return an object that groups all the child keys in an array by parent key

const createParentChildTree = (parentChildKeyArray) => {
  const parent = [];
  const children = [];
  const result = { children, parent };

  parentChildKeyArray.forEach(([parentKey, childKey]) => {
    if (!result.parent.includes(parentKey)) {
      result.parent.push(parentKey);
    }
    if (!result.children[parentKey]) {
      result.children[parentKey] = [];
    }
    result.children[parentKey].push(childKey);
  });

  return result;
};