/* decided to use ES5 */
/* added it to the array prototype although is not recomended for a prod release, just as a example */
(function(proto) {
  function isArray(val) {
    return (Object.prototype.toString.call(val) === '[object Array]') ? true : false;
  }

  function flat(a,b) {
    return a.concat(isArray(b) ? b.reduce(flat, []) : [b]);
  }

  proto.flatten = function() {
    return this.reduce(flat, [])
  }
})(Array.prototype)

/* use
 * [43,3,[3,35,64,[34,3,[433]]]].flatten();
 */