Clean Express controllers using currying and promises

README.md

Purpose

Demonstrate how to write clean express controllers using function currying, functionnal programming and promises.

• Promises helps avoiding callback hell
• Currying helps code reuse
• Functional programing makes data flow clearer

Aknowledgment

Yes, this sample shows obvious over-engineering. The provided API could be provided by simplier, clearer code. However, this code will scale on business complexity:

• handle more 'head' and 'tail' possible values: √
• asynchronously retrieve player data from playerName: √
• chain more actions: √
• add more game parameters won't change game signature: √

Install, run, test

git clone https://gist.github.com/ce14ba85518e7809b0cd.git express_curry_promise
cd express_curry_promise
npm install
node app.js

... in another shell

curl -v http://localhost:3000/flip/head
curl -v http://localhost:3000/flip/tail
curl -v http://localhost:3000/flip/head
curl -v http://localhost:3000/flip/tail

app.js

var express = require('express');
var app = express();

var flipController = require('./flip_controller');

app.get('/flip/:bet', flipController.play);
app.get('/players/:playerName/flip/:bet', flipController.play);

app.listen(3000);

flip_coin.js

var Promise = require('bluebird'); // Awesome promise library
var _ = require('lodash');

module.exports = function(game){
  return Promise.resolve(_.tap({
      game: game,
      got: Math.round(Math.random()) ? 'head' : 'tail'
    }, function(result){
      result.success = result.got === game.bet.toLowerCase();
      result.message = [game.playerName, (result.success ? 'wins' : 'loses')].join(' ')
    })
  );
};

flip_controller.js

var _ = require('lodash'); // underscore++
var Promise = require('bluebird');
var flipCoin = require('./flip_coin'); // Your actual handler, usually async

var VALID_BET_VALUES = ['head', 'tail'];

var getBetFromRequest = function(req){
  var bet = req.params.bet.toLowerCase();

  if(! _.contains(VALID_BET_VALUES, bet) ) {
    throw new Error("missing or invalid parameter `bet`, expects 'head' or 'tail' string but got: " + bet);
  }

  return bet;
};

var getPlayerName = function(req){
  return req.params.playerName || 'anonymous';
};

var getGameOpts = function(req){
  return {
    bet: getBetFromRequest(req),
    playerName: getPlayerName(req)
  };
}

var respond = _.curry(function(res, status, payload){
  res.status(status).send(payload).end();
});

var controller = {
  play: function(req, res){
    Promise.try(getGameOpts, req) // expect exceptions
      .then(flipCoin)
      .then(respond(res, 200)) // on success, send a 200 and forward flipCoin result
      .catch(_.compose(
        respond(res, 400),
        _.partialRight(_.pick, 'message') // pick only error message
      ));
  }
}

module.exports = controller;

package.json

{
  "name": "curry_promise_express",
  "version": "1.0.0",
  "description": "Example of clean Express controllers using currying and promises",
  "main": "app.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "repository": {
    "type": "git",
    "url": "https://gist.github.com/8472f35350a7031e2cb2.git"
  },
  "keywords": [
    "curry",
    "express",
    "promise"
  ],
  "author": "Arnaud Rinquin",
  "license": "MIT",
  "dependencies": {
    "bluebird": "^2.3.6",
    "express": "^4.9.8",
    "lodash": "^2.4.1"
  }
}