二分查找法优缺点：

数组有序的情况下：

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title></title>
    </head>
    <body>
        <script type="text/javascript">
            //顺序必须有序
             var num = 100000;
             var randomNum = Math.ceil(Math.random()*num);
             document.title = randomNum;
             var arr = [];
             for(var i = 0 ;i<num;i++)
             {
                 arr.push(i)
             }
             console.log(randomNum);
              //======第一种方式=========
             function show1(arr,randomNum)
             {
                 console.time(2)
                   for (var i = 0; i <arr.length; i++) {
                       if(arr[i] === randomNum)
                       {
                           console.timeEnd(2);
                           return arr[i];
                       }
                   }
             }
             show1(arr,randomNum);
             //======第二种方式=========
             function show(arr,randomNum)
             {
                 console.time(1)
                 var first = 0;
                 var last = arr.length -1;
                 
                 while(first <= last)
                 {
                     var mindex = Math.floor((first + last)/2);
                     if(randomNum < arr[mindex]) //大于中间值
                     {
                         last = mindex -1;
                     }else if(randomNum > arr[mindex]){
                         first = mindex +1;
                     }else{
                         console.timeEnd(1)
                          return arr[mindex];
                     }
                 }
             }
             console.log(show(arr,randomNum));
        </script>
    </body>
</html>

数组无序的情况下：

使用二分先把数组排列成有序------然后二分所花的时间比普通for循环的时间要长

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title></title>
    </head>
    <body>
        <script type="text/javascript">
            //顺序必须有序
             var num = 100000;
             var randomNum = Math.ceil(Math.random()*num);
             document.title = randomNum;
             var arr = [];
             for(var i = 0 ;i<num;i++)
             {
                 arr.push(i)
             }
             //无序的排序
             arr.sort(function(){
                 return Math.random() -0.5;
             })
             console.log(randomNum);
              //======第一种方式=========
             function show1(arr,randomNum)
             {
                 console.time(2)
                   for (var i = 0; i <arr.length; i++) {
                       if(arr[i] === randomNum)
                       {
                           console.timeEnd(2);
                           return arr[i];
                       }
                   }
             }
             show1(arr,randomNum);
             //======第二种方式=========
             function show(arr,randomNum)
             {
                 console.time(1)
                 arr.sort(function(n1,n2){
                     return n1 -n2;
                 })
                 var first = 0;
                 var last = arr.length -1;
                 
                 while(first <= last)
                 {
                     var mindex = Math.floor((first + last)/2);
                     if(randomNum < arr[mindex]) //大于中间值
                     {
                         last = mindex -1;
                     }else if(randomNum > arr[mindex]){
                         first = mindex +1;
                     }else{
                         console.timeEnd(1)
                          return arr[mindex];
                     }
                 }
             }
             console.log(show(arr,randomNum));
        </script>
    </body>
</html>