index.js

var separateBy;
var displayBy;
var sortUsing;
var sortOrder;

function seperateMethod() {
   var radioButtons = document.querySelector(
      'input[name="separate-by"]:checked'
   ).value;
   switch (radioButtons) {
      case "separate-new-lines":
         separateBy = /\n/g;
         break;
      case "separate-commas":
         separateBy = ",";
         break;
      case "separate-custom":
         separateBy = document.getElementById("separate-custom-input").value;
         break;
   }
}

function displayMethod() {
   var radioButtons = document.querySelector('input[name="display-by"]:checked')
      .value;
   switch (radioButtons) {
      case "display-new-lines":
         displayBy = "<br />";
         break;
      case "display-commas":
         displayBy = ", ";
         break;
      case "display-custom":
         displayBy = document.getElementById("display-custom-input").value;
         break;
   }
}

function sortUsingMethod() {
   var radioButtons = document.querySelector('input[name="sort-using"]:checked')
      .value;
   switch (radioButtons) {
      case 'no-sort':
         sortUsing = function(array) {
            return array;
         }
         break;
      case "alphabetically":
         sortUsing = function(array) {
            return array.sort();
         };
         break;
      case "numerically":
         sortUsing = function(array) {
            return array.sort((a, b) => a - b);
         };
         break;
      case "word-length":
         sortUsing = function(array) {
            return array.sort((a, b) => a.length - b.length);
         };
         break;
   }
}

function sortOrderMethod() {
   var radioButtons = document.querySelector('input[name="sort-order"]:checked')
      .value;
   switch (radioButtons) {
      case "increasing-order":
         sortOrder = function(array) {
            return array;
         };
         break;
      case "decreasing-order":
         sortOrder = function(array) {
            return array.reverse();
         };
         break;
   }
}

function listSort() {
   toggleCopyButton();
   seperateMethod();
   displayMethod();
   sortUsingMethod();
   sortOrderMethod();

   var listInput = document.getElementById("list-input").value;
   var sortedList = document.getElementById("sorted-list-op");

   var separateArray = listInput.split(separateBy);

   separateArray = document.getElementById("remove-duplicates").checked
      ? [...new Set(separateArray)]
      : separateArray;
   separateArray = document.getElementById("ignore-characters")
      ? separateArray.map(x =>
           x.substr(document.getElementById("ignore-characters-input").value)
        )
      : separateArray;

   var sortArray = sortUsing(separateArray.filter(x => x != ""));
   var orderedArray = sortOrder(sortArray);
   var jointArray = orderedArray.join(displayBy);
   var breaklineList = jointArray.replace(/<br \/\>/g, "\n");
   sortedList.innerHTML = breaklineList;
}

function copySortedList() {
   var sortedList = document.getElementById("sorted-list-op");
   sortedList.select(); //selects everything within the textarea where id = "sorted-list-op"
   if (document.execCommand("copy")) {
      alert("copied");
   }
}

function toggleCopyButton() {
   let listInput = document.getElementById("list-input");
   let copyButton = document.getElementById("copy-sorted-button");

   if (listInput.value !== "") {
      copyButton.disabled = false;
   } else {
      copyButton.disabled = true;
   }
}