downloadStravaGPX.js

//
// 1. Go to https://www.strava.com/athlete/training
//
// 2. Open the Chrome developer console
//
// 3. Paste into the console the code from
// https://github.com/eligrey/FileSaver.js/blob/master/src/FileSaver.js
//
// 4. Paste the two functions below into the console
//
// 5. Type run getActivities(true) or getActivities(false)
//
// With true as the parameter it will wait for the activity to download before
// starting the next download. 
//
// With false it will send the requests off for all at the same time. This is 
// much faster but may fail - more testing needed.
//
// To get more activities select another page of activities and run getActivities()
// again.

function getActivity(i) {
   return new Promise(function(resolve, reject) {
       var attr = activityCollection.models[i].attributes
       var filename = attr.start_time + '-' + attr.name + ".tcx"
       var xhttp = new XMLHttpRequest();
       xhttp.open("GET", attr.activity_url + "/export_tcx", true);
       xhttp.responseType = 'blob'
       xhttp.onload = function() {
           if (xhttp.responseURL === attr.activity_url) { // 302 redirect back to the activity page
               console.log (i 
                 + ": Looks like Strava can't download this activity. Does it have a distance?\n"
                 + "Try manually downloading this url " 
                 + attr.activity_url + "/export_tcx");
           } else {
               saveAs(xhttp.response, filename);
               console.log(i + ':' + filename);
           }
           resolve();
       }
       xhttp.onerror = function() {
           console.log("Error: " + i + ':' + filename);
           reject();
       }
       xhttp.send();
   });
}

async function getActivities(wait) {
   for (var i = 0; i < activityCollection.models.length; i++) {
       wait ? await getActivity(i) : getActivity(i);
   }
   console.log("all done")
}

I had an issue with saveAs not being defined – everything worked when I pasted the contents of https://cdnjs.cloudflare.com/ajax/libs/FileSaver.js/1.3.8/FileSaver.js into the console as well (found via https://stackoverflow.com/a/54527783)