flatten.js

/*
 * The function flattens an arbitrarily nested array of k-dimensions using recursion.
 * The implementation is quite fast (i.e. O(n) where n is the total number of 
 * elements in the k-th dimensional array where k >= 1).
 * However stack overflow errors could occur in cases where k is particularly large - 
 * thus requiring a very deep level of recursion. 
 *
 * @param {number[][]} - A k-dimensional array where k >= 1
 * @returns {number[]} - A 1-dimensional flattened array
 */
const flatten = function(arr) {
  if (!Array.isArray(arr)) {
    throw new TypeError('Argument is not an array'); 
  }
  // a closure and an IIFE is used to 'cache' the partly flattened array until completion
  // of the recursively called innerFlatten()
  const flattenedArray = [];
  return (function innerFlatten(arr) {
    for (let i = 0, length = arr.length; i < length; i++) {
      const value = arr[i];
      if (Array.isArray(value)) {
        innerFlatten(value);
      } else {
        flattenedArray.push(value);
      }
    }
    return flattenedArray;
  })(arr);
};

module.exports = flatten;