How to resolve next is not a function error message? - node.js

Question

I am trying to create a MEAN stack app, I'm currently working on the UPDATE functionality.
My code is currently failing when it runs into this method:
businessRoutes.route('/update/:id').post(function (req, res) {
Business.findById(req.params.id, function (err, business) {
if (!business)
return next(new Error('Could not load Document'));
else {
business.person_name = req.body.person_name;
business.business_name = req.body.business_name;
business.business_gst_number = req.body.business_gst_number;
business.save().then(business => {
res.json('Update complete');
console.log('Update Complete');
})
.catch(err => {
res.status(400).send("unable to update the database");
});
}
});
});
The error message being displayed in the console is:
TypeError: next is not a function
It's failing on this line of code:
return next(new Error('Could not load Document'));
Can someone please tell me why this is occurring & how I can resolve it?

Answer 1

The second parameter to findById expects a callback that has two arguments, err and <entity>. There's no middleware or something else in place, what you call next(...) tries to call your found entity.
From the docs
Adventure.findById(id, function (err, adventure) {});
You see, in your case, business is always undefined, and adventure or next is never a function.

Answer 2

Here is what you probably meant:
The problem is that the param that you mean to be "next" actually comes from express.js (which means it comes in on the first function call back that you ran (after .post(---this function--- has 3 params that you may chose to use:
req the request that the user made to your server
res the response that you are getting ready to send
next the 3rd param is optional and allows you to send to the next middleware or if you send it a parameter it will send it to the error handler which will send your error as a response.
On the other hand:
you placed the next param randomly in the middle of the mongoose function that you attempted to call: the mongoose function actually only takes (err, item)...
businessRoutes.route('/update/:id').post(function (req, res, next) {
// note that express exposes the "next" param
// this is a callback indicating to run the `next` middleware
Business.findById(req.params.id, function (err, business, whoKnows) {
// note this is from mongoose.js: the callback function actually only has 2 parameters (err, and foundObject)
if (!business)
return next(new Error('Could not load Document'));
else {
business.person_name = req.body.person_name;
business.business_name = req.body.business_name;
business.business_gst_number = req.body.business_gst_number;
business.save().then(business => {
res.json('Update complete');
})
.catch(err => {
res.status(400).send("unable to update the database");
});
}
});
});
Be advised, that I'd recommend using async await and that would make the whole thing much easier to understand:
businessRoutes.route('/update/:id').post(async (req, res, next) => {
try {
const foundBusiness = await Business.findById(req.params.id)
//... do stuff
catch (e) {
next(throw new Error(e))
// .. other stuff
}
})