JS Y-combinator

y.js

const Y = a=>(a=>a(a))(b=>a((...a)=>b(b)(...a)));

// Usage
/*
 * var result = Y((the function itself) => (...parameters) => {
 *   return result;
 * })(...parameters);
 *
 */

// Example

const gcd = Y(gcd => (n1, n2, ...ns) => {
  if (n2) return gcd(n2, n1 % n2, ...ns);
  return n1;
});

const lcm = Y(lcm => (n1, n2, ...ns) => {
  if (n2) lcm(n1 * n2 / gcd(n1, n2), ...ns);
  return n1;
});