I want to understand a mechanism of callbacks, so i try to compute a summary size for files in one directory. Here is my code:

import path from 'path';
import fs from 'fs';
import async from 'async';

export const getDirectorySize = (directory) => {
  fs.readdir(directory, (error, files) => {
    if (error) throw error;

    const counter = (el, callback) => { 
      fs.stat(path.join(directory, el), (error2, result) => {
        if (error2) throw error2;
        if (result.isFile()) callback(null, result.size);
      });
    }

    async.map(files, counter, (error3, results) => {
        if (error3) throw error3;
        console.log(results.reduce((prev, current) => prev + current, 0));
      });
    });
}
getDirectorySize('/usr/src/app');

So when i run code above, the third argument (callback) of async.map doesn't call. But function "counter" is called (There were no errors)! So how to see the results of async.map?