docs: function concat() {
        var length = arguments.length;
        if (!length) {
          return [];
        }
        var args = Array(length - 1),
            array = arguments[0],
            index = length;
  
        while (index--) {
          args[index - 1] = arguments[index];
        }
        return arrayPush(isArray(array) ? copyArray(array) : [array], baseFlatten(args, 1));
      }