Add button not working properly

//problem: user interation doesnt provide desired results 
//soltuion: add interactivity so the user can manage daily tasks

var taskInput = document.getElementById("new-task"); //new-task
var addButton = document.getElementsByTagName("button")[0];     //firs button
var incompleteTaskHolder =   document.getElementById("incomplete-tasks"); //incomplete-tasks
var completeTasksHolder = 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");
    //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.class = "edit";
    deleteButton.innerText = "Delete";
    deleteButton.class = "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 with the text from the #new-task
  var listItem = createNewTaskElement(taskIncomplete.value);

  //append listItem to 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 inputs value
    //else
      //switch to .editMode 
      //input value becomes the labels text

  //toggel .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
    completeTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskIncomplete);
}

//mark a task as incomplete
var taskIncomplete = function(){
  console.log("task incomplete...");
    //append the 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 the editTask to edit button
        editButton.onclick = editTask;

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

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


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

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

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