This is an example application that uses the Spotify app SDK to access the realtime audio data. A simple visualisation is generated using Processing.js. As the Spotify boilerplate application was used as a starting point, the notes below are adapted from the boilerplate notes.
I wrote a blogpost about this application, explaining how it works. You can read it here.
To test the application your Spotify account needs to have the developer flag set. You can find out how to activate the developer flag here.
Clone this repository in your Spotify workspace directory. If you haven't got a Spotify directory, read about where to create it here. Cloning will create a subfolder called audio-fft-processing.
You can access the application (restart the Spotify desktop client if you have not done so) by typing in spotify:app:audio-fft-processing in the search field. Read more about this in the Spotify Developer Guidelines.
Helpful links: