When I run the console, I get this error message: Uncaught TypeError: Cannot read property 'parentNode' of undefined.

//Problem: User interraction doesnt provide required results
//Solution: Add interactivity so the user can manage daily tasks

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

//New task list item
var createNewTaskElement = function(taskString) {
  //Create list Item
  var listItem = document.createElement("li");
   //input (checkbox)
  var checkBox = document.createElement("input");
  //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 needs modifying

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

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

  label.innerText = taskString;

  //each element needs to be 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 with the text from #new task 
  var listItem = createNewTaskElement(taskInput.value);
//Append listItem to the incompleteTaskHolder
  incompleteTaskHolder.appendChild(listItem);
  bindTaskEvents(listItem, taskCompleted);
}

//Edit an existing task
var editTask = function() {
  console.log("Edit task...");
//When the Edit button is pressed
  //if the class of the parent is .editMode
    //Switch from .editMode
    //label text become the input's value
  //else
    //Switch to .editMode
    //input value becomes the label's text

    //toggle .editMode on the parent
}

//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("Task Complete...");
 //Append the task list item to the #completed-tasks
 var listItem = this.parentNode;
  completedTasksHolder.appendChild(listItem);
  bindTaskEvents(listItem, taskIncomplete);
}

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

var bindTaskEvents = function(taskListItem, checkBoxEventHandler){
  console.log("Bind list item events");
  //select taskListItem's 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 delete button
  deleteButton.onclick = deleteTask();

    //bind taskCompleted to checkbox
  checkBox.onchange = checkBoxEventHandler;
}

//Set the click handler to the add task function
addButton.onclick = addTask;

var ajaxRequest = function(){
 console.log("AJAX request"); 
}
addButton.onclick = ajaxRequest;

//Cycle over the incompleteTaskHolder ul list items
for (var i = 0; i < incompleteTaskHolder.children.length; i++) {
//bind events to list item's children (taskCompleted)
  bindTaskEvents(incompleteTaskHolder.children[i], taskCompleted);
}   

//Cycle over the completeTaskHolder ul list items
  for (var i = 0; i < completedTasksHolder.children.length; i++) {
//bind events to list item's children (taskInComplete)
  bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
}