Todo App - Undefined error

//User Interaction doesn't provide desired results
//Solution: Add interactivity so user can manage daily task

var taskInput = document.getElementById("new-task"); //new-task
var addButton = document.getElementsByTagName("button")[0]; //first button
var incompleteTasksHolder = document.getElementById("incomplete-tasks"); //incomplete-task
var completedTasksHolder = document.getElementById("completed-tasks"); //completed-task

 //New Task List Item
  var createNewTaskElement = function(taskString) {
    //Create List Item
    var listItem = document.createElement("li");
    //Input (checkboxes)
    var checkBox = document.createElement("input"); //checkbox
    //label
    var label = document.createElement("label");
    //Input (text)
    var editInput = document.createElement("input");
    //Button.edit
    var editButton = document.createElement("button");
    //Button.delete
    var deleteButton = document.createElement("button");

    //Each Element need modifying 

    checkBox.type = "checkbox";
    editInput.type = "text";

    editButton.innerText = "Edit";
    editButton.className = "edit";
    deleteButton.innerText = "Delete";
    deleteButton.className = "delete";

    label.innerText = taskString;


    //Each Element needs appending
    listItem.appendChild(checkBox);
    listItem.appendChild(label);
    listItem.appendChild(editInput);
    listItem.appendChild(editButton);
    listItem.appendChild(deleteButton);

    return listItem;
  }

//Add a new task
var addTask = function() {
  console.log("Add Task...");

  //Create a new list item from the #new-task
  var listItem = createNewTaskElement(taskInput.value);

  //Appen listItem to IncompletTasksHolder
  incompleteTasksHolder.appendChild(listItem);
  bindTaskEvents(listItem,taskCompleted);
  taskInput.value = "";
}

//Edit an existing task
var editTask = function() {
  console.log("Edit Task...");  

  var listItem = this.parentNode;
  var editInput = listItem.querySelector("input[type=text]");
  var label = listItem.querySelector("label");

  var containsClass = listItem.classList.contains("editMode");

  //If the class of parent is .editMode

  if(containsClass){
    //Switch from .editMode
    //label text becomes the input value
    label.innerText = editInput.value;
} else {
    //Switch to .editMode
    //Input value becomes the label's text
  editInput.value = label.innerText;
}
  //Toggle edit mode on the listItem
  listItem.classList.toggle("editMode");
}

//Delete an existing task
var deleteTask = function() {
  console.log("Delete Task...");  
  var listItem = this.parentNode;
  var ul = listItem.parentNode;

  //Remove the parent list item from the Ul
  ul.removeChild(listItem);
}

//Mark a task as complete
var taskCompleted = function() {
  console.log("Complete Task...");
    //Append the task li to the #complete-task
  var listItem = this.parentNode;
  completedTasksHolder.appendChild(listItem);
  bindTaskEvents(listItem,taskIncomplete);
}

//Mark a task as incomplete
var taskIncomplete = function() {
  console.log("Incomplete Task...");
  //Append the task to li to #incomplete-task
  var listItem = this.parentNode;
  incompleteTasksHolder.appendChild(listItem);
  bindTaskEvents(listItem,taskCompleted);
}

var bindTaskEvents = function(taskListItem, checkBoxEventHandler) {
  console.log("Bind list items events");
  //select taskListItems children
  var checkBox = taskListItem.querySelector("input[type=checkbox]");
  var editButton = taskListItem.querySelector("button.edit");
  var deleteButton = taskListItem.querySelector("button.delete");

    //bind editTask to edit button
    editButton.onclick = editTask;

    //bind deleteTask to the delete button
    deleteButton.onclick = deleteTask;

    //bind checkBoxEventHandlerto the checkbox
    checkBox.onchange = checkBoxEventHandler;
}

var ajaxRequest = function(){
  console.log("Ajax Request");
}

//Set the click handler to the addTask function
addButton.addEventListener("click", addTask);
addButton.addEventListener("click", ajaxRequest);


//cycle over incompleteTasksHolder ul list items
for(var i = 0; i < incompleteTasksHolder.children.length; i++){
    //bind events to list iitems children (taskCompleted)
    bindTaskEvents(incompleteTasksHolder.children[i], taskCompleted);
}  
//cycle over completeTaskHolder ul list items
for(var i = 0; i < completedTasksHolder.children.length; i++){
    //bind events to list items children(taskIncomplete)
    bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
}