I wrote a custom PNG encoder designed specifically for QR codes in JS. While the space is well served, PNGs created by my code come in at about 25% the size of those created by leading alternatives.

Really it was a vehicle to learn how PNGs work. It occurred to me that I know plenty about vector formats, but didn't know anything about how raster images work. It was a fascinating journey.

I also learnt a lot about typed arrays and data views in JS, and got to use some old knowledge of bit masking from my C days.

https://www.npmjs.com/package/qrcode-png