Hexlet logo

JS: Trees

Theory: Aggregation 2

Let's practice one more data aggregation technique on file systems. Let's write a function that takes a directory as input and returns a list of directories on the first level of nesting and the number of files inside each of them, including all subdirectories.

const tree = mkdir('/', [
  mkdir('etc', [
    mkdir('apache'),
    mkdir('nginx', [
      mkfile('nginx.conf'),
    ]),
  ]),
  mkdir('consul', [
    mkfile('config.json'),
    mkfile('file.tmp'),
    mkdir('data'),
  ]),
  mkfile('hosts'),
  mkfile('resolve'),
]);

console.log(getSubdirectoriesInfo(tree));
// => [['etc', 1], ['consul', 2]]

This task breaks down into two smaller ones:

  • Implementing the function counting files in a directory
  • Calling this function for each of the subdirectories

Let's start by counting the number of files. This is a classic aggregation task:

const getFilesCount = (node) => {
  if (isFile(node)) {
    return 1;
  }

  const children = getChildren(node);
  const descendantCounts = children.map(getFilesCount);
  return _.sum(descendantCounts);
};

The next step is to extract all the children from the initial node and apply a count to each of them:

const getSubdirectoriesInfo = (tree) => {
  const children = getChildren(tree);
  const result = children
    // We're only interested in directories
    .filter(isDirectory)
    // Counting in each directory
    .map((child) => [getName(child), getFilesCount(child)]);

  return result;
};

I.e., we accessed the children directly by first filtering them, and mapped them to the desired array, which contained the name and number of files for each directory.

Recommended programs

Navigation
Theory
Exercise