Flatten multi-dimensional arrays in JavaScript

flattenArray.js

(function() {
  if (!Array.prototype.flatten) {
    Array.prototype.flatten = function(array) {
      'use strict';
      
      // check if called on null object
      if (this == null) {
        throw new TypeError('Array.prototype.flatten must not be called on a null or undefined object.');
      }
      
      // check if argument is an Array
      if (!array instanceof Array) {
        throw new TypeError(array + ' is not an Array.');
      }
      
      // check if argument is empty
      if (!array.length) {
        throw new RangeError('Cannot flatten empty Array.');
      }
      
      var result = array;
      
      // reduce array by concatenation, funnel any
      // sub-Arrays into this function to handle recursively
      result = array.reduce(function(start, next) {
        return start.concat(Array.isArray(next) ? Array.prototype.flatten(next) : next);
      }, []);
      
      return result;
    }
  }
})();

Use as Array.prototype.flatten(arrayToFlatten). For example:

var arr = [1, 2, [4, 5], 2, 3, [4, 5, [6]]];
Array.prototype.flatten(arr);
// [1, 2, 4, 5, 2, 3, 4, 5, 6]