Array.prototype.flatten = Array.prototype.flatten || function flatten(){
  const result = this.reduce((flatArray, element) => Array.isArray(element) ?
      [...flatArray, ...flatten.call(element)] :
      [...flatArray, element],
  []);

  return result;
};


[1,'a', undefined, [undefined, 1, [2], 3], null].flatten(); // [ 1, 'a', undefined, undefined, 1, 2, 3, null ]
[[1,2,[3]],4].flatten();  // [ 1, 2, 3, 4 ]