NodeJS trouble sending messages to client - node.js

Question

I am trying to learn Node.JS, but making a REST API using this tutorial:
https://medium.freecodecamp.org/building-a-simple-node-js-api-in-under-30-minutes-a07ea9e390d2
I am having a very stupid little issue I can't seem to fix. In my user_routes.js file, I am trying to write several messages to the express() app, however it never works after the first res.send() call. Why is this? I can't find anywhere in my code where I am closing the connection or whatever, so why can't I write more than once to the request?
My user_routes.js
module.exports = function(app, db) {
app.post('/user', function (req,res) {
res.send("User Request Recieved via POST");
// Add the user to the database, if they don't already exist
const firstName = req.body.firstName;
const lastName = req.body.lastName;
const email = req.body.email;
const password = req.body.password;
const user = {
firstName: firstName,
lastName : lastName,
email : email,
password : password
};
if (db.collection('users').find({'email':email}).count() == 0) {
res.send('Unique Email');
db.collection('users').insert(user, (err, result) => {
if (err) {
console.log("error");
} else {
console.log(result.ops[0])
}
});
} else {
res.send("Email already in use")
}
})
};
Any my server.js:
const express = require('express');
const MongoClient = require('mongodb').MongoClient;
const bodyParser = require('body-parser');
const app = express();
const port = 6969;
const db = require('./config/db')
// We need to decode data from the url using the body-parser lib
app.use(bodyParser.urlencoded({ extended: true }));
MongoClient.connect(db.url).then( function(db) {
require('./app/routes')(app, db);
app.listen(port, () => {
console.log('We are live on ' + port);
});
}).catch (function (err) {
console.log(err);
});
module.exports = app;
I don't seem to close the connection anywhere, so why is it I am only able to write one message to the client?

Answer 1

res.send() is the last thing your function should be doing. Think of it like a return for your function, you can't return multiple times.

Answer 2

res.send() == return()
res.send() is equivalent to "return" for your post -- you can only do it once per call.
Multiple Messages per res.send()
If you want to send multiple messages through one call, you need to compile an object/array of messages you want to send and send that object/array through your res.send(). Example:
ret_msg = [];
ret_msg.push("Here's your first message.");
ret_msg.push("Here's your second message.");
ret_msg.push("Here's your third message.");
res.send(ret_msg);