//Implementação Dr. Axel
function findKeyAxel(object, callback, thisValue) {
  for (const [key, value] of Object.entries(object)) {
    if (callback.call(thisValue, value, key, object)) {
      return key;
    }
  }
  return undefined;
}

//Minha implementação
function findKey(obj, callback){
    const key = Object.entries(obj).find(callback)[0];
    return ( key !== undefined ? key:undefined );
}