Is this valid syntax for node/promisify async function?

Question

I accidentally typed await(await stat(content... and it worked. Not sure if this is valid syntax, or there is a better way to do it? I'm trying to read all the files that are directories and do not match my regex.

const fs = require('fs')
const path = require('path')
const content = path.resolve('.') + '/docs' + '/'
const util = require('util');
const stat = util.promisify(fs.stat)
const readDir = util.promisify(fs.readdir)
const directories = 'docs/';
const exclude = new RegExp(/^(adir|\.somedir)/,'i');
let newFiles = {}

async function main(){
    const ls = await readDir(directories)
    console.log('starting....');
    let newArray = []
     for (let index = 0; index < ls.length; index++) {
               let x =  await (await stat(content + ls[index])).isDirectory()
               let file = ls[index]
               if (x && !(exclude.test(file))){newArray.push(file)}
               console.log('x is ',x);
        }   
    console.log('new filtered array: ', newArray);
}

Answer 1

ls

My advice would be not to put all of your eggs in one basket. We can write an ultra fast ls function using Node's fs.Dirent objects and bypass the need for a slow fs.stat call on each file -

// fsext.js

import { readdir } from "fs/promises"
import { join } from "path"

async function* ls (path = ".")
{ yield { dir: path }
  for (const dirent of await readdir(path, { withFileTypes: true }))
    if (dirent.isDirectory())
      yield *ls(join(path, dirent.name))
    else
      yield { file: join(path, dirent.name) }
}

async function toArray (iter)
{ const r = []
  for await (const v of iter)
    r.push(v)
  return r
}

export { ls, toArray }

// main.js

import { ls, toArray } from "./fsext.js"

toArray(ls("./node_modules")).then(console.log, console.error)

To test it out, let's add some popular npm packages so we have a large hierarchy to test our our program. We'll install the lot and count the number of directories and files -

$ npm install async chalk commander debug express immutable lodash moment prop-types react react-dom request webpack

$ find ./node_modules | wc -l

5453

Now let's run our program and time it -

$ time node main.js

[
  { dir: './node_modules' },
  { dir: 'node_modules/.bin' },
  { file: 'node_modules/.bin/acorn' },
  { file: 'node_modules/.bin/browserslist' },
  { file: 'node_modules/.bin/loose-envify' },
  { file: 'node_modules/.bin/mime' },
  { file: 'node_modules/.bin/sshpk-conv' },
  { file: 'node_modules/.bin/sshpk-sign' },
  { file: 'node_modules/.bin/sshpk-verify' },
  { file: 'node_modules/.bin/terser' },
  { file: 'node_modules/.bin/uuid' },
  { file: 'node_modules/.bin/webpack' },
  { file: 'node_modules/.package-lock.json' },
  { dir: 'node_modules/@types' },
  { dir: 'node_modules/@types/eslint' },
  { file: 'node_modules/@types/eslint/LICENSE' },
  { file: 'node_modules/@types/eslint/README.md' },
  { file: 'node_modules/@types/eslint/helpers.d.ts' },
  { file: 'node_modules/@types/eslint/index.d.ts' },
  { dir: 'node_modules/@types/eslint/lib' },
   ... 5433 more items
]

node main.js  0.09s user 0.02s system 116% cpu 0.099 total

dirs

If we only want directories, we can write dirs as a simple specialization of our generic ls -

// fsext.js (continued)

async function* dirs (path)
{ for await (const f of ls(path))
    if (f.dir)
      yield f.dir
}

$ find ./node_modules -type d | wc -l

457

Now compare it against our program

// main.js

import { dirs, toArray } from "./fsext.js"

toArray(dirs("./node_modules")).then(console.log, console.error)

$ time node.main.js

[
  './node_modules',
  'node_modules/.bin',
  'node_modules/@types',
  'node_modules/@types/eslint',
  'node_modules/@types/eslint/lib',
  'node_modules/@types/eslint/lib/rules',
  'node_modules/@types/eslint/rules',
  'node_modules/@types/eslint-scope',
  'node_modules/@types/estree',
  'node_modules/@types/json-schema',
  'node_modules/@types/node',
  'node_modules/@types/node/assert',
  'node_modules/@types/node/dns',
  'node_modules/@types/node/fs',
  'node_modules/@types/node/stream',
  'node_modules/@types/node/timers',
  'node_modules/@types/node/ts3.6',
  'node_modules/@webassemblyjs',
  'node_modules/@webassemblyjs/ast',
  'node_modules/@webassemblyjs/ast/esm',
  ... 437 more items
]

node main2.js  0.09s user 0.02s system 108% cpu 0.099 total

exclude

If we want to exclude certain directories or files, we can write it generically as well -

// fsext.js (continued)

async function* exclude (iter, test)
{ for await (const v of iter)
    if (Boolean(test(v)))
      continue
    else
      yield v
}

// main.js

import { dirs, exclude, toArray } from "./fsext.js"

toArray(exclude(dirs("./node_modules"), v => /@/.test(v)))
  .then(console.log, console.error)

$ time node main.js

[
  './node_modules',
  'node_modules/.bin',
  'node_modules/accepts',
  'node_modules/acorn',
  'node_modules/acorn/bin',
  'node_modules/acorn/dist',
  'node_modules/ajv',
  'node_modules/ajv/dist',
  'node_modules/ajv/lib',
  'node_modules/ajv/lib/compile',
  'node_modules/ajv/lib/dot',
  'node_modules/ajv/lib/dotjs',
  'node_modules/ajv/lib/refs',
  'node_modules/ajv/scripts',
  'node_modules/ajv-keywords',
  'node_modules/ajv-keywords/keywords',
  'node_modules/ajv-keywords/keywords/dot',
  'node_modules/ajv-keywords/keywords/dotjs',
  'node_modules/ansi-styles',
  'node_modules/array-flatten',
  ... 351 more items
]

node main.js  0.09s user 0.02s system 105% cpu 0.104 total

reorganize

In our file system extensions module, fsext, we wrote two functions that work on any iterables, not just the ls or dirs. I would suggest breaking these out into their own iter module. This type of reorganization helps decouple concerns and maximize code reuse throughout your entire program -

// iter.js

async function* empty () {}

async function* exclude (iter = empty(), test = Boolean)
{ for await (const v of iter)
    if (Boolean(test(v)))
      continue
    else
      yield v
}

async function toArray (iter = empty())
{ const r = []
  for await (const v of iter)
    r.push(v)
  return r
}

export { empty, exclude, toArray }

// fsext.js

import { readdir } from "fs/promises"
import { join } from "path"

async function* ls (path = ".")
{ yield { dir: path }
  for (const dirent of await readdir(path, { withFileTypes: true }))
    if (dirent.isDirectory())
      yield *ls(join(path, dirent.name))
    else
      yield { file: join(path, dirent.name) }
}

async function* dirs (path)
{ for await (const f of ls(path))
    if (f.dir)
      yield f.dir
}

async function* files (path)
{ for await (const f of ls(path))
    if (f.file)
      yield f.file
}

export { ls, dirs, files }

// main.js

import { dirs } from "./fsext.js"
import { exclude, toArray } from "./iter.js"

const somePath = "..."
const someTest = v => ...

toArray(exclude(dirs(somePath), someTest))
  .then(console.log, console.error)

search

Looking for a specific file or folder? Read on in this Q&A to implement search.

Answer 2

Much credit goes to gorg and certainPerformance. This is a simple solution.

const { stat, readdir } = require('fs').promises;

async function main() {
    try {
        const getFiles = await readdir(directories, { withFileTypes: true })
        let foo = getFiles.filter(x=> x.isDirectory() && ! excludeDir.test(x.name))
        .map(f=>f.name);
    } catch (err) {
    console.error(err);
    }
}

Answer 3

Since it works, the syntax is valid - but the code is confusing and probably shouldn't be used without some tweaks. The important thing to know here is that it's valid to await something that isn't a Promise. If the expression on the right is a Promise, the whole thing will resolve to the value of the resolved Promise; if the expression on the right is not a Promise, the whole expression will resolve to that value. That is:

await Promise.resolve(5)

is essentially the same as

await 5

but while the second works, it's confusing - better to only await things that are Promises. fs.isDirectory doesn't return a Promise, so it'd be a good idea to remove the await from it.

There's also a better approach altogether for what you're doing: use Promise.all instead, so that all items in the directory can be searched through at once, instead of having to wait for them one-by-one. Your current code will take a long time if there are a whole lot of items in the directory - this is not necessary.

You can also simplify the regex by using a regex literal instead of new RegExp.

const exclude = /^(?:adir|\.somedir)/i;

async function main() {
    const filenames = await readDir(directories);
    const newArray = await Promise.all(
        filenames.map(async (filename) => {
            const fileStat = await stat(content + filename);
            if (fileStat.isDirectory && !(exclude.test(file))) {
                return filename;
            }
        })
    )
    const results = newArray.filter(Boolean);
    console.log('new filtered array: ', results);
}

You can also consider using fs.promises instead, instead of using util.promisify.

const { stat, readdir } = require('fs').promises;

Answer 4

isDirectory returns a boolean, not a Promise<boolean> so the second await is superfluous, you could just write (await stat(content + ls[index])).isDirectory()