TypeError: callback is not a function in waterfall nodejs

Question

Recently i tried using this waterfall method in my mongoose but unfortunately it throwed error,which i am not able to figure out can anyone suggest me help.......

TypeError: callback is not a function

My code,

      exports.waterfall = function (err, res) {
  async.waterfall([
    function (callback) {

        var item = {
          'status': '1',
          'type': 'categories'
        };
      Categories.find(item, function (err, result) {
        if (err) {
          return
        }
        if (result) {
          callback(null, result)
        }
      });
    },
    function (sol,callback) {
      var response = [];
       for (var i in sol) {
        var id = sol[0]._id,
          item = {
            'categoryid': id,
            'status': '1',
            'type': 'topics'
          }; 
        Categories.find(item, function (err, soll) {
          if (err) {
            return
          }
          if (soll) {
            console.log(soll)
          }
        });
      }    
      callback(null,soll)
     }
  ], 
  function (err, ress) {
   if(err){
     console.log('err')
     return
   }
   if(res){
   res.json(ress)
   }
  });
};

Recently i tried using this waterfall method in my mongoose but unfortunately it throwed error,which i am not able to figure out can anyone suggest me help.......

Answer 1

function (callback, sol) {

Should be function (sol, callback), the callback is always the last argument.