app4.js

function draw (e) {
  if(!isDrawing) return ;
  console.log(e);
  ctx.beginPath();
  ctx.moveTo(lastX, lastY);
  ctx.lineTo(e.offsetX, e.offsetY);
  ctx.stroke();
  [lastX, lastY] = [e.offsetX, e.offsetY];
}

canvas.addEventListener('mousemove', draw);
canvas.addEventListener('mousedown', (e) => {
    isDrawing = true;
    [lastX, lastY] = [e.offsetX, e.offsetY];
});
canvas.addEventListener('mouseup', ()=> isDrawing = false);
canvas.addEventListener('mouseout', () => isDrawing = false);