How does the Spotify web browser button interact with the Spotify app?

Question

Check out this play button, written entirely it seems, using javascript: spotify play button.

Notice that pressing play will cause Spotify to start playing music. OK

Answer 1

The desktop runs a server at *.spotilocal.com on your computer, then on the web, the embed player (example) will choose a subdomain name (example: fdxubmxkqp.spotilocal.com).

It then makes request to the local server's API, examples:

https://fdxubmxkqp.spotilocal.com:4370/service/version.json
https://fdxubmxkqp.spotilocal.com:4370/simplecsrf/token.json

And it just returns some JSON:

{
"version": 9, 
"client_version": "1.0.10.107.gd0dfca3a"
}

See this screenshot for more info, or you can get the same info by going to news.spotify.com and using the Network inspector with spotilocal as a filter.