Javascript: flatten function on Array

arr-flatten.js

// Defining flatten method on array

Array.prototype.flatten = function () {
  var result = [];
  (function arrayFlatten(arr) {
    arr.forEach(function(element) {
      element.constructor === Array ? arrayFlatten(element) : result.push(element);
    })
  })(this);
  return result;
};

// Examples
var arr = [1, [2, 3], [4, 5], [6, 7, 8]];
arr.flatten(); // [1, 2, 3, 4, 5, 6, 7, 8]

arr = [1, [2, 3, [4, 5, [6, 7]]], [8], [9, 10]]
arr.flatten(); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]