y combinator

snippets.js

/**
 * Creates a recursive function from one that isn't
 * @param  {Function} x A non-recursing function
 * @return {Function}   A recursive version of x
 */
function yCombinator(x) {
  return (function(procedure) {
    return x(function(arg) {
      return procedure(procedure)(arg);
    });
  })(function(procedure) {
    return x(function(arg) {
      return procedure(procedure)(arg);
    });
  });
}

// Usage
var factorial = yCombinator(function(recurse) {
  return function(x) {
    return x === 0 ? 1 : x * recurse(x - 1);
  };
});

var result = factorial(6);
console.log(result); // logs 720