How to work with promises in this example?

Question

JavaScript

There is a code that counts the size of all files in the current folder:

'use strict';

const fs = require('mz/fs');

fs.readdir(__dirname)
  .then(function(filesNames) {
    return Promise.all(
      filesNames.map(fileName => fs.stat(fileName))
    )
  })
  .then(function(stats) {
    return stats.filter(stat => stat.isFile())
  })
  .then(function(stats) {
    return stats.reduce((sum, stat) => sum + stat.size, 0)
  })
  .then(console.log);

But what if after checking stat.isFile()we need to perform various manipulations with the contents of the file? Those. needs to be executed fs.readFile(), and we have already lost the file name.
How to do it?

Answer 1

Decision:

'use strict';

const fs = require('mz/fs');

fs.readdir(__dirname)
  .then(filesNames => Promise.all(
      filesNames.map(fileName => {
          return fs.stat(fileName).then(stat => {
              return {name: fileName, stat: stat};
          });
      })
    )
  )
  .then(stats => stats.filter(
    statsObj => statsObj.stat.isFile()
  ))
  .then(stats => stats.reduce(
    (sum, statsObj) => {
      console.log(`${statsObj.name}: ${statsObj.stat.size}`);
      return sum + statsObj.stat.size;
    }, 0
  ))
  .then(console.log)
  .catch(error => console.log(error));

Answer 2

Emptyform Have you ever read about promises?
You pass in then through resolve What you need, in reject what you need to send in catch
in your case, the result is passed to resolve filesNames.map(fileName => fs.stat(fileName))
Refer to the JS documentation