Flatten an array of arbitrarily nested arrays of integers into a flat array of integers

flattenArray.js

/**
 * Function that  will flatten an array of arbitrarily nested arrays of integers into a flat array of integers
 * @param arr The array to flatten
 * @param removeNonItems A flag to remove `undefined` and `null` values if set to true
 * 
 * @returns Array
 */
function flatten(arr, removeNonItems = false) {
  var nested = false;

  var flattened = arr.reduce(function(initial, item) {
    // Skip `undefined` and `null` items if the flag removeNonItems = true
    // check if they are `undefined` and `null` by using `== null`, we still want zeros
    if (removeNonItems && item == null) {
      return initial;
    }
    if (Array.isArray(item)) {
      nested = true;
    }
    return initial.concat(item);
  }, []);

  return nested ? flatten(flattened) : flattened;
}

var test = [
  [1, 2, 4, [12, [6]]],
  3, 0, null, undefined
];

var flattened = flatten(test); // [ 1, 2, 4, 12, 6, 3, 0, null, undefined ]
var flattened2 = flatten(test, true); // [ 1, 2, 4, 12, 6, 3, 0 ]