Enabling Dark Mode On Websites Based On Surrounding Light

Now you can switch between a "dark mode" and "normal mode" on a website based on the surrounding light! It can really help in curating a better user experience based on the conditions users are viewing your website.

So, here's how you do it-

Adding the short and simple javascript.

if ( 'AmbientLightSensor' in window ) {
  const sensor = new AmbientLightSensor();
  sensor.onreading = () => {
    console.log('illuminance level :', sensor.illuminance);
      if(sensor.illuminance < 15) {
            document.querySelector('body').classList.add('dark');
      }
      else {
            document.querySelector('body').classList.remove('dark');
      }
  };
  sensor.onerror = (event) => {
    console.log(event.error);
  };
  sensor.start();
}

Then, add specific css for your dark mode under the class you've just set. Like here I am adding and removing the class dark based on the level of ambient light.
You can play around with the illuminance value and see what works better in your case.

I also found this cool experiment by @tomlokhorst, where he changed font weight based on ambient light using variable fonts.
However, he mentions further in the thread that he did not use the aforementioned AmbientLightSensor but it still makes a pretty good use case.

---

Disclaimer: The browser support for this feature is low.
On chrome it's only available under a flag. Check the caniuse stats for more details. Despite that the future of web is bright or in a dark mode, however you like it! :)

EDIT-
If you're getting an error, something like AmbientLightSensor does not exist on type Window while trying to make this work in Angular, refer to this comment for a solution.

Reference

• Ambient Light Sensor W3C Spec
• Since I mentioned variable fonts, here is an interesting read on that

Comments

[Ben Halpern]

Woah!

[Ananya Neogi]

🎉

[Antony Garand]

Very nice!

I'll try making a POC where the website becomes progressively darker based off the illuminance level, using css custom properties, if anyone is interested in joining me!

If someone wants to out the snippet using a real website, Brawldb is dark by default, but light when the body has the theme-light class.

It is usually toggled via the top-right menu, but should also work via this snippet.

[Ananya Neogi]

Definitely, go for it! :)

I also made a very simple demo web page sometime back. Check that out, if you wish, to see this code work instantly.

[Andy Zhao (he/him)]

WHAT THE HECK that's amazing! I wonder how the browser detects the light???

[Andrew Bone]

It relies on the device having a sensor.

developer.mozilla.org/en-US/docs/W...

[Andy Zhao (he/him)]

Ah, okay. For some reason I thought it was solely based on software.

[Saulo Vargas]

Usually cellphones or some notebooks have hardware to detect light and auto-adjust brightness, so it can be used for this.

[Ananya Neogi]

Yes, this is based on the sensors that are present on your device. The Sensors API exposes those device sensors on to the web platform.

[Ananya Neogi]

It is based on the Sensors API.
Also, to use this sensor, browser needs permission from the user, that is done via the Permissions API.

[Aldrin Navarro]

I read a similar article last year which convinced me that Ambient Light Sensor API is very cool! Check it out!

[Rijubrata Bhaumik]

Hello Ananya,
Great that you liked the ALS feature on Chrome. We are planning to ship ALS on Chrome by default sometime soon. It would be great to get more inputs
from creative webdev folks. Feel free to ask for more details :)

Riju,
(ALS/sensors -chrome+W3C)

Rijubrata Bhaumik

@rijubrata

📣 How would you use Ambient Light Sensor for the Web platform ?

✅Adaptive Dimming (dark/dim mode(s))
✅Camera calibration (ISO..)
✅Reader mode (change fonts ?)

we want to ship ALS on Chrome, please share your ideas💡in the comments or GH
📖 w3c.github.io/ambient-light/…

13:26 PM - 12 Jul 2019

2 2

[XYZ]

If there is no sensor available then you might also just use the local time. Maybe not for a complete dark mode but for something like a night shift mode. The same way the OSs are doing it.

[M. Andrew Darts]

So stoked to see someone writing about this!
The web APIs are growing and I am so ready to see what people build with these!

Great work!

[Ananya Neogi]

Thanks! Indeed, web APIs are growing and it's exciting!

[MsumanP-hexaware]

How can i implement the same in angular ?
I tried but Its giving error.

[Ananya Neogi]

This is just plain JavaScript so it should work. Can you elaborate what error it is?
Maybe it's something specific to angular and not related to this particular bit of code.

[MsumanP-hexaware]

I am getting error like Property 'AmbientLightSensor' does not exist on type 'Window' & Cannot find name 'AmbientLightSensor'.

My Code:
if (window.AmbientLightSensor) {
const sensor = new AmbientLightSensor();
sensor.onreading = () => console.log(sensor.illuminance);
sensor.onerror = event => console.log(event.error.name, event.error.message);
sensor.start();
}

[Ananya Neogi]

You need to extend the existing Window interface to tell it about this new property.
So, before your class implementation add this code -

declare global {
    interface Window {
        AmbientLightSensor: any;
    }
}

Then you can add your AmbientLightSensor code in the following way -

        if (window.AmbientLightSensor) {
            const sensor = new window.AmbientLightSensor();
            sensor.onreading = () => console.log(sensor.illuminance);
            sensor.onerror = event => console.log(event.error.name, event.error.message);
            sensor.start();
        }

I hope this will solve your problem.

[MsumanP-hexaware]

Now error is not coming but window.AmbientLightSensor is coming as undefined.

[Juneau Lim]

It’s mind-blowing. I never imaged such a thing is possible.
Furthermore such a small amount of code without asking permission. Love it.
Thank you so much for letting me know.