Category Archives: Code

Binding to an specific host with express.js

The documentation on the listen method is really vague, but the way you can make your server listen to an specific address only is this:

var server = app.listen(3000, '', 511, onServerListening);

In this case I want the server to respond only to connections using the host name. Not, and not localhost. Only

The reason for this is that I am playing with Twitter’s OAuth and I was stumbling upon this issue because express insisted in binding to by default (i.e. any address), but it wouldn’t work well when Twitter redirected back after authenticating.

For reference, another thing I tried and didn’t work:

var server = app.listen(3000, '', onServerListening);

Notice that the 511 parameter is missing. So effectively it would take onServerListening as if it was the 511 parameter, which according to the documentation is “the maximum length of the queue of pending connections” and it wouldn’t call the callback. Booo.


If you want to write code that looks nice and crispy in high density displays, you have to take the device pixel density into account. This is exposed to us in JavaScript as the window.devicePixelRatio property, which returns a number that describes how many physical pixels does it take to represent a device independent pixel. The more pixels it takes, the finer the display will look, and the harder it is to actually “see” individual pixels (as in, individual, physical light dots) in the screen. This is famously known now as the “Retina” effect, but not only Mac products exhibit this behaviour-most relatively high end phones with “touch screen” usually have a display density higher than 1.

The interesting bit here is that you can’t assume that once a document is opened in your browser, the devicePixelRatio will stay constant. If you drag the window to a screen with a different density, this value will change–as seen in this video where I am dragging my test page window between the Retina display of the laptop, with ratio = 2, and the external monitor whose ratio is 1. For comparison, devicePixelRatio is 3 on my Nexus 5, 1.5 on a Firefox OS Flame, and 1 on a Geeksphone Keon.

Try it out, and maybe leave a comment with what you got!

There isn’t any event triggered when this value changes, so that’s why in this test case I set a periodic interval to keep updating the value:

setInterval(updateDPR, 50);

function updateDPR() {
    p.textContent = window.devicePixelRatio;

Perhaps not the most efficient piece of code, but this is also a bit of a corner case maybe, and a test page, so just don’t copy it literally :-)


What does the Battery API report on a desktop computer?

I was discussing with Chris Mills how to build an example for Web APIs that was clear enough yet showed some sort of API usage in action. He had chosen the Battery API which is, effectively, simple enough. But I had a question: what does this API report when you run it in a device without battery?

Nothing better than building an example, so that’s what I did.

On a laptop

It reports the battery level and discharging / charging, plus the time left, if not at the maximum level already.

battery api on a laptop

On a desktop

It reports battery level at 100%, charging, and 0 seconds to charge.

battery api on a desktop

Mystery solved!

A final note/remark: while going through the API docs I found it so very synchronous and unpromise-y! If this API was written nowadays it would probably be done with Promises or some sort of asynchronicity by default. I wonder if that will be retrofitted.

Failproof AJAX requests in Firefox OS

Have correct values in the manifest

The app type has to be at least privileged. No, web (hosted) apps won’t work with different domains if CORS is not enabled there.

Add the systemXHR permission too, or your XMLHttpRequests to domains without CORS enabled will instafail.

Example of the syntax for these two things in the manifest:

  "type": "privileged",
  "permissions": {
    "systemXHR": {
      "description": "Allows loading remote content"

Add the “magic word” when creating XMLHttpRequest objects

The magic word being the mozSystem property:

request = new XMLHttpRequest({ mozSystem: true });

You should now be able to use the request in the usual way. Remember to add an error handler in case it fails!

We are using these superfailproof requests in our privileged app template to load definitions from MDN. Have a look!

For more info

Firefox now implementing the latest Custom Element spec

This week we saw bug 856140 Update document.register to adhere to the latest Custom Element spec fixed. So you can now do things such as declaring and instancing <fancy-button>s, which you couldn’t do with the previous implementation, because it would require your custom element’s name to start with an x-, following the old spec. Also, UI libraries such as Brick and other X-Tag based libraries won’t break because the callbacks are never called when web components are enabled. Those are all old bad nightmares from the past!

However the spec isn’t finalised yet–hence you can’t be done implementing something which hasn’t been written yet. And we need people to experiment with web components in Firefox and report bugs if they find them (you can file them under Product = Core, Component = DOM).

If you want to try this out, you can download a Nightly copy and follow the instructions in my Shadow DOM post to enable web components support (it’s disabled by default to prevent “breaking the world”, as Potch would put it).