Y-Combinator in modern JavaScript

index.js

// Y-Combinator implemented in JavaScript
// Inspired (mostly copied) from http://kestas.kuliukas.com/YCombinatorExplained/ 
// Thank you!

const Y = g =>
  (f => f(f))(f =>
    g(x => f(f)(x)))

const factorial = given => 
  n => n < 2
    ? 1
    : n * given(n - 1)

// Y(factorial)(5) => 120

Follow up with multiple args:

const Y = g =>
  (f => f(f))(f =>
    g((...args) =>
      f(f)(...args)))

const isPrime = given => (n, s = 2) => n === s
  ? true
  : (n % s !== 0) && given(n, ++s)

console.log(Y(isPrime)(2));   // -> true
console.log(Y(isPrime)(17));  // -> true
console.log(Y(isPrime)(24));  // -> false