An implementation of the Y Combinator in Javascript

y-combinator.js

const Y = (fn) => (
  (_fn) => fn((arg) => (_fn(_fn))(arg))
)(
  (__fn) => fn((arg) => (__fn(__fn))(arg))
);

var factorial = (fact) => (n) => n === 0 ? 1 : n * fact(n - 1);

Y(factorial)(7);
//> 5040

/* 
 * const _factorial = Y(factorial)
 *
 * _factorial = (n) => n === 0 ? 1 : n * _fact(n - 1);
 * where _fact is (arg) => (_fn(_fn))(arg)
 * and where _fn is (__fn) => factorial((arg) => (__fn(__fn))(arg))
 * 
 */