Call built-in from overridden/overwritten function of Object in javascript

Hi All.
Do any body know how to call built in function of object like substr from overridden/overwritten version. I am trying following example but the function called itself recursively not the function being overridden/overwritten.
String.prototype.substr= function(str)
{

return str.substr(0,5);
}

I shall be very thankful if some one could give me some hints.
Sultan Wadood.