advent.js

const input = `$ cd /
$ ls
dir a
14848514 b.txt
8504156 c.dat
dir d
$ cd a
$ ls
dir e
29116 f
2557 g
62596 h.lst
$ cd e
$ ls
584 i
$ cd ..
$ cd ..
$ cd d
$ ls
4060174 j
8033020 d.log
5626152 d.ext
7214296 k
`;

const MAX_FILE_SIZE = 100000;
const FOLDERS_PATH_DIVIDER = ".";

const moveForward = (currentPath, dirName) =>
  (currentPath += `${FOLDERS_PATH_DIVIDER}${dirName}`);
const moveBack = (currentPath) =>
  currentPath.slice(0, -currentPath.lastIndexOf(FOLDERS_PATH_DIVIDER));

const parse = (string) => {
  const stringArr = string.split("\n");
  const resObj = {};

  let currentPath = "";

  stringArr.map((s) => {
    if (s.includes("$ cd")) {
      const dirName = s.split(" ")[2];

      currentPath =
        dirName === ".."
          ? moveBack(currentPath)
          : moveForward(currentPath, dirName);
    } else if (!s.includes("$ ls") && !s.includes("dir")) {
      const fileSize = +s.split(" ")[0];

      resObj[currentPath] = resObj[currentPath] + fileSize || fileSize;
    }
  });

  // not the best solution from the time complexity perspective, but i didn't find any other
  // I guess it will be better to use the tree instead of flat object with full paths
  Object.entries(resObj).map(([key, value]) => {
    let parentFolder = key;

    while (parentFolder.length > 2) {
      parentFolder = moveBack(parentFolder);
      resObj[parentFolder] = resObj[parentFolder] + value;
    }
  });

  const sumOfFolderSizes = Object.entries(resObj).reduce(
    (acc, [_, value]) => (value < MAX_FILE_SIZE ? acc + value : acc),
    0
  );

  return sumOfFolderSizes;
};

console.log(parse(input));