CODEWITHSUNDEEP
node.jsexpress
Kin
Kin

Reputation: 4596

How Express recorgonizes middlewares?

I'm novice in Express and a little bit confused about how it handles middlewares? So basically I have two middlewares which looks like:

app.use(require('_/app/middlewares/errors/404'))
app.use(require('_/app/middlewares/errors/500'))

404

var log = require('_/log')

module.exports = function (req, res, next) {
    log.warn('page not found', req.url)
    res.status(404).render('errors/404')
}

500

var log = require('_/log')

module.exports = function (er, req, res, next) {
    log.error(er.message)

    res.locals.error = er
    res.status(500).render('errors/500')
}

So now I want to add my custom middleware app.use(require('_/app/middleware/shareLocals')) which looks like:

module.exports = function (req, res, next) {
    res.locals.base_url = req.protocol + '://' + req.get('host');

    next();
}

The main problem is that now when I try to use base_url I get 404 error...

So how Express understands what middleware do? That is between my middleware and 404 are no visual differences:

Appears the feeling the middlewares in Express are made for errors (when excepts err as first param) and for 404 (when there is no first err)...

P.S. Is there any difference defining middlewares before or after routes?

Upvotes: 0

Views: 46

Answers (1)

Stian
Stian

Reputation: 695

P.S. Is there any difference defining middlewares before or after routes? Yes.

The order in which you register your middlewares (and routes) have a lot to say.

Image express as a giant list. Starting at the first element in the list, you have the first middleware OR route you have defined, next is the second, etc.

When express gets a request, it appears to be matching your route/name of route/middleware, and if it's a hit, it executes the middleware/route and potentially waits for a "next()" call.

So if you have a route "/test" it will only be executed if you have a request matching "/test". routes with different names obviously wont get triggered. middlewares can also have names: app.use("/test", middlewareA). This will also only trigger if "/test" is requested. The way you do it, all requests (within the routes namespace) will be triggered app.use(middlewareA). It's like a wildcard.

Now, to the implications of things being ordered: Your 404 middleware should only be used AFTER all routes have been defined. that way, when the list reached the 404 middleware, no routes have actually been found.

returning/sending result/not calling next() at the end of a middleware will all potentially create problems in your flow. I wont go into details about this, but be aware of it.

I am guessing your own middleware is added after the 404 middleware. That is probably the problem. If not, you should surrender more of your code so we can take a better look. But remember, order is everything :)

Upvotes: 1

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