Callback function 'next' is executed automatically without making a call to it - express

Question

I have this POST method which uses FetchURL middleware to fetch data from the url submitted by the user.
router.post('/', FetchURL, (req, res) => {
console.info('data received');
...
})
Everything works with response.ok being true, but the contrary case doesn't quite work as expected.
I don't want next to be called when response.ok equals false.
But I get to see "data received" logged to the console which means the next function does get called on its own.
fetch_url.js
function FetchURL(req, res, next) {
fetch(req.body.input_url)
.then(response => {
if(response.ok)
return response.json();
// else render error message on the client machine
res.status(response.status)
.render('index', {
errStatus: [response.status, response.statusText]
});
/* Throwing an Error here is the only way I could prevent the next callback */
// throw new Error(`Request failed with status code ${response.status}.`);
})
.then(data => {
req.data = data;
next();
})
.catch(err => console.error(err));
}
I could not find anything relevant on the documentation of expressjs middleware. The only way I could prevent next from being called is by throwing an Error on the server.
What happens behind the scene here?

Answer 1

try making a second check before next is called like following
function FetchURL(req, res, next) {
fetch(req.body.input_url)
.then(response => {
if(response.ok) // wrap your response in a temporary object.
return { fail: false, data: response.json() } ;
// else render error message on the client machine
res.status(response.status)
.render('index', {
errStatus: [response.status, response.statusText]
});
/* Instead of throwing an Error, return something indicating error */
return { fail: true };
})
.then(data => {
// check if previous procedure has failed.
if(!data.fail) {
req.data = data.data;
next();
}
})
.catch(err => console.error(err));
}