text: const MODERN_ACTIVITY = 15;
  const HALF_LIFE_PERIOD = 5730;
  const radioactiveDecayConst = 0.693 / HALF_LIFE_PERIOD;
  
  module.exports = function dateSample(sampleActivity) {
    if (typeof sampleActivity !== 'string') return false;
    sampleActivity = parseFloat(sampleActivity);
    if (sampleActivity <= 0 ||
      sampleActivity > MODERN_ACTIVITY ||
      sampleActivity === undefined ||
      isNaN(sampleActivity)) return false //  type of NaN is number,so unsuitable; NaN === NaN false, so isNaN to be used
    const ratioActivities = MODERN_ACTIVITY / sampleActivity;
    let result = Math.ceil(Math.log(ratioActivities) / radioactiveDecayConst);
    return result;
    //  alternative way, inverted conditons, to return false
    //return (
    //  sampleActivity > 0 &&
    //  sampleActivity < MODERN_ACTIVITY &&
    //  sampleActivity !== undefined &&
    //  !isNaN(sampleActivity) &&
    //  result);
  };
  