y.js

const y = (t) => {
  const b = ({b, t}) => {
    return (n) => {
      const a = () => {
        return {b, t}
      }

      return t(b(a()))(n)
    }
  }
  const a = () => {
    return {
      b: ({b, t}) => {
        return (n) => {
          const a = () => {
            return {b, t}
          }

          return t(b(a()))(n)
        }
      },
      t
    }
  }
  return t(b(a()))
}

const fact = y((self) => (n) => {
  if (n == 0) {
    return 1
  }
  return n * self(n - 1)
})
console.log(fact(5))