Tracks the battery on load and before unload

trackBatteryUsage.js

function collectBatteryUsage(stage){
  navigator.getBattery().then(function(batteryManagerData){
    ga('send', 'event', 'battery usage', stage, String(batteryManagerData.level));
  });
}

(function trackBatteryUsage(){
  if(!window.Promise){
    ga('send', 'event', 'browser support features', 'Promises', 'unable to handle Promises');
    return;
  }
  
  if(typeof(navigator.getBattery) !== 'function'){
    ga('send', 'event', 'browser support features', 'Battery API', 'unable to handle Battery API');
    return;
  }
  
  // Collect battery level when user enters the page.
  collectBatteryUsage('initial');
  
  // Collect battery level when user exits the page.
  window.onbeforeunload(function(){
    collectBatteryUsage('before exit');
    return null; 
  });
  
})();