如何获取背景以显示全图

body {
  background-image: url('背景图片的URL');
  background-size: cover;
  background-repeat: no-repeat;
}

<div class="background"></div>

.background {
  background-image: url('背景图片的URL');
  background-size: cover;
  background-repeat: no-repeat;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
}

document.body.style.backgroundImage = "url('背景图片的URL')";
document.body.style.backgroundSize = "cover";
document.body.style.backgroundRepeat = "no-repeat";