var  str = new c_str();
str.setvalue("456");
str.setabc("789");
alert(str.getvalue() + "\r\n" + str.getabc());

//------------
function c_str()
{
  this.value = "xyz";
  this.abc = "123";
  this.getvalue = function () { return this.value; }
  this.setvalue = function (s) { this.value = s; }
  this.getabc = function () { return this.abc; }
  this.setabc = function (s) { this.abc = s; }
  //...
}

String.prototype.xyz = c_xyz;

var str = new String("123");

function c_xyz()
{
	   return "<h1>" + this.valueOf() + "</h1>";
}

alert(str.xyz());