function bsearch(A,x){

        //l：查找范围左 r:查找范围右

        let l = 0, //查询范围左边界

            r = A.length-1, //查找范围右边界

            guess //中间猜测位置

        while(l <= r){

            //guess等于l,r中间位置

            guess = Math.floor( (l+r)/2 )       

            //判断当前位置是否为要查找的值，是则返回下标

            if(A[guess] === x) return guess

            //如果当前位置值大于要查询的值 右边界位置减一

            else if(A[guess] > x) r = guess -1

            //否则左边界位置加1

            else l = guess + 1

        }

        //找不到返回-1

        return -1

    }

    const a = [3,5,4,7,12,42,47,66,71,78];

    console.log(bsearch(a,12))//

    console.log(bsearch(a,4))//-1

    console.log(bsearch(a,66))//

    console.log(bsearch(a,5))//