This is a small script I wrote that uses the Fitbit API to download all my old runs with GPS data so I can upload them to Strava.
You'll need to find your user ID as well as your Bearer Auth token and places these in the provided ENV file. I didn't sign up for an API key, I just pulled these off the Fitbit dashboard by looking at the requests it was sending to its API when loading data.
Once these ENV variables are added the script can be run using npm start. The TCX files will be downloaded to the "runs" folder and needed according to their activity ID. These files can then be manually uploaded to Strava via their website.