Came accros a sequence of code execution which I found unusual here is the code:
server.js
const Actions_Single_PVC = require('./routes/Actions_single_PVC.js');
app.use('/Actions_single_PVC', Actions_Single_PVC);
app.use((err, req, res, next) => {
console.log('invalid token');
});
Actions_single_PVC.js
router.post('/', asyncMW(async (req, res, next) => {
throw new Error();
}));
router.use((err, req, res, next) => {
console.log('error');
}
And in case you have never seen this construction before here is asyncMW:
const asyncMiddleware = fn =>
(req, res, next) => {
Promise.resolve(fn(req, res, next))
.catch(next);
};
module.exports = asyncMiddleware;
What I didn't understand was that when an error is thrown (I reproduced it here with throw new Error();) that the error handling middleware in the server.js file is executed. I expected that the error handling middleware of the Actions_single_PVC.js would get executed.
Why is the error middlware in server.js executed and not the error middlware in Actions_single_PVC.js?
It is because the following code applies middleware to only request with base path matching Actions_single_PVC.
app.use('/Actions_single_PVC', Actions_Single_PVC);
Whereas following code is apply middleware to all global requests.
app.use((err, req, res, next) => {
console.log('invalid token');
});
If you'll hit the url /Actions_single_PVC then the middlewares in Actions_single_PVC will get hit.