A blog and website by Peter Bengtsson

TypeScript function keyword arguments like Python

Python, JavaScript

To do this in Python:

def print_person(name="peter", dob=1979):

# prints: name=peter   dob=1979

# prints: name=Tucker  dob=1979

# prints: name=peter   dob=2013

# TypeError: print_person() got an unexpected keyword argument 'sex' TypeScript:

function printPerson({
  name = "peter",
  dob = 1979
}: { name?: string; dob?: number } = {}) {

// prints: name=peter  dob=1979

// prints: name=peter  dob=1979

printPerson({ name: "Tucker" });
// prints: name=Tucker dob=1979

printPerson({ dob: 2013 });
// prints: name=peter  dob=2013

printPerson({ gender: "boy" })
// Error: Object literal may only specify known properties, and 'gender' 

Here's a Playground copy of it.

It's not a perfect "transpose" across the two languages but it's sufficiently similar.
The trick is that last = {} at the end of the function signature in TypeScript which makes it possible to omit keys in the passed-in object.

By the way, the pure JavaScript version of this is:

function printPerson({ name = "peter", dob = 1979 } = {}) {

But, unlike Python and TypeScript, you get no warnings or errors if you'd do printPerson({ gender: "boy" }); with the JavaScript version.

Please post a comment if you have thoughts or questions.

How to use letsencrypt-acme-challenge.conf in Nginx


Because I always forget, if you're using certbot to create certs for your Nginx server, you'll need to it up so it works on HTTP as well as HTTPS. But once you're done, you're going to want all HTTP traffic to redirect to HTTPS. The correct syntax is:

server {
    include /etc/nginx/snippets/letsencrypt-acme-challenge.conf;
    location / {
      return 301$request_uri;

And that letsencrypt-acme-challenge.conf looks like this (code comments stripped):

location ^~ /.well-known/acme-challenge/ {
    default_type "text/plain";
    root         /var/www/html;
location = /.well-known/acme-challenge/ {
    return 404;

This way, a GET request for will be 200 OK if there's a file called /var/www/html/.well-known/acme-challenge/test.html. And will 404 Not Found.

But all and any other GET request will redirect. E.g. -- 301 Moved Permanently -->

Please post a comment if you have thoughts or questions.

How I upload Firebase images optimized

JavaScript, Web development, Firebase

I have an app that allows you to upload images. The images are stored using Firebase Storage. Then, once uploaded I have a Firebase Cloud Function that can turn that into a thumbnail. The problem with this is that it takes a long time to wake up the cloud function, the first time, and generating that thumbnail. Not to mention the download of the thumbnail payload for the client. It's not unrealistic that the whole thumbnail generation plus download can take multiple (single digit) seconds. But you don't want to have the user sit and wait that long. My solution is to display the uploaded file in a <img> tag using URL.createObjectURL().

The following code is most pseudo-code but should look familiar if you're used to how Firebase and React/Preact works. Here's the FileUpload component:

interface Props {
  onUploaded: ({ file, filePath }: { file: File; filePath: string }) => void;
  onSaved?: () => void;

function FileUpload({
}: Props) => {
  const [file, setFile] = useState<File | null>(null);

  // ...some other state stuff omitted for example.

  useEffect(() => {
    if (file) {
      const metadata = {
        contentType: file.type,

    const filePath = getImageFullPath(prefix, item ? :, file);
    const storageRef = storage.ref();

    uploadTask = storageRef.child(filePath).put(file, metadata);
      (snapshot) => {
        // ...set progress percentage
      (error) => {
      () => {
        onUploaded({ file, filePath });  // THE IMPORTANT BIT!

          .add({ filePath })
          .then(() => { onSaved() })

  }, [file])

  return (
        accept="image/jpeg, image/png"
        onInput={(event) => {
          if ( {
            const file =[0];

The important "trick" is that we call back after the storage is complete by sending the filePath and the file back to whatever component triggered this component. Now, you can know, in the parent component, that there's going to soon be an image reference with a file path (filePath) that refers to that File object.

Here's a rough version of how I use this <FileUpload> component:

function Images() {

  const [uploadedFiles, setUploadedFiles] = useState<Map<string, File>>(
    new Map()

  return (<div>  
      onUploaded={({ file, filePath }: { file: File; filePath: string }) => {
        const newMap: Map<string, File> = new Map(uploadedFiles);
        newMap.set(filePath, file);

    <ListUploadedPictures uploadedFiles={uploadedFiles}/>

function ListUploadedPictures({ uploadedFiles}: {uploadedFiles: Map<string, File>}) {

  // Imagine some Firebase Firestore subscriber here
  // that watches for uploaded pictures. 
  return <div>
    { => (
      <Picture picture={picture} uploadedFiles={uploadedFiles} />

function Picture({ 
}: {
  uploadedFiles: Map<string, File>;
  picture: {
    filePath: string;
}) {
  const thumbnailURL = getThumbnailURL(filePath, 500);
  const [loaded, setLoaded] = useState(false);

  useEffect(() => {
    const preloadImg = new Image();
    preloadImg.src = thumbnailURL;

    const callback = () => {
      if (mounted) {
    if (preloadImg.decode) {
      preloadImg.decode().then(callback, callback);
    } else {
      preloadImg.onload = callback;

    return () => {
      mounted = false;
  }, [thumbnailURL]);

  return <img
      width: 500,
      height: 500,
      "object-fit": "cover",
        ? thumbnailURL
        : file
        ? URL.createObjectURL(file)

Phew! That was a lot of code. Sorry about that. But still, this is just a summary of the real application code.

The point is that; I send the File object back to the parent component immediately after having uploaded it to Firebase Cloud Storage. Then, having access to that as a File object, I can use that as the thumbnail while I wait for the real thumbnail to come in. Now, it doesn't matter that it takes 1-2 seconds to wake up the cloud function and 1-2 seconds to perform the thumbnail creation, and then 0.1-2 seconds to download the thumbnail. All the while this is happening you're looking at the File object that was uploaded. Visually, the user doesn't even notice the difference. If you refresh the page, that temporary in-memory uploadedFiles (Map instance) is empty so you're now relying on the loading of the thumbnail which should hopefully, at this point, be stored in the browser's native HTTP cache.

The other important part of the trick is that we're using const preloadImg = new Image() for loading the thumbnail. And by relying on preloadImage.decode ? preloadImage.decode().then(...) : preload.onload = ... we can be informed only when the thumbnail has been successfully created and successfully downloaded to make the swap.

Please post a comment if you have thoughts or questions.

10 years a Mozillian, always a Mozillian

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...

  • Elmo: The web application for managing the state of Firefox localization
  • Socorro: When Firefox crashes and asks to send a crash dump, this is the storage plus website for analyzing that
  • Peekaboo: When people come to visit a Mozilla office, they sign in on a tablet at the reception desk
  • Balrog: For managing what versions are available for Firefox products to query when it's time to self-upgrade
  • Air Mozilla: For watching live streams and video archive of all recordings within the company
  • MozTrap: When QA engineers need to track what, and the results, of QA testing Firefox products
  • Symbol Server: Where all C++ debug symbols are stored from the build pipeline to be used to source-map crash stack traces
  • Buildhub: To get a complete database of all and every individual build shipped of Firefox products
  • Remote Settings: Managing experiments and for Firefox to "phone home" for smaller updates/experiments between releases
  • MDN Web Docs: Where web developers go to look up all the latest and most detailed details about web APIs

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!

Please post a comment if you have thoughts or questions.

How to fadeIn and fadeOut like jQuery but with Cash


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.

Please post a comment if you have thoughts or questions.

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


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.

Please post a comment if you have thoughts or questions.