server.js

'use strict';
// server.js
// load the things we need
//This is to parse the arguments from the cmd line
const optimist = require('optimist');

//listen for our initial argument.
const abArray = optimist.argv["array"];
let insideArray = [1,2,3,[1,2,[4,5],3,4],2];
let passedInput = true;

const app = {

    //main function
    runThisThing: function (arrayInput){
        const arrayToFlatten = this.sanitizeInput(arrayInput);
        const result = this.flattenArray(arrayToFlatten);

        if (!passedInput){
            console.log("You need to pass the array to flatten as an argument: --array arrayToFlatten");
        }

        console.log(result);
    },

    sanitizeInput: function(arg){
        //Check to see if user passed any arguments
        if (arg && arg.length > 0){
            //Because the intial argument from optimist is always a string, we gast
            //convert to an array.
            try {
                insideArray = JSON.parse(arg);
            } catch (e) {
                console.log("Sorry, this is likely an invalid array object " + e);
                insideArray = [];
            }
        } else {
            passedInput = false
        }

        return insideArray;
    },

    flattenArray: function(abArray){
        //Check to see if the parsed value is a valid array still.
        let finalValue = []
        if (Array.isArray(insideArray)){
            //do the actual flattening, then add "[]" to end and front of the resulting string
            //to make the parsing back to an array complete
            const result = "[" + insideArray.toString() + "]";
            finalValue = JSON.parse(result);  
        } 

        return finalValue;
    }

}

app.runThisThing(abArray);

module.exports = app;

serverTests.js

// load the things we need
var expect = require('chai').expect;
const app = require("../server.js");


describe('Run Things', function () {
    it('This should return a valid array', function () {
      
      const values = "[2, 4]";
      const result = app.sanitizeInput(values);
      expect(Array.isArray(result)).to.be.equal(true);
  
    });

    it('This should return a valid array', function () {
      
      const values = "[2, 4]";
      const result = app.flattenArray(values);
      expect(Array.isArray(result)).to.be.equal(true);
  
    });
});