Problem with async error handling with Express

Question

I am trying to avoid the try catch block in route handlers. I am trying to catch the error in the following manner:

const catchAsync = (fn) => {

    // why this function does not have access to req,res,next?
    // I am passing async(req,res,next) as an argument, hence
    // req,res,next should be known to this function

    fn(req, res, next).catch((err) => next(err));
  
};

and then in the route handler:

exports.createTour = catchAsync(async (req, res, next) => {
  const newTour = await Tour.create(req.body);
  res.status(201).json({
    status: "success",
    data: {
      tour: newTour,
    },
  });
});

Now the problem is that, I don't understand why the function fn(req,res,next) inside the catchAsync function block, does not have access to (req,res,next) when called ? All I understand is that I am passing async(req,res,next) as an argument to catchAsync function. Hence when the function gets called inside the catchAsync block, it should have access to req,res,next

Where/how are you using `createTour`? Is it being used in an express route? — Nick Parsons, Sep 07 '22 at 10:52
I think what you're after is similar to the second code block from the bottom of this answer: [Handling errors in express async middleware](https://stackoverflow.com/a/51391081) — Nick Parsons, Sep 07 '22 at 10:55

score 2 · Accepted Answer · answered Sep 07 '22 at 10:52

req,res,next are parameters of the fn function. These are variables that are local to the function's own execution context. When you call a function, you're supposed to provide values for these parameter variables. You cannot access those variables themselves, as they don't exist yet before the call is getting executed.

Problem with async error handling with Express

1 Answers1