My add task button isn't working

//Problem: User interaction doesn't provide desired 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
var incompleteTasksHolder = 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"); //CheckBox
  //label
  var label = document.createElement("label");
  //input (text)
  var editInput = document.createElement("input"); //text
    //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";
  deleteButton.className = "delete";


  //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 with the text from #new-task:
  var ListItem = createNewTaskElement(TaskInput.value);

  //Append ListItem to incompleteTasksHolder
  incompleteTasksHolder.appendChild(ListItem);
  bindTaskEvents(ListItem, taskIncomplete);

  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 the parent is .editMode
  if(containsClass) {
      //Switch from .editMode
      //label text become input's value
    label.innerText = editInput.value;
  } else {
      //Switch to .editMode
      //input value becomes the label's text
editInput.value = label.innerText;
  }
     //Toggle .editMode on the list item
  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("task complete");
  //When the checkbox is checked
    //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");
  //When the checkbox is unchecked
    //Append the task list item to the #completed-tasks
  var listItem = this.parentNode;
  incompleteTasksHolder.appendChild(listItem);
  bindTaskEvents(listItem, taskCompleted);
}

var bindTaskEvents = function(taskListItem, checkBoxEventHandler) {
  console.log("Bind list items events");
  //select taskListItem's childeren
  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 taskCompleted to the checkbox
  checkBox.onchange = checkBoxEventHandler;
}

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


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

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