Flatten multi-dimensional arrays recursively in Javascript

flatten.js

function flattenArray(array) {
  let result = [];
  array.forEach(function(item, i) {
    if (Array.isArray(item)) {
      result = result.concat(flattenArray(array[i]));
    }
    else {
      result = result.concat([item]);
    }
  });
  return result;
}

console.log(flattenArray([1,2,[3,[4]], 5])); // [1,2,3,4,5]