function orbitalPeriod(arr) {
  var GM = 398600.4418;
  var earthRadius = 6367.4447;
    arr.forEach(element => {
        element.orbitalPeriod = Math.round(2 * Math.PI * Math.sqrt(Math.pow(earthRadius + element.avgAlt, 3) / GM));
        delete element.avgAlt;
    });
  return arr;
}

console.log(orbitalPeriod([{name : "sputnik", avgAlt : 35873.5553}]));