how to cross out the text when clicking checkBox javascript

I'm trying to cross out text next to the checkbox button when the checkbox is clicked by the user. But when I test it, for some reason nothing is happening. I want to check if the box is checked. If it is then I want to to cross that item next to that check box. This function, however does not work. Can someone please explain what I'm doing wrong? thanks.

function myFunction() {
  var editButton = document.createElement("button");
  //button.delete
  var deleteButton = document.createElement("button");
  var item = document.getElementById("todoInput").value
  var checkBox = document.createElement("input");
  checkBox.type = "checkbox";
  checkBox.id = "checkbox"
  var text = document.createTextNode(item)
  var newItem = document.createElement("li")
  newItem.className = "addedClass"

  newItem.appendChild(text)
  if (item === "") {
    alert("please fill in the blanks");
  } else {
    var crap = document.getElementById("todoList")
    crap.appendChild(newItem)
    var addhere = document.getElementById("todoList")
    addhere.appendChild(checkBox);
  }

  function updateItem() {
    if (document.getElementById(checkbox).checked) {
      document.getElementById(todoList).style.textDecoration = "line-through"
    }
  }
}

<form name="myForm" id="todoForm">
  <input id="todoInput" name="fname" required>
  <button type="button" onclick="myFunction()">OK</button>
</form>
<ol id="todoList"></ol>