaScript), it is easy to hand make a working solution. With JavaScript it's near impossible apart of just observing why it fails and guessing how it could work.

How do we overcome the lack of access to JavaScript?

One possible solution is to create a local proxy on your laptop and to use a man in the middle technique to capture the HTTP requests and replace the website JavaScript HTTP request by a call to a local file of your choice. On macOs for example, you could use Charles Proxy or on Unix you may want to use MITM

This is tedious and not always successful, specifically when JavaScript is written and minified on the fly.

Last week, a lunch in Tokyo

During a lunch with Jason Laster (@jasonlaster11), Brian Birtles (@brianskold) and Arai-san (@arai_unmht), I was specifically discussing about my woes when debugging websites and how cool it would be to just be able edit the JavaScript file inside the devtools debugger. Arai-san said « Oh but I have something a bit similar to this. » So I told them… « Ship it! »

The afternoon went on with the current work in the Mozilla Tokyo office. But just before dinner, Arai-san decided to be my hero. They worked during the afternoon on packaging properly their tool and makes it an add-on for the Firefox devtools.

And indeed it was shipped by the end of the day! Discover Developer In The Middle. You can select a resource, prettify it in your favorite editor, change some logic, save it inside the UI and just reload the page. Magical and so much more practical.

This is the first version, please do use it! Try it and send feedback about the tool.

Thanks Arai-san!


I never stop to be amazed by the quality, expertise and kindness of the Mozilla Japan team. I'm coming to the office once a week, because I live a bit far from the Tokyo office, but I'm definitely blessed to meet them every week.