How does the return of control flow work in a chain of Express middleware functions? - express

Question

I actually have a couple of questions about Express middleware chaining and thought I'd group them in this single post since they're closely related.
Q1: In a middleware function, if I send() the Response, pass to the next function, or just return, does any of the remaining code get executed?
router.get('/a', (req, res, next) => {
res.sendStatus(200);
// Does this ever get executed?
});
router.get('/b', (req, res, next) => {
next();
// Does this ever get executed?
});
router.get('/c', (req, res, next) => {
return;
// Does this stop the middleware chain? What happens to the request/response?
});
Q2: Can you pass on to another middleware function from inside of the preceding one? My reason for this is that I have to supply an argument to create a certain middleware, and that argument is dependent on which Express app object is the owner of the Request. So I need to access a property of the Request before it is passed to the middleware that needs to be created using that property. Would this work: ?
router.get('/d', (req, res, next) => {
var middlewareFunction = createDynamicMiddleware();
middlewareFunction(req, res, next); // Will this still work as expected?
});
Q3: How are the Request/Response arguments passed to the next middleware when you call next() without any arguments?

Answer 1

Q1: In a middleware function, if I send() the Response, pass to the next function, or just return, does any of the remaining code get
executed?
router.get('/a', (req, res, next) => {
res.sendStatus(200);
// Does this ever get executed?
// Yes, but don't call next() or res.send() or it will throw an error
});
router.get('/b', (req, res, next) => {
next();
// Does this ever get executed?
// Yes, but don't call next() or res.send() or it will throw an error
});
router.get('/c', (req, res, next) => {
return;
// Does this stop the middleware chain? What happens to the request/response?
// The request get stuck and after a while it will fail due 'timeout'
});
Q2: Can you pass on to another middleware function from inside of the preceding one? My reason for this is that I have to supply an
argument to create a certain middleware, and that argument is
dependent on which Express app object is the owner of the Request. So
I need to access a property of the Request before it is passed to the
middleware that needs to be created using that property.
Your factory of dynamic middleware should return a new function
Functions that returns new functions, are also knows as curried functions, here is a practical example:
// based on 'someId' we will return a specific middleware
req.get('/user/:someId',
createDynamicMiddleware,
(req,res) => {
res.send('ok')
})
function createDynamicMiddleware(req,res,next){
if(req.params.someId === 'me') {
return function(req, res, next){
// do some stuff if someId is me
return next();
}
} else {
return function(req, res, next){
// do some other stuff otherwise
return next();
}
}
}
Q3: How are the Request/Response arguments passed to the next middleware when you call next() without any arguments?
Express handle this for you!
Also, if you add a property to req or res objects, it will be present in the next middlewares
For example:
router.get('/', (req,res,next) => {
req.customProperty = 'hello!';
return next();
}, (req, res) => {
console.log(req.myCustomProperty)
// hello!
})