Extending javascript Function prototype object

Function.prototype.twice = function() {
var fn = this;
return function() {
return fn.call(null, fn.apply(null, arguments));
};
};

Function.prototype.twice2 = function() {
var fn = this;
return function() {
console.log(this); // Window Object
return fn.call(this, fn.apply(this, arguments));
};
};

function plus1(x) { return x + 1; }
var plus2 = plus1.twice();
var plus3 = plus1.twice2();
console.log(plus2(10)); // 12
console.log(plus3(10)); // 12

Reference:http://osteele.com/talks/ajaxian-2008/samples/idioms-9.js.html