fixed-point (Y) combinator in JS

test.js

// compute factorial of 5
Y(function(self){
  return function(x){
    return x == 0 ? 1 : x * self(x - 1);
  };
})(5); // === 120

with_letrec.js

var Y = function(f){
  return function recurse(){
    return f(function(){ return recurse().apply(this, arguments); });
  }();
};

without_letrec.js

var Y = function(f){
  var recurse = function(self){
    return f(function(){ return self(self).apply(this, arguments); });
  };
  return recurse(recurse);
};