Watching for the battery getting low

low-battery.js

let warningShowed = false;

if ('getBattery' in navigator) {
  navigator.getBattery().then(battery => {
    checkBatteryLevel();
    battery.onchargingchange = checkBatteryLevel;
    battery.onchargingtimechange = checkBatteryLevel;
    battery.ondischargingtimechange = checkBatteryLevel;
    battery.onlevelchange = checkBatteryLevel;
    
    function checkBatteryLevel () {
      // dischargingTime is seconds until battery is empty. 1200 is 20 minutes
      // level is a percentage of 100. So 0.2 is 20%
      if (battery.dischargingTime < 1200 || battery.level <= 0.2 && !warningShowed) {
        alert(`Warning, battery level is at ${toPercentage(battery.level)}.`);
        warningShowed = true;
      }
    }
  });
}

/* Convert a decimal like 0.2 to a percentage: 20% */
function toPercentage(number) {
  return `${Math.round(number * 100)}%`;
}