Factorial (Javascript)

factorial.js

var factorial = function(num) {
  return !num ? 1 : num *= factorial(num - 1);
}

// Shorter ES6 variant
// const factorial = num => !num ? 1 : num *= factorial(num-1);

alert(factorial(6)); // 720
// https://jsfiddle.net/2ary59t0/157/