title: Simple Node/Express app, the functional programming way (How to handle side-effects in JavaScript?)
body: <p>There are many good articles about the theory of functional programming in JavaScript. Some even contain code examples showing the difference between imperative/object-oriented programming and declarative/functional programming. But I have found none that show, with simple JavaScript code examples, how to handle side-effects in a web app. No real world application can entirely avoid side-effects (database calls, logging to console, saving to a file, drawing to the screen etc.) and I have a hard time figuring out how it is done in practice.</p>
  
  <p>There are blog articles and S/O answers (like this one: <a href="https://stackoverflow.com/questions/18172947/how-to-perform-side-effects-in-pure-functional-programming">How to perform side-effects in pure functional programming?</a>) that touch on the subject of handling side-effects in the real world but they are usually far from simple, don't include code example or include code example in other languages (Haskell, Scala, etc.). I haven't found one for Node/JavaScript.</p>
  
  <p>So... given the following very simple example Node/Express app with MongoDB database, what code changes must be implemented so that this piece of code fully reflect current JavaScript functional programming best practices. Especially when it comes to the routes/functions handling database calls. I'm hoping your answers will help me, and others, better understand the practical application of the 'avoiding side-effects' concept of Functional Programming in real-world JavaScript.</p>
  
  <pre><code>/*app.js*/
  
  const express = require('express')
  const app = express()
  var mongoose = require('mongoose');
  mongoose.connect('mongodb://localhost/test');
  
  var greetingSchema = mongoose.Schema({
      greeting: String
  });
  
  var Greeting = mongoose.model('Greeting', greetingSchema);
  
  app.get('/', function (req, res) {
    Greeting.find({greeting: 'Hello World!'}, function (err, greeting){
      res.send(greeting);
    });  
  });
  
  app.post('/', function (req, res) {
    Greeting.create({greeting: 'Wasssssssssssuuuuppppp'}, function (err, greeting){
    res.send(greeting);
    });      
  });
  
  app.listen(3000, function () {
    console.log('Example app listening on port 3000!')
  })
  </code></pre>