"abcd"反转为"dcba"，

1、利用正则和数组反转，无循环

function reverse(oldstr){

　　var oldStrArr = oldstr.replace(/\w/g,‘$&,‘).split(‘,‘);

　　return oldStrArr.reverse().toString().replace(/,/g,‘‘);

}

var aa = "abcd";

var bb = reverse(aa);

console.log(bb);//"dcba"

思路："abcd" -> "a,b,c,d," -> ["a","b","c","d",""] ->["","d","c","b","a"] ->"d,c,b,a" -> "dcba"

　　"abcd" -> "a,b,c,d," －－－：replace(/\w/g,‘$&,‘) //正则

　　"a,b,c,d," -> ["a","b","c","d",""]－－－：split(‘,‘) //字符串基本API

　　["a","b","c","d",""] ->["","d","c","b","a"]－－－：reverse() //数组基本API

　　["","d","c","b","a"] ->"d,c,b,a"－－－：toString() //数组基本API

　　"d,c,b,a" -> "dcba"－－－：replace(/,/g,‘‘) //正则

2、循环遍历字符串

JS小方法－字符串反转