flatten.js

// `flatten` will flatten an array of arbitrarily nested arrays of integers
// into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4]
var flatten = (arr) => {
  // if arr isn't an array, just return an empty array... for now
  // there should be better rules around this
  if (!arr || !Array.isArray(arr)) [];

  var result = arr.reduce((acc, curr) => {
    // if the current value (curr) is an array, we need to
    // recursively call `flatten`
    if (Array.isArray(curr)) {
      return acc.concat(flatten(curr));
    } else {
      return acc.concat([curr]);
    }
  }, []);

  // filter out any non-number values
  return result.filter((el) => typeof el === 'number');
}