Errors surfaced in DOM code (for Effective TypeScript)

dom-errors.ts

function handleDrag(eDown: Event) {
  const targetEl = eDown.currentTarget;
  targetEl.classList.add('dragging');
// ~~~~~~~           Object is possibly 'null'.
//         ~~~~~~~~~ Property 'classList' does not exist on type 'EventTarget'.
  const dragStart = [
    eDown.clientX, eDown.clientY
       // ~~~~~~~                'clientX' does not exist on 'Event'.
       //                ~~~~~~~ 'clientY' does not exist on 'Event'.
  ];
  const handleUp = (eUp: Event) => {
    targetEl.classList.remove('dragging');
 // ~~~~~~~~           Object is possibly 'null'
 //          ~~~~~~~~~ Property 'classList' does not exist on type 'EventTarget'
    targetEl.removeEventListener('mouseup', handleUp);
 // ~~~~~~~~ Object is possibly 'null'.
    const dragEnd = [
      eUp.clientX, eUp.clientY
       // ~~~~~~~              Property 'clientX' does not exist on 'Event'.
       //              ~~~~~~~ Property 'clientY' does not exist on 'Event'.
    ];
    console.log('dx, dy = ', [0, 1].map(i => dragEnd[i] - dragStart[i]));
  }
  targetEl.addEventListener('mouseup', handleUp);
// ~~~~~~~ Object is possibly 'null'.
}

   const div = document.getElementById('surface');
   div.addEventListener('mousedown', handleDrag);
// ~~~ Object is possibly 'null'.