y-combinator-es3.js

function Y(le) {
  return (function (f) {
    return f(f);
  }(function (f) {
    return le(function (x) {
      return f(f)(x);
    });
  }));
}

var factorial = Y(function (fac) {
  return function (n) {
    return n <= 2 ? n : n * fac(n - 1);
  };
});

factorial(5); // 120

y-combinator-es6.js

let Y =
  (le => (f => f(f))
    (f => le((...args) => f(f)(...args))));

let factorial =
  Y(f => (n =>
    (n < 2 ?
      1 :
      n * f(n - 1))));

factorial(5); // 120

y-combinator-scheme.rkt

(define (Y f)
  ((lambda (x) (x x))
   (lambda (g)
     (f (lambda args (apply (g g) args))))))

(define fac
  (Y
    (lambda (f)
      (lambda (x)
        (if (< x 2)
          1
          (* x (f (- x 1))))))))

(fac 5) ; 120