Can't get onclick event attached to add button/firing correctly

//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
//Add a new task
var addTask = function() {
  console.log("Add task...");
  alert("Add task");
  //When the button is pressed
  //Create a new list item with teh text from #new-task:
    //Input (checkbox)
    //Label
    //Input (text)
    //Button.edit
    //Button.delete
    //Each element 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 is .editMode
        //Switch from .editMode
        //Label text become the input's value
      //else
        //Switch to .editMode
        //Input value become's 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 Checkbox is checked
      //Append the task list item to the #completed-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
}

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