function replaceChat(source,pos,newChar){
     if(pos<0||pos>=source.length||source.length==0){
         return "invalid parameters...";
     }
     var iBeginPos= 0, iEndPos=source.length;
     var sFrontPart=source.substr(iBeginPos,pos);
     var sTailPart=source.substr(pos+1,source.length);
     var sRet=sFrontPart+newChar+sTailPart;
     return sRet;
 }
    alert(replaceChat("happy",1,"b"));