chaining `fs.readdir` with a `.then` to return an array

Question

I am trying to create an array of specific files in a directory; which will go through a few test cases to make sure it fits a given criteria.

I'm using the fs.readdir method, but it doesn't return a promise meaning I cannot push to an array.

My idea was to populate an array (arr) with the files I actually want to output and then do something with that array. But because readdir is asynchronous and I can't chain a .then() onto it, my plans are quashed.

I've also tried the same thing with readdirSync to no avail.

const Promise = require('bluebird');
const fs = Promise.promisifyAll(require('fs'));

var arr = [];

fs.readdirAsync(folder).then( files => {
  files.forEach(file => {
    fs.stat(folder + '/' + file, (err, stats) => {
       if(!stats.isDirectory()) {
         arr.push(file);
        return;
      }
     });
   });
})
.then( () => {
  console.log(arr);
});

Answer 1

fs.readdir is callback based, so you can either promisify it using bluebird or Node.js util package (or writing a simple implementation of it yourself), or simply wrap the call in a promise, like so:

// Wrapped in a promise
new Promise((resolve, reject) => {
    return fs.readdir('/folderpath', (err, filenames) => err != null ? reject(err) : resolve(filenames))
})

Or the custom promisify function:

// Custom promisify
function promisify(fn) {
  /**
   * @param {...Any} params The params to pass into *fn*
   * @return {Promise<Any|Any[]>}
   */
  return function promisified(...params) {
    return new Promise((resolve, reject) => fn(...params.concat([(err, ...args) => err ? reject(err) : resolve( args.length < 2 ? args[0] : args )])))
  }
}

const readdirAsync = promisify(fs.readdir)
readdirAsync('./folderpath').then(filenames => console.log(filenames))