express order difference make result different - express

Question

const express = require('express')
const app = express();
app.use('/users', (req, res, next) =>{
res.send('Hello from express Again.')
});
app.use('/', (req, res, next) =>{
res.send('Hello from express.')
});
app.listen(4000, function () {
console.log('console server listening on 4000');
})
when i go to to http://localhost:4000/users it will render: Hello from express Again..
BUT if I change the code order like:
app.use('/', (req, res, next) =>{
res.send('Hello from express.')
});
app.use('/users', (req, res, next) =>{
res.send('Hello from express Again.')
});
Now I do it again: http://localhost:4000/users The result is Hello from express.
I guess it's configuration issue. But I just don't know which configuration.

Answer 1

Yes, the order you declare your routes in Express is important. Routes are matched from first to last in order of declaration and the first route that matches gets the request. If it sends a response and doesn't call next(), then the following routes do not get a chance to run.
Also relevant here is that app.use(somePath, ...) will match any route that starts with somePath. So, this:
app.use('/', (req, res, next) =>{
res.send('Hello from express.')
});
will match every single possible route because they all start with "/".
So, when you do this:
app.use('/', (req, res, next) =>{
res.send('Hello from express.')
});
app.use('/users', (req, res, next) =>{
res.send('Hello from express Again.')
});
And, send a request for /users, then the first route will match everything and thus the second route is never called.
Whereas, when do this:
app.use('/users', (req, res, next) =>{
res.send('Hello from express Again.')
});
app.use('/', (req, res, next) =>{
res.send('Hello from express.')
});
The /users route gets the first chance to match so when you send a request for /users, that /users route gets matched first and takes the request.
Note: You should only be using app.use() when you either want to match all possible http verbs or you WANT the greedy partial match that is in play here. If you don't want one of those, then use the verb-specific version such as app.get(). In your example, if you make both of these app.get(), then it is no longer order-sensitive because app.get() only matches complete path matches and does not do partial matches.
Both of these work the same:
app.get('/', (req, res, next) =>{
res.send('Hello from express.')
});
app.get('/users', (req, res, next) =>{
res.send('Hello from express Again.')
});
and:
app.get('/users', (req, res, next) =>{
res.send('Hello from express Again.')
});
app.get('/', (req, res, next) =>{
res.send('Hello from express.')
});
Because only one of them will match / and only one will match /users.