How to loop through the file system?

Question

Node.js

There is a directory and directories nested in it that contain photos, I need to go through all the files in these directories, process the photos and put them in another directory, but at the same time preserve the structure. If there is only one directory and files in it, then this is easy to do:

var fs = require('fs')
  , gm = require('gm');

    var data = fs.readdirSync('img');


     data.map(function(item, i, arr) {
       gm('img/' + item)
         .write('opt/' + item, function (err) {
          if (err) console.log(err);
        });

     });

Tell me how to loop through the nested directories, while maintaining this hierarchy