splitAt.js

const items = [1,52,14,54,[10,1,3,5],14,6];

const flatten = (arr) => {
  const flat = [].concat(...arr)
  return flat.some(Array.isArray) ? flatten(flat) : flat;
}

const splitAt = (arr, predicate) => {
  var selected = [];
  var others = [];
  for (var i = 0; i < arr.length; i++) {
    (predicate(arr[i]) ? selected : others).push(arr[i]);
  }
  return [selected, others];
};


console.log(flatten(splitAt(items, x => x === 14)));

// [14,14,1,52,54,10,1,3,5,6]