Uncaught TypeError:

//Problem: user interaction does not 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

// add a new task.
var addTask = function() {
  console.log("Add task...");
  //When the button is pressed
  //Create a new list item with the text from #new-task:
    //input (checkbox)
    //label
    //input (text)
    //button.edit
    //button.delete
    //Each elements, needs modified and appended
}



// Edit an existing task.
var editTask = function() {
  console.log("Edit task...");
  //When the edit button is pressed
  //if the class of the parent i .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...");
    //When the delete button is pressed
      //Remove the parent list item from the ul

}


// Mark a task as complete.
var taskCompleted = function() {
  console.log("Task complete...");
  //When the Chockbox is checked
    //Append the task list item to the #complete-tasks

}



// Mark a task as Incomplete.
var taskIncomplete = function() {
  console.log("Task incomplete...");
  //When the checkbox is unchecked
    //Append the task list item to the #incomplete-tasks

}



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 deleteButton
  deleteButton.onclick = deleteTask;

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

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