Why does express middleware override my api's response? - node.js

Question

I am new to nodejs/express and have followed a blog article "Build Node.js RESTful APIs in 10 Minutes". It provided detailed steps to create simple REST apis. After finishing every code mentioned in the article, the app have not been working properly, it would just return
{
"url": "/transactions not found"
}
I found that the culprit was in the last part of the article, which told me to add this line to server.js:
Having done all these, what happens if we entered a wrong route? say
you entered 'http://localhost:3000/task', It responds with a message
“Cannot GET /task”. Let’s add express middleware which could be used
to return more interactive messages.
app.use(function(req, res) {
res.status(404).send({url: req.originalUrl + ' not found'})
});
It seems to hardcode a HTTP status code of 404 no matter what does my api returns. After removing this line, the app return meaningful response.
Here is my server.js:
var express = require('express'),
app = express(),
port = process.env.PORT || 3000,
mongoose = require('mongoose'),
Transaction = require('./api/models/transactionModel'), //created model loading here
bodyParser = require('body-parser');
// mongoose instance connection url connection
mongoose.Promise = global.Promise;
mongoose.connect('mongodb://localhost/transactionDb');
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());
app.use(function(req, res) {
res.status(404).send({url: req.originalUrl + ' not found'})
});
var routes = require('./api/routes/transactionRoutes'); //importing route
routes(app); //register the route
app.listen(port);
console.log('Transaction List RESTful API server started on: ' + port);
And the controller:
'use strict';
var mongoose = require('mongoose'),
Transaction = mongoose.model('Transactions');
exports.list_all_transactions = function (req, res) {
Transaction.find({}, function (err, transaction) {
if (err)
res.send(err);
res.json(transaction);
});
};
exports.create_a_transaction = function (req, res) {
var new_transaction = new Transaction(req.body);
new_transaction.save(function (err, transaction) {
if (err)
res.send('Error creating a new transaction!');
res.json(transaction);
});
};

Answer 1

it wasn't override your response, just because it returned in the middle before touch your api. The flow of request is running from top to bottom, example in your code above:
[coming request] --> bodyParser --> bodyParser --> 404 handler (returned here) -x-> api (cannot reach).
Just use() 404 middleware to the bottom and everything works fine.
mongoose.Promise = global.Promise;
mongoose.connect('mongodb://localhost/transactionDb');
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());
var routes = require('./api/routes/transactionRoutes'); //importing route
routes(app); //register the route
app.use(function(req, res) {
res.status(404).send({url: req.originalUrl + ' not found'})
});

Answer 2

I suspect your
app.use(function(req, res) {
res.status(404).send({url: req.originalUrl + ' not found'})
});
should be lower: the server is finding that piece of code first, while the actual routes are loaded later.
app.use means that it always executes that code, so it handles every request by sending a 404 error and saying it can't find the URL.
Put that code under all of the other routes (right above app.listen(port)) and it should work!