The Tor Browser / file revision

 /* This Source Code Form is subject to the terms of the Mozilla Public

  * License, v. 2.0. If a copy of the MPL was not distributed with this

  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

 // The Y combinator, applied to the factorial function

 // Return the function that is the fixed point of f.

 var Y = f => (x => f(v => x(x)(v)))

              (x => f(v => x(x)(v)));

 // The factorial function is the fixed point of this:

 var f = fac => n => (n <= 1) ? 1 : n * fac(n - 1);

 print("5! is " + Y(f)(5));