how is deleteButton.onclick = deleteTask running if its in a function that hasn't been called?

//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");  // type of 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 

  //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("Some New Task");
    //Append listItem to incompleteTaskHolder
  incompleteTasksHolder.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 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...");
  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;  
  completedTasksHolder.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;
  incompleteTasksHolder.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 delete button
  deleteButton.onclick = deleteTask;

  //bind checkBoxEventHandler to 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 < completedTasksHolder.children.length; i++) {
  //bind events to list item's children (taskIncomplete)
  bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
}