parameters in the callback functions fed into asyncHandler

Hey Tim Wang, great question. The callbacks in the route handlers don't have to have that third parameter unless you're going to be calling the next() method specifically and directly inside of the route handlers' callback.

And the route handlers' callbacks not having the next parameter shouldn't impede the asynchHandler helper function from utilizing its own next() method.

For an example, here's a version of that asyncHanler helper function that I have in a project:

/* Async try/catch */
exports.asyncHandler = (cb) => {
  return async (req,res,next) => {
    try {
      await cb(req,res,next);
    } catch(err) {
      next(err);
    }
  }
}

For the above to work, we just need to pass a callback function to it. The next() method called in the catch block is a reference to the first next parameter in the return statement. Remember, with the way this asyncHandler is being used, it is itself a callback function to another method like .get or .post, which passes the req and res to it. And Express sets it up so it has access to that next() method as the third parameter if its present. But that first next parameter in the example should impact the second next parameter. If the first one was missing, then you wouldn't be able to call the next() method in the catch block above. And if the second next parameter was missing, then maybe you wouldn't be able to call the next() method in the callback.

To test this out and get a better feel and understanding for it, try changing the name of the first next parameter, and then give the next method in the catch block the same name, and see if it all still works. Hope that helps.