A blog and website by Peter Bengtsson

10 years a Mozillian, always a Mozillian

30 August 2021 2 comments   Web development, Mozilla, MDN

As of September 2021, I am leaving Mozilla after 10 years. It hasn't been perfect but it's been a wonderful time with fond memories and an amazing career rocket ship.

In April 2011, I joined as a web developer to work on internal web applications that support the Firefox development engineering. In rough order, I worked on...

This is an incomplete list because at Mozilla you get to help each other and I shipped a lot of smaller projects too, such as Contribute.json, Whatsdeployed, GitHub PR Triage, Bugzilla GitHub Bug Linker.

Reflecting back, the highlight of any project is when you get to meet or interact with the people you help. Few things are as rewarding as when someone you don't know, in person, finds out what you do and they say: "Are you Peter?! The one who built XYZ? I love that stuff! We use it all the time now in my team. Thank you!" It's not a brag because oftentimes what you build for fellow humans it isn't engineering'ly brilliant in any way. It's just something that someone needed. Perhaps the lesson learned is the importance of not celebrating what you've built but just put you into the same room as who uses what you built. And, in fact, if what you've built for someone else isn't particularly loved, by meeting and fully interactive with the people who use "your stuff" gives you the best of feedback and who doesn't love constructive criticism so you can become empowered to build better stuff.

Mozilla is a great company . There is no doubt in my mind. We ship high-quality products and we do it with pride. There have definitely been some rough patches over the years but that happens and you just have to carry on and try to focus on delivering value. Firefox Nightly will continue to be my default browser and I'll happily click any Google search ads to help every now and then. THANK YOU everyone I've ever worked with at Mozilla! You are a wonderful bunch of people!

How to fadeIn and fadeOut like jQuery but with Cash

24 August 2021 0 comments   JavaScript

Remember jQuery? Yeah, it was great. But it was also horrible in its own ways but only when compared to the more powerful tools that we have now as of 2021. I still (almost) use it here on my site. Atually, I use "fork" of jQuery called Cash which calls itself: "An absurdly small jQuery alternative for modern browsers."
Cash is written in TypeScript, which gives me peace of mind, and as a JS bundle, it's only 19KB minified (5.3KB Brotli compressed) whereas jQuery is 87KB minified (27KB Brotli compressed).

But something that jQuery has, that Cash doesn't, is animations. E.g. $('myselector').fadeIn(). If you need to do this with Cash you can use the following pure JavaScript solution:

// Example implementation

const msg = $('<div class="message">')
  .text(`Random message: ${Math.random()}`)
  .css("opacity", 0)
  .css("transition", "opacity 600ms")
setTimeout(() => msg.css("opacity", 1), 0);

setTimeout(() => {
  msg.css("transition", "opacity 1000ms").css("opacity", 0);
  setTimeout(() => msg.remove(), 1000);
}, 3000);

What this application demonstrates is the creation of a <div> that's immediately injected into the DOM but slowly fades into view. And 3 seconds later it fades out and is removed. Full demo/sample application here.

Sample application using cash like jQuery's $.fadeIn().

The point of the demo is how you can cause the fade-in effect with just Cash but still relies on CSS for the actual animation.
The trick is to, ultimately, create it first like this:

<div class="message" style="opacity:0; transition: opacity 600ms">
  Random message: 0.6517198324628395

and then, right after it's been added to the DOM, change the style=... to:

-<div class="message" style="opacity:0; transition: opacity 600ms">
+<div class="message" style="opacity:1; transition: opacity 600ms">

What's neat about this is that you use the transition shortcut so it's done entirely with CSS instead of a requestAnimationFrame and/or while-loop like jQuery's effects.js does it.

Note! This is not a polyfill since jQuery's fadeIn() (etc.) can do a lot more such as callbacks. The example might not be great but I hope this little solution becomes useful for someone else who needs this.

Shut the door! How to automate getting the kids to close the door

23 August 2021 0 comments   Family

Like any responsible parent, I get heart palpitations when my kids (and especially their friends) rush through the door, in or out, and just leave the door wide open letting all that sweet sweet air-conditioned coldness rush out like a prison break. My wife gave me these Gibcloser "Safety Door Closer" as a present and we installed them today on all 3 doors (front door, to-basement door, back porch door).


They're ~$18 on and it appears they come in many different colors. Our doors were white so that that's what we used. It took less than 30 minutes to install on all 3 doors. All you need is a screwdriver and, I would suggest, some thin foam command strips.

Door held at 90° and released.

I know it might sound silly but I've wanted this for so long and it never occurred to me that such a simple solution might exist. Clearly, we've all seen door closers like this before, but almost always bigger. Especially on commercial buildings. But they all seem so complicated and expensive-looking and definitely strike fear in you of: "that'd never work on my home doors". What's neat about these is that they are gentle. It's just a gentle (albeit accelerating) push but if someone/something was to get stuck as it closes, it wouldn't chop off a finger or a foot.

Truth be told, they don't close the door all the way. In the above video, it closes nicely because I held the door open at 90° which is a bit unrealistic angle that someone would forget to close the door. But at least it'll close it enough to stop the flow of cold/warm air going through.

How to submit a form with Playwright

03 August 2021 0 comments   JavaScript

Because it was driving me insane, and because I don't want to ever forget...

Playwright is a wonderful alternative to jest-puppeteer for doing automated headless browser end-to-end testing. But one I couldn't find in the documentation, Google search, or Stackoverflow was: How do you submit a form without clicking a button?. I.e. you have focus in an input field and hit Enter. Here's how you do it:

await page.$eval('form[role="search"]', (form) => form.submit());

The first part is any CSS selector that gets you to the <form> element. In this case, imagine it was:

<form action="/search" role="search">
  <input type="search" name="q">

You, or my future self, might be laughing at me for missing something obvious but this one took me forever to solve so I thought I'd better blog about it in case someone else gets into the same jam.

UPDATE (Sep 2021)

I found a much easier way:


This obviously only works when you've typed something into an input so the focus is on that <input> element. E.g.:

await page.fill('input[aria-label="New shopping list item"]', "Carrots");

How to install Python Poetry in GitHub Actions in MUCH faster way

27 July 2021 0 comments   Python

We use Poetry in a GitHub project. There's a pyproject.toml file (and a poetry.lock file) which with the help of the executable poetry gets you a very reliable Python environment. The only problem is that adding the poetry executable is slow. Like 10+ seconds slow. It might seem silly but in the project I'm working on, that 10+s delay is the slowest part of a GitHub Action workflow which needs to be fast because it's trying to post a comment on a pull request as soon as it possibly can.

Installing poetry being the slowest partt
First I tried caching $(pip cache dir) so that the underlying python -v pip install virtualenv -t $tmp_dir that does would get a boost from avoiding network. The difference was negligible. I also didn't want to get too weird by overriding how the works or even make my own hacky copy. I like being able to just rely on the snok/install-poetry GitHub Action to do its thing (and its future thing).

The solution was to cache the whole $HOME/.local directory. It's as simple as this:

- name: Load cached $HOME/.local
  uses: actions/cache@v2.1.6
    path: ~/.local
    key: dotlocal-${{ runner.os }}-${{ hashFiles('.github/workflows/pr-deployer.yml') }}

The key is important. If you do copy-n-paste this block of YAML to speed up your GitHub Action, please remember to replace .github/workflows/pr-deployer.yml with the name of your .yml file that uses this. It's important because otherwise, the cache might be overzealously hot when you make a change like:

       - name: Install Python poetry
-        uses: snok/install-poetry@v1.1.6
+        uses: snok/install-poetry@v1.1.7

...for example.

Now, thankfully (which is the recommended way to install poetry by the way) can notice that it's already been created and so it can omit a bunch of work. The result of this is as follows:

A fast install poetry

From 10+ seconds to 2 seconds . And what's neat is that the optimization is very "unintrusive" because it doesn't mess with how the snok/install-poetry workflow works.

But wait, there's more!

If you dig up our code where we use poetry you might find that it does a bunch of other caching too. In particular, it caches .venv it creates too. That's relevant but ultimately unrelated. It basically caches the generated virtualenv from the poetry install command. It works like this:

- name: Load cached venv
  id: cached-poetry-dependencies
  uses: actions/cache@v2.1.6
    path: deployer/.venv
    key: venv-${{ runner.os }}-${{ hashFiles('**/poetry.lock') }}-${{ hashFiles('.github/workflows/pr-deployer.yml') }}


- name: Install deployer
  run: |
    cd deployer
    poetry install
  if: steps.cached-poetry-dependencies.outputs.cache-hit != 'true'

In this example, deployer is just the name of the directory, in the repository root, where we have all the Python code and the pyproject.toml etc. If you have yours at the root of the project you can just do: run: poetry install and in the caching step change it to: path: .venv.

Now, you get a really powerful complete caching strategy. When the caches are hot (i.e. no changes to the .yml, poetry.lock, or pyproject.toml files) you get the executable (so you can do poetry run ...) and all its dependencies in roughly 2 seconds. That'll be hard to beat!

An effective and immutable way to turn two Python lists into one

23 June 2021 7 comments   Python

tl;dr; To make 2 lists into 1 without mutating them use list1 + list2.

I'm blogging about this because today I accidentally complicated my own code. From now on, let's just focus on the right way.

Suppose you have something like this:

winners = [123, 503, 1001]
losers = [45, 812, 332]

combined = winners + losers

that will create a brand new list. To prove that it's immutable:

>>> combined.insert(0, 100)
>>> combined
[100, 123, 503, 1001, 45, 812, 332]
>>> winners
[123, 503, 1001]
>>> losers
[45, 812, 332]

What I originally did was:

winners = [123, 503, 1001]
losers = [45, 812, 332]

combined = [*winners, *losers]

This works the same and that syntax feels very JavaScript'y. E.g.

> var winners = [123, 503, 1001]
[ 123, 503, 1001 ]
> var losers = [45, 812, 332]
[ 45, 812, 332 ]
> var combined = [, ...losers]
[ 123, 503, 1001, 45, 812, 332 ]
> combined.pop()
> losers
[ 45, 812, 332 ]

By the way, if you want to filter out duplicates, do this:

>>> a = [1, 2, 3]
>>> b = [2, 3, 4]
>>> list(dict.fromkeys(a + b))
[1, 2, 3, 4]

It's the most performant way to do it if the order is important.

And if you don't care about the order you can use this:

>>> a = [1, 2, 3]
>>> b = [2, 3, 4]
>>> list(set(a + b))
[1, 2, 3, 4]
>>> list(set(b + a))
[1, 2, 3, 4]