This is an unofficial organization. I am in no way affiliated with Nintendo.
This organization provides the source code that used to be hosted on https://nintendo-playground.com. Now, the playground is no longer hosted online, but you can still host it locally if you want.
The playground lets you test custom client implementations without contacting Nintendo's servers. This can be useful because invalid requests can easily get your device banned, and buying a new one is expensive.
Here are some examples that show why custom clients can be useful:
I also wanted to practice with web technology (see infrastructure).