You can see a demo on https://jmperezperez.com/beats-audio-api
You can search for any track in the Spotify's catalog, and display a diagram of the detected peaks. Then, click on 'Play' to listen to the song while seeing an indicator on top of the peaks diagram. Apart from the detected tempos, the tempo provided by Spotify is shown.
Run it yourself
There are no dependencies to install, and you don't need to register any app on Spotify. Just clone the project and start a server.