js替换指定位置字符串

var str='QWER';//替换WE
newstr=replacepos(str,1,2,'XX');
console.log(newstr);//QXXR;
function replacepos(text,start,stop,replacetext){
        mystr = text.substring(0,stop-1)+replacetext+text.substring(stop+1);
        return mystr;
}