Get URL parameter by name JS prototype

/*
@Mamdouh Freelancer
SO account: http://stackoverflow.com/users/7040620/
Github: https://github.com/Mamdouh-Freelancer
*/

//usage: location.href.param("key");

String.prototype.param = function(str) {
  e = this.indexOf("?" + str + "=") > -1 ? "?" + str + "=" : "&" + str + "=";
  return this.indexOf(e) == -1 ? undefined : this.split(e)[1].split("&")[0];
}

var url = "http://sub.domain.com/?key=value&option=value";
var k = url.param("key");