Log function not working in cli app using vorpal js

I am creating a cli app using node js & vorpal. There is no syntax error or warnings and everything works perfectly except that vorpal log doesn't work. Below is a small code snippet.

//functions.js

const mkdir = require('mkdirp');

function makeDir(dirname,location) {
    let p = `${location}/${dirname}`;
    mkdir(p, function(err) {
        if (err) return err;
        return `Directory ${dirname} created !`;
    });
}
module.exports.makeDir = makeDir;

//main.js

const app = require('vorpal')();
const functions = require('./functions');

app
    .command('newdir <name> <location>', 'Create new database')
    .action(function(args,cb) {
        let name = args.name;
        let location = args.location;
        functions.makeDir(name,location,function(err,msg) {
            if (err) this.log(err);
            this.log(msg);  //nothing gets logged
        });
        cb();
    });

app
.delimiter('app $')
.show();

As I said everything works fine & the directory is created but no log is displayed.

What I've Tried : Using app.log & using app.session.log. It does not even log custom strings, example : this.log('Hello')

System : Windows

The makeDir function should be coded to accept a callback and call it when done with the appropriate information passed, like below:

const mkdir = require('mkdirp');

function makeDir(dirname,location, callback /* accept a callback */ ) {
    let p = `${location}/${dirname}`;
    mkdir(p, function(err) {
        if (callback)
          callback(err ? err : null, err? null : `Directory ${dirname} created !`);
    });
}
module.exports.makeDir = makeDir;

Then you can do:

functions.makeDir(name,location,function(err,msg) {
  if (err) this.log(err);
  this.log(msg);  //nothing gets logged
} /* callback now is accepted */ );

In your action.

PS make sure you bind the this reference to your callback, for example using Function.prototype.bind so you dont get funny results when using this.log inside the callback