I can't get my add task button working/tweaked. Stuck.

//Problem: User interaction doesn't provide desired results.
//Solution: Add interactivity so the user can manage daily tasks.

var taskInput; //new-task
var addButton = document.getElementsByTagName('button')[0]; //first button
var incompleteTaskHolder = document.getElementById('incomplete-tasks'); //incomplete-tasks
var completedTaskHolder = 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

  //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() {
  taskInput = '';
  console.log('Add task...');
//Create a new list item with the text from #new-task
  var listItem = createNewTaskElement(taskInput.value);

//Append listItem to incompleteTasksHolder
  incompleteTaskHolder.appendChild(listItem);
  bindTaskEvents(listItem, taskCompleted);


}


//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 the 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...');
  //Append the task list item to the #completed-tasks
  var listItem = this.parentNode;
  completedTaskHolder.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 the delete button
  deleteButton.onclick = deleteTask;
    //bind taskCompleted to checkbox
  checkBox.onchange = checkBoxEventHandler;
}

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

addButton.onclick 

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