JS flatten 简单实现

function steamroller(arrs) {
  if (!arrs || !arrs.length) throw new ReferenceError();
  var arr = [];

  (function flatten (items) {
    items.forEach(function(item){
      if (item !== undefined && item !== null) {
        if (Array.isArray(item)) {
          arr.push(flatten(item));
        } else {
          arr.push(item);
        }
      }
    });
  }(arrs));

  arr = arr.filter(function(item){
    return item;
  });

  return arr;
}

steamroller([1, [2], [3, [[4]]]]);