flatten Arrays

flatten.js

// https://stackoverflow.com/questions/10865025/merge-flatten-an-array-of-arrays-in-javascript/39000004#39000004

const flatten = function(arr, result = []) {
  for (let i = 0, length = arr.length; i < length; i++) {
    const value = arr[i];
    if (Array.isArray(value)) {
      flatten(value, result);
    } else {
      result.push(value);
    }
  }
  return result;
};