How can I inspect the set of Express.js middleware that is being used?

Question

Backstory: I'm trying to debug an issue in one piece of middleware that I think is coming from other piece. But, I'm not sure. So anyway, I would like to be able to check what middleware is actually being called, because I'm not sure of the ordering.

Is it possible to inspect the set of middleware that is currently being used?

I have tried to find any piece of internal state where Express might be storing the middleware, but I was not able to.

Answer 1

How middleware works:

The middlewares are generally used to transform a request or response object, before it reaches to other middlewares.

If you are concerned with the order in which the middlewares are called, express calls the middleware, in the order in which they are defined.

Example,

app.use(compression());
app.use(passport.initialize());
app.use(bodyParser());
app.use(cookieParser());

the order is

1. compression,
2. passport,
3. bodyParser,
4. cookieParser

(plus I think your bodyParser and cookieParser middlewares should be before the other middlewares like passport).

That is the reason why the error handling middlewares are kept at last, so that if it reaches them, they give an error response.

So basically, request drips down the middlewares until one of them says that it does not want it to go any further(get, post methods are such middlewares, that stop the request).

Now, the debugging part:

You may not be able to inspect the middleware properly internally, but you can check whether middleware has worked properly by inserting your own custom middleware in between and then put a breakpoint on it.

Let's say you want is to debug what happened to your request after the bodyParser middlewares does it tasks, you can do is put your custom middleware in between and check the request and response whether they are modified properly or not. how you do this is by following example.

Example,

app.use(bodyParser());

//custom middleware to inspect which middleware is not  working properly
app.use(function(req,res,next){
   console.log("check something!"); //do something here/put a breakpoint
   next(); 
   /*this function is the third parameter 
   and need to be called by the middleware 
   to proceed the request to the  next middleware,
   if your don't write this line, your reqest will just stop here.
   app.get/app.post like methods does not call next*/
});

app.use(cookieParser());

This is one way in which you move this custom debugger in between the middlewares, until you figure out which one is giving faulty outputs.

Also, if you want to check the middlewares functionality, you can look at the documentation of those middlewares. They are quite good.

Check by playing with your custom middleware.