makeItModular.js

/**
 * Created by mmarinescu on 1/13/2017.
 *  ## MAKE IT MODULAR (Exercise 6 of 13)

 This problem is the same as the previous but introduces the concept of
 modules. You will need to create two files to solve this.

 Create a program that prints a list of files in a given directory,
 filtered by the extension of the files. The first argument is the
 directory name and the second argument is the extension filter. Print the
 list of files (one file per line) to the console. You must use
 asynchronous I/O.

 You must write a module file to do most of the work. The module must
 export a single function that takes three arguments: the directory name,
 the filename extension string and a callback function, in that order. The
 filename extension argument must be the same as what was passed to your
 program. Don't turn it into a RegExp or prefix with "." or do anything
 except pass it to your module where you can do what you need to make your
 filter work.

 The callback function must be called using the idiomatic node(err, data)
 convention. This convention stipulates that unless there's an error, the
 first argument passed to the callback will be null, and the second will be
 your data. In this exercise, the data will be your filtered list of files,
 as an Array. If you receive an error, e.g. from your call to
 fs.readdir(), the callback must be called with the error, and only the
 error, as the first argument.

 You must not print directly to the console from your module file, only
 from your original program.

 In the case of an error bubbling up to your original program file, simply
 check for it and print an informative message to the console.

 These four things are the contract that your module must follow.

 » Export a single function that takes exactly the arguments described.
 » Call the callback exactly once with an error or some data as described.
 » Don't change anything else, like global variables or stdout.
 » Handle all the errors that may occur and pass them to the callback.

 The benefit of having a contract is that your module can be used by anyone
 who expects this contract. So your module could be used by anyone else who
 does learnyounode, or the verifier, and just work.
 */
// CALL the function with
let myModule = require('./modularChild')
let dir = process.argv[2] || './'
let fileExtension = process.argv[3] || 'txt'
myModule(dir, fileExtension, (err, files) => {
    if(err)
        console.log(err)
    else
        files.forEach((file) => {
            console.log(file)
        })
})