Uncaught TypeError: undefined is not a function app.js

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 the delete button
  deleteButton.onclick = deleteTask;

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

//************************** "Wire up the variables & Functions" *************************

//Set the click handler to the addTask function


addButton.onclick = addTask; //by not adding in the () at this point, we ensure the function isn't executed when code hits this point. It should only execute once the button is clicked.

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

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