ES6中Promise.race

<script type="text/javascript">
   let p1 = new Promise(function(resolve,reject){
      setTimeout(function(){
         resolve("450毫秒后执行")
      },450)
   });
   let p2 = new Promise(function(resolve,reject){
      setTimeout(function(){
         resolve("300毫秒后执行")
      },300)
   });
   let p3 = new Promise(function(resolve,reject){
      setTimeout(function(){
         resolve("400毫秒后执行")
      },400)
   });
   let p4 = new Promise(function(resolve,reject){
      setTimeout(function(){
         resolve("500毫秒后执行")
      },500)
   });
   
   let arr = [p1,p2,p3,p4];
   //有多个异步操作,只有1个返回给结果,就立即执行.
   let result = Promise.race(arr);
   result.then(function(res){
      console.log(res)
   })
</script>