Flattens an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4]

flatten.js

function flatten(array)  {
  let flat_array = [];
  let len_array = array.length;
  
  for (let i = 0; i < len_array; i++) {
    if (Array.isArray(array[i])) {
      flat_array = flat_array.concat(flatten(array[i]));
    } else {
      flat_array.push(array[i]);
    }
  }
  return flat_array;
}