FAQ: Middleware - Route-Level app.use() - Single Path


This community-built FAQ covers the “Route-Level app.use() - Single Path” exercise from the lesson “Middleware”.

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This exercise can be found in the following Codecademy content:

Web Development

Learn Express

FAQs on the exercise Route-Level app.use() - Single Path

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Just wondering, in the description of step 6 it says this:

app.use('/sorcerer', (req, res, next) => {
  console.log('User has hit endpoint /sorcerer');

In the example above the console will print 'User has hit endpoint /sorcerer' , if someone visits our web page’s ‘/sorcerer’ endpoint. Since the method app.use() was used, it won’t matter if the user is performing a GET ,a POST , or any other kind of HTTP request. Since the path was given as an argument to app.use() , this middleware function will not execute if the user hits a different path (for instance: '/spells' or '/sorcerer/:sorcerer_id' ).

However, in the exercise it seems that the middleware defined on path ‘/beans/:beanName’ will also respond to requests to ‘/beans/:beanName/add’ and ‘/beans/:beanName/remove’.

So, what exactly is the rule here? will a middleware respond to any request with a URL that is underneath the given path?


Aha. From the documentation, it appears that app.use() affects both the chosen path, and all derivative paths:

the middleware function is executed when the base of the requested path matches path .

If you read the documentation here you can see that you can be really creative, even using a regular expression to say what the app.use() applies to.

It goes on to say:

Since path defaults to “/”, middleware mounted without a path will be executed for every request to the app.

So, just as whether or not you type app.use('/', aFunction) or app.use(aFunction) will affect all paths, typing app.use('/users', aFunction) will also be invoked on, for example, app.get('/users/:aUser', aFunction)

In short, it appears to me at least that app.use() is a way of drastically reducing lines of code by acting as a ‘catch all’, for example: if the path matches [this complicated regular expression] then do this stuff. That way, you can capture all sorts of paths (for example any path that has three digits followed by three letters) and still get the code you want executing, executed. Hope this helps someone else out there :slight_smile: