drawLines();

function getHeight(){
  return window.innerHeight
|| document.documentElement.clientHeight
|| document.body.clientHeight;
}

function drawLines(){
  const lines = document.getElementsByClassName('line');
  if(lines.length) {
    for (let i = 0; i < lines.length; i++) {
        document.body.removeChild(lines[i]);
    }
  }
  
  for(i = 0; i < getHeight()/10; i++){
    const line = document.createElement("div");  
    line.className = `line line-${i}`;
    line.style.top = `${i * 10}px`;
    const time = Math.random() * 5;
    line.style.animation = `lines ${time}s infinite`;
    document.body.appendChild(line) ;
  }
}

window.onresize = function(event) {
  drawLines();
};