script.js

window.onload = function() {
  var tslaBodyTag = document.body,
      tslaMenuMask = document.getElementById('tsla-header-mask'),
      tslaMenuHeader = document.getElementById('tsla-header-main'),
      tslaMenuCheckbox = document.getElementById('tsla-header-main--trigger'),
      tslaHeaderCheckboxes = document.getElementsByClassName('tsla-header-checkbox');

  window.addEventListener('resize', tslaResize);

  // if the browser window is greater than or equal to 640, uncheck checkboxes
  function tslaResize() {
    if (window.innerWidth >= 640) {
      uncheckCheckboxes(tslaHeaderCheckboxes);
    }
  }

  // uncheck all the checkboxes
  function uncheckCheckboxes(elements) {
    if(elements.length) {
      for (var i = 0; i < elements.length; i++) {
        elements[i].checked=false;
      }
    }
  }

  // if user clicks on the 'mask', uncheck all the checkboxes
  tslaMenuMask.addEventListener('click', function() {
    if (tslaMenuCheckbox.checked) {
      uncheckCheckboxes(tslaHeaderCheckboxes);
      tslaBodyTag.classList.remove('tsla-prevent-scroll');
    }
  });

  // if user clicks on the menu opener, add/remove a class to the body tag
  tslaMenuCheckbox.addEventListener('click', function() {
    if (tslaMenuCheckbox.checked) {
      tslaBodyTag.classList.add('tsla-prevent-scroll');
    } else {
      tslaBodyTag.classList.remove('tsla-prevent-scroll');
    }
  });
}