zcombinator.js

const z = (f) => {
  const g = (x) => {
    return f((v) => {
      const xx = x(x);
      return xx(v);
    });
  };
  return g(g);
};

const fact = z((fact1) => {
  return (x) => {
    if (x === 0) {
      return 1;
    } else {
      return x * fact1(x - 1);
    }
  };
});

console.log(fact(5)); // 120