Javascript String printf(obj)

String.prototype.printf = function (obj) {
 var useArguments = false;
 var _arguments = arguments;
 var i = -1;//ww w .ja va 2s .c o  m
 
 if (typeof _arguments[0] == "string") {
  useArguments = true;
 }
 
 if (obj instanceof Array || useArguments) {
  return this.replace(/\%s/g,
   function (a, b) {
    i++;
    if (useArguments) {
     if (typeof _arguments[i] == 'string') {
      return _arguments[i];
     }
     else {
      throw new Error("Arguments element is an invalid type");
     }
    }
  
   return obj[i];
  });
 }
 else {
  return this.replace(/{([^{}]*)}/g,
   function (a, b) {
    var r = obj[b];
    return typeof r === 'string' || typeof r === 'number' ? r : a;
  });
 }
};