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Is it in Nightly yet?

Are you a Mozilla developer? Has your manager, itching to try out your latest feature, ever asked you, “Is it in the latest Nightly build yet?” I asked myself a little too often so I built a thing:


It’s quite simple - pass in a changeset id and get an answer! Try it out at!

Lessons learned


I’ve been thinking about ways to construct more robust applications in less time. To this end, React is mentioned a lot and this seemed like a great opportunity to finally get my toes wet. Here are some things I liked:

To be fair, I’ll need to build a more complex app to really experience & understand what the benefits of React are – and more importantly, its trade-offs!


While I implemented this project, I was faced with a dilemma: write tests because I’m creating a program I’d expect others to use2 or don’t because it’s just a small project. In the end, I decided to write a few tests.

I came to discover that, even on tiny projects, testing is invaluable.

The server response in the initial implementation was slow when a changeset was not in Nightly because the server didn’t return quickly for invalid changeset IDs. I decided to add a fast fail implementation to mozhg.isRevisionSetValid, for which I already wrote some tests that passed the first time – you know, the kind that you have to intentionally break to make sure they’re actually working – and, given how simple the code I added was3, I was surprised to find these tests failing, despite the application seeming to perform correctly with manual testing.

Testing is great: I intend to do more.

Code re-use

I originally built this with a server component4 and later found out I could do this all in a standalone HTML page. Given how much easier it is to host HTML content, I decided to make the conversion.

My server implementation was in node.js and, using browserify, I found it incredibly easy to move my code from the server to the browser. Given that my my code was already divided into the mozhg node module and my client-side React code, I simply had to rewrite the entry point of the page to delegate calls from React directly into my node module, rather than passing these requests to the server. If I had written the server in Python, it would have been a lengthy conversion!

Writing server-side and client-side code in the same language can be extremely valuable if it’s unclear where the separation of concerns will lie. That being said, I could have done more research about what was possible, but there are trade-offs: we don’t always have time to do all the research.


To find the latest Nightly changeset ID, I dive into Mozilla’s latest Nightly file dump5. Once there, I take the moz_source_stamp attribute from the fennec*.json file. moz_source_stamp represents the topmost commit that made it into that night’s build.

With changeset ID in hand, I access Mozilla’s hgweb service. There are JSON APIs available for most of the commands you’d find in a local hg repository and, fortunately for me, anything involving revision sets isn’t one of them. As such, I scrape the html interface for a mozilla-central changelog revision set query. This query goes from the changeset specified by the user to the latest Nightly changeset. If the query returns an empty list, the user-specified changeset is either 1) not in mozilla-central or 2) appears after the last Nightly changeset and thus is not in Nightly. If it returns a list of changesets, the changeset is in mozilla-central before the last Nightly changeset and thus is in Nightly.

As I elaborated on in the testing section above, in order to return quickly from this method, I validate the changesets before passing them to the revision set function.

Curious for more? You can find the source on Github.

1: e.g. compared to web development without UI frameworks, but this is also a general statement about how important encapsulation is.

2: It helps that my application depends on external factors like website scraping too.

3: It turns out I did not intend to return r1 && r2, a coercion of my string input arguments. Instead, I wanted isR1Valid && isR2Valid, which are two booleans. Oh, Javascript…

4: I make XSS requests (to get the revision content) and got blocked by CORS. I created a server component to make the requests on the client’s behalf before I realized the requests were only blocked when accessing the Mozilla properties via http, as opposed to https.

5: The file dump is specific to Firefox for Android, which may or may not share the same latest commit for its nightly builds with other projects (e.g. desktop Firefox). If you’re interested in using this for other projects, file an issue!

This post is written by Michael Comella, a developer at Mozilla currently working on Firefox performance.
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
Creative Commons License