Convert XML files to JSON and move them

What the code does (tested basic, no errors, usecase):

• Reads all XML files (ending in .yin) from a directory.
• converts each to JSON
• store it in another directory, with the same basename as source file
• ... with .yin replaced by .json

Overt issues:

• The code has too many nested functions
• walk module's next() method in multiple places
• lack of robust error checks and ensuring no uncaught errors left behind

Q. How to refactor the code using Node.js best practices? (things like):

• flatten out the nesting.
• avoiding having to remember to call next(), on all if(err) scenarios.
• does any other (than those included) module improve readability/efficiency?

Two Node.js files, a module and a main file invoking that module, are attached.

File 1: Main:

// main.js
var path = require ('path');
var yin2json = require ('./yin2json');

console.log ("__dirname: " + __dirname);
var templateDir = path.resolve (__dirname, "resources", "mgmtDataTemplates");
var yinDir = path.resolve (templateDir, "yins");
var jsonDir = path.resolve (templateDir, "jsons");

yin2json.convertYins (yinDir, jsonDir);

File 2: Module:

// yin2json.js

    var fs = require('fs');
    var path = require('path');
    var walk = require('walk');
    var xml2js = require('xml2js');

    var jsonDir ;
    var convertYins = function (yin_dir, json_dir) {
        jsonDir =  json_dir;

        var walker = walk.walk(yin_dir, { followLinks: true });
        walker.on("errors", fDirWalkError);
        walker.on("end", fDirWalkEnd);
        walker.on("file", fDirWalkFile);

    }

    function fDirWalkError (err) {
        console.log ("fDirWalkError: " + err);
        next (err);
    }

    function fDirWalkEnd  () {
        console.log ("======= End of directory walk");
    }

    function fDirWalkFile (root, fileStat, next) {
        if (fileStat.name.indexOf(".yin") < 0) {
            console.log ("skipping file " + fileStat.name + " (does not end in .yin)");
            return;
        } else {
            var yin_file = path.resolve(root, fileStat.name);
            console.log ("yin file: " + yin_file);

            fs.readFile(yin_file, function (err, data) {
                if (err) {
                    console.log ("error reading file:" + yin_file);
                    next (err);
                }
                xml2js.parseString (data, function (err, json_obj) {
                    if (err) {
                        console.log (err);
                        next (err);
                    }
                    var json_string = JSON.stringify(json_obj, null, 2);
                    var json_file = path.resolve (jsonDir, path.basename(yin_file).replace(/\.yin$/, ".json"));
                    console.log ("json file: ", json_file);

                    fs.writeFile(json_file, json_string, "utf8", function (err) {
                        if (err) {
                            console.log ("error converting yin (%s) to json(%s)", yin_file, json_file);
                            next (new Error ("error converting yin(" + yin_file + ") to json(" + json_file + ")"));
                        }
                        else {
                            console.log ("Converted yin (%s) to json(%s)", yin_file, json_file);
                        }
                    });
                });
            });
        }
        next ();
    }

module.exports.convertYins = convertYins;