citrusbyte

var crazyArray = [[1,2,[3]],4];

function flatten(crazyArray, items = []) {
  var item;
  while(crazyArray.length != 0){
    item = crazyArray.pop();
    if(Array.isArray(item)) {
      flatten(item, items);
    } else {
       items.push(item);
    }
   
  }

  return [...items].reverse();
}

console.log(flatten(crazyArray));

console.log(flatten([[1],[2],[3],[4],[5,6]]))