js替换字符串

var v = "abbbabbbbab";
var pos = v.lastIndexOf("a");
var vv = v.substring(0, pos) + "c" + v.substring(pos+1, v.length);
alert(vv);

var str = "adksdjfldjfoeoxl";
var t = str.lastIndexOf("d");
str = str.replace(/d/, "c");
alert(str);