Questions about MAKE IT MODULAR in Learnyounode tutorial

I have several questions that I’m stuck on. First here’s the code for the Module I have written so far:

var fs = require("fs");
var path = require("path");
module.exports = function (input, filter, cback) {
  fs.readdir(input, function doneReading(err, list) {
    if (err) return callback(err)
    for (var i = 0; i < list.length; i++) {
      var extension = list[i].split(".").pop();
      if (extension == filter && list[i].split(".").length != 1) {

Here’s the code for the Main file that calls the Module:

var program = require("./program");
program(process.argv[2], process.argv[3], function() {});

First, I know it’s a placeholder and isn’t right since there has to be more to it but in the Main file I don’t understand what I’m supposed to do with the passed in callback so I did a placeholder since my code passes all the tests except for the test I have a question on below. So first thing is how does a passed in callback to a module work?

Second question, the test results are these:

Your submission results compared to the expected:

                 ACTUAL                                 EXPECTED                

   ""                      ==    ""                     
   ""                        ==    ""                       
   ""                         ==    ""                        
   ""                                  ==    ""                                 



 Submission results match expected


 Additional module file exports a single function


 Additional module file exports a function that takes 3 arguments


 Your additional module file [program.js] does not appear to pass back an
 error received from fs.readdir(). Use the following idiomatic Node.js
 pattern inside your callback to fs.readdir(): if (err) return

 # FAIL Your solution to MAKE IT MODULAR didn't pass. Try again!

If you see the code above I did include an err check inside the readdir callback but for some reason it doesn’t get counted correctly and I don’t know why.

In your main file you are passing an empty function as your cback. Also, look carefully at what you have named your variable for your call back function.