script.js

document.addEventListener('DOMContentLoaded', function() {

  // Select the submit button and input to be used later
  const submit = document.querySelector('#addButton');
  const newTask = document.querySelector('.inputText');

  // Disable submit button by default:
  submit.disabled = true;

  // Listen for input to be typed into the input field
  newTask.onkeyup = () => {
      if (newTask.value.length > 0) {
          submit.disabled = false;
      }
      else {
          submit.disabled = true;
      }
  }

  // Listen for submission of form
  document.querySelector('form').onsubmit = () => {

      // Find the task the user just submitted
      const task = newTask.value;

      // Create a list item for the new task and add the task to it
      const li = document.createElement('li');
      li.innerHTML = task;

      // Add new element to our unordered list:
      document.querySelector('#myUL').append(li);

      // Clear out input field:
      newTask.value = '';

      // Disable the submit button again:
      submit.disabled = true;

      // Stop form from submitting
      return false; }

      var list = document.querySelector('ul');
list.addEventListener('click', function(ev) {
  if (ev.target.tagName === 'LI') {
    ev.target.classList.toggle('checked');
  }
}, false);

  }
);