<style>
 .box{width: 4px;height: 5px;background: wheat;position: absolute;border-radius: 50%;}
 body{background: url(../img/bg.jpg) ;}
</style>

<script>
class Glowworm{
 constructor(){
 // 獲取屏幕的可視區(qū)域的寬高，用作將來(lái)的隨機(jī)范圍
 this.clientW = document.documentElement.clientWidth;
 this.clientH = document.documentElement.clientHeight;
 // 假設(shè)螢火蟲的寬高
 this.w = 20;
 this.h = 20;
 }
 createEle(){
 var div = document.createElement("div");
 div.className = "box";
 document.body.appendChild(div);
 // 在創(chuàng)建元素之前一定得先生成隨機(jī)坐標(biāo)
 div.style.left = this.x + "px";
 div.style.top = this.y + "px";
 // 元素創(chuàng)建好之后，立即運(yùn)動(dòng)
 this.move(div);
 }
 randomPos(){
 // 隨機(jī)生成坐標(biāo)
 this.x = random(0,this.clientW - this.w);
 this.y = random(0,this.clientH - this.h);
 }
 move(ele){
 // 開始運(yùn)動(dòng)之前，還得隨機(jī)生成目標(biāo)
 this.randomPos();
 // 開始運(yùn)動(dòng)
 move(ele,{
  left:this.x,
  top:this.y
 },()=>{
  // 一個(gè)動(dòng)畫結(jié)束后，重復(fù)開啟當(dāng)前動(dòng)畫，即可
  this.move(ele);
 })
 }
}

for(var i=0;i<60;i++){
 // 先得到實(shí)例
 var g = new Glowworm();
 // 生成隨機(jī)坐標(biāo)
 g.randomPos();
 // 再創(chuàng)建元素
 g.createEle();
}

function random(a,b){
 return Math.round(Math.random()*(a-b)+b);
}
</script>

function move(ele,obj,cb){
 clearInterval(ele.t);
 ele.t = setInterval(() => {
 var i = true;
 for(var attr in obj){
  if(attr == "opacity"){
  var iNow = getStyle(ele,attr) * 100;
  }else{
  var iNow = parseInt(getStyle(ele,attr));
  }
  let speed = (obj[attr] - iNow)/10;
  speed = speed < 0 ? Math.floor(speed) : Math.ceil(speed);
  // 只要有一個(gè)屬性沒到目標(biāo)：絕對(duì)不能清除計(jì)時(shí)器
  if(iNow !== obj[attr]){
  i = false;
  }
  if(attr == "opacity"){
  ele.style.opacity = (iNow + speed)/100;
  }else{
  ele.style[attr] = iNow + speed + "px";
  }
 }
 if(i){
  clearInterval(ele.t);
  if(cb){
  cb();
  }
  // cb && cb();
 }
 }, 30);
}

function getStyle(ele,attr){
 if(ele.currentStyle){
 return ele.currentStyle[attr];
 }else{
 return getComputedStyle(ele,false)[attr];
 }
}