var str="aabbdhxhsa";
while(~str.indexOf('a')) str=str.replace('a','b');
console.log(str);

这绝对是流氓问题，为什么不能用正则？不过还是可以使用内置函数的：

 function replaceAll(target, search, replacement){
    return target.split(search).join(replacement);
 }

 //测试
 var target = 'abcabc';
 var search = 'a';
 var replacement = 'A';
 var s = replaceAll(target, search, replacement);
 console.log(s);//AbcAbc

"aabbcc".split("a").join("b")

    var str="aabbdhxhsa";
    console.log(str.replace(/a/g, "b")); // bbbbdhxhsb

试试看

var str="aabbdhxhsa"
var newStr = str.replace("a","b"); 
alert(newStr);