Express中间件功能链中的控制流返回如何工作？

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!
})