Tag Archives: web apis


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 :-)


Invest in the future: build for the web!

I spoke at GOTO Amsterdam a few weeks ago. I was really thrilled to be back in the Netherlands after so many years! So thanks to Sergi Mansilla, who curated the HTML5 track, and the organisation in general for bringing me there!

The talk wasn’t recorded, but I made a screencast just in case you really want to listen to me. I am also posting the outline/notes I wrote, and they differ in places because I don’t read them during the talk (I don’t even have them handy) and I sometimes went a bit off topic, but that’s the beauty of improvisation!

Here are the slides, and the slides source code just in case you wanted it too.

On to the notes-expect some MASSIVE GIFs and amazingly clever photomanipulation! tee hee hee!
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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.