Copy from https://gist.github.com/igorlima/b31f1a26a5b100186a98

Author: igorlima

README.md

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+[![Gittip Donate Button](http://img.shields.io/gratipay/igorribeirolima.svg)](https://gratipay.com/igorribeirolima/)
+
+# JS unit testing using dependency injection
+
+You probably know that to do JavaScript testing is good and some hurdles to overcome is how to test our code in a manner to *(i)* inject mocks for other modules, *(ii)* to leak private variables or *(iii)* override variables within the module.
+
+[rewire](https://github.com/jhnns/rewire) is a tool for helping us on overcoming these hurdles. It provides us an easy way to dependency injection for unit testing and adds a special setter and getter to modules so we can modify their behaviour for better unit testing. What [rewire](https://github.com/jhnns/rewire) does is to not load the file and eval the contents to emulate the load mechanism.
+
+To get started with dependency injection, we'll create [a twitter rest api server](https://gist.github.com/igorlima/b31f1a26a5b100186a98) and do unit tests using mocks and overriding variables within modules. This example will focus on back-end unit testing but if you want to use [rewire](https://github.com/jhnns/rewire) also on the client-side take a look at [client-side bundlers](https://github.com/jhnns/rewire#client-side-bundlers).
+
+## An example
+
+This example is a public HTTP API to retrieve Twitter user timelines. It has basically two files: [server.js](https://gist.github.com/igorlima/b31f1a26a5b100186a98#file-server-js) and [twitter.js](https://gist.github.com/igorlima/b31f1a26a5b100186a98#file-twitter-js).
+
+The first file creates [an basic instance](http://expressjs.com/starter/hello-world.html) of [express](http://expressjs.com) and defines [a route for a GET request method](http://expressjs.com/starter/basic-routing.html), which is ``/twitter/timeline/:user``.
+
+The second one is a module responsible for retrieving data from Twitter. It requires:
+
+* [**twit**](https://github.com/ttezel/twit): Twitter API Client for node
+* [**async**](https://github.com/caolan/async): is a utility module which provides straight-forward, powerful functions for working with asynchronous JavaScript
+* [**moment**](http://momentjs.com): a lightweight JavaScript date library for parsing, validating, manipulating, and formatting dates.
+
+Part of these modules will be mocked and overridden in our tests.
+
+## Running the example
+
+This example is already running [in a cloud](https://social-media-rest-api.herokuapp.com). So you can reach the urls below and see it working:
+
+* [igorribeirolima timeline](https://social-media-rest-api.herokuapp.com/twitter/timeline/igorribeirolima)
+* [strongloop timeline](https://social-media-rest-api.herokuapp.com/twitter/timeline/strongloop)
+* [tableless timeline](https://social-media-rest-api.herokuapp.com/twitter/timeline/tableless)
+
+To run it locally, clone [this gist](https://gist.github.com/igorlima/b31f1a26a5b100186a98) by ``git clone https://gist.github.com/b31f1a26a5b100186a98.git twitter-rest-api-server`` and set five environment variables. Those envs are listed below. For secure reason I won't share my token. To get yours, access [Twitter developer documentation](https://dev.twitter.com/overview/documentation), [create a new app](https://apps.twitter.com) and set up your credentials.
+
+For [Mac users](http://stackoverflow.com/questions/7501678/set-environment-variables-on-mac-os-x-lion), you can simply type:
+
+```
+export TwitterConsumerKey="xxxx"
+export TwitterConsumerSecret="xxxx"
+export TwitterAccessToken="xxxx"
+export TwitterAccessTokenSecret="xxxx"
+export MomentLang="pt-br"
+```
+
+For [Windows users](http://stackoverflow.com/questions/21606419/set-windows-environment-variables-with-commandline-cmd-commandprompt-batch-file), do:
+
+```
+SET TwitterConsumerKey="xxxx"
+SET TwitterConsumerSecret="xxxx"
+SET TwitterAccessToken="xxxx"
+SET TwitterAccessTokenSecret="xxxx"
+SET MomentLang="pt-br"
+```
+
+After setting up the environment variables, go to ``twitter-rest-api-server`` folder, install all node dependencies by ``npm install``, then run via terminal ``node server.js``. It should be available at the port ``5000``. Go to your browser and reach ``http://localhost:5000/twitter/timeline/igorribeirolima``.
+
+![running express app example locally](http://i1368.photobucket.com/albums/ag182/igorribeirolima/running%20express%20app%20example%20locally_zpsndaidg4w.png)
+
+## Writing unit tests
+
+[Mocha](http://mochajs.org) is the JavaScript test framework running we gonna use. It makes asynchronous testing simple and fun. [Mocha](http://mochajs.org) allows you to use any assertion library you want, if it throws an error, it will work! In this example we are gonna utilize [node's regular assert](https://nodejs.org/api/assert.html) module.
+
+Imagine you want to test this code [twitter.js](https://gist.github.com/igorlima/b31f1a26a5b100186a98#file-twitter-js):
+
+```javascript
+var Twit   = require('twit'),
+    async  = require('async'),
+    moment = require('moment'),
+    T      = new Twit({
+      consumer_key: process.env.TwitterConsumerKey || '...',
+      consumer_secret: process.env.TwitterConsumerSecret || '...',
+      access_token: process.env.TwitterAccessToken || '...',
+      access_token_secret: process.env.TwitterAccessTokenSecret || '...'
+    }),
+    
+    mapReducingTweets = function(tweet, callback) {
+      callback(null, simplify(tweet));
+    },
+
+    simplify = function(tweet) {
+      var date = moment(tweet.created_at, "ddd MMM DD HH:mm:ss zz YYYY");
+      date.lang( process.env.MomentLang );
+      return {
+        date: date.format('MMMM Do YYYY, h:mm:ss a'),
+        id: tweet.id,
+        user: {
+          id: tweet.user.id
+        },
+        tweet: tweet.text
+      };
+    };
+
+module.exports = function(username, callback) {
+  T.get("statuses/user_timeline", {
+    screen_name: username,
+    count: 25
+  }, function(err, tweets) {
+    if (err) callback(err);
+    else async.map(tweets, mapReducingTweets, function(err, simplified_tweets) {
+      callback(null, simplified_tweets);
+    });
+  })
+};
+```
+
+To do that in a easy and fun way, let load this module using [rewire](https://github.com/jhnns/rewire). So within your test module [twitter-spec.js](https://gist.github.com/igorlima/b31f1a26a5b100186a98#file-twitter-spec-js):
+
+```javascript
+var rewire  = require('rewire'),
+    assert  = require('assert'),
+    twitter = rewire('./twitter.js'),
+    mock    = require('./twitter-spec-mock-data.js');
+```
+
+[rewire](https://github.com/jhnns/rewire) acts exactly like *require*. Just with one difference: Your module will now export a special setter and getter for private variables.
+
+```javascript
+myModule.__set__("path", "/dev/null");
+myModule.__get__("path"); // = '/dev/null'
+```
+
+This allows you to mock everything in the top-level scope of the module, like the *twitter module* for example. Just pass the variable name as first parameter and your mock as second.
+
+You may also override globals. These changes are only within the module, so you don't have to be concerned that other modules are influenced by your mock.
+
+```javascript
+describe('twitter module', function(){
+
+  describe('simplify function', function(){
+    var simplify;
+   
+    before(function() {
+      simplify = twitter.__get__('simplify');
+    });
+
+    it('should be defined', function(){
+      assert.ok(simplify);
+    });
+
+    describe('simplify a tweet', function(){
+      var tweet, mock;
+     
+      before(function() {
+        mock = mocks[0];
+        tweet = simplify(mock);
+      });
+
+      it('should have 4 properties', function() {
+        assert.equal( Object.keys(tweet).length, 4 );
+      });
+
+      describe('format dates as `MMMM Do YYYY, h:mm:ss a`', function() {
+
+        describe('English format', function() {
+          before(function() {
+            revert = twitter.__set__('process.env.MomentLang', 'en');
+            tweet = simplify(mock);
+          });
+
+          it('should be `March 6th 2015, 2:29:13 am`', function() {
+            assert.equal(tweet.date, 'March 6th 2015, 2:29:13 am');
+          });
+
+          after(function(){
+            revert();
+          });
+
+        });
+
+        describe('Brazilian format', function() {
+          before(function() {
+            revert = twitter.__set__('process.env.MomentLang', 'pt-br');
+            tweet = simplify(mock);
+          });
+
+          it('should be `Março 6º 2015, 2:29:13 am`', function() {
+            assert.equal(tweet.date, 'Março 6º 2015, 2:29:13 am');
+          });
+
+          after(function(){
+            revert();
+          });
+          
+        });
+
+      });
+
+    });
+
+  });
+  
+  describe('retrieve timeline feed', function() {
+    var revert;
+    before(function() {
+      revert = twitter.__set__("T.get", function( api, query, callback ) {
+        callback( null, mocks);
+      });
+    });
+
+    describe('igorribeirolima timeline', function() {
+      var tweets;
+      before(function(done){
+        twitter('igorribeirolima', function(err, data) {
+          tweets = data;
+          done();
+        });
+      });
+
+      it('should have 19 tweets', function() {
+        assert.equal(tweets.length, 19);
+      });
+
+    });
+
+    after(function() {
+      revert();
+    });
+
+   });
+});
+```
+
+`__set__` returns a function which reverts the changes introduced by this particular `__set__` call.
+
+
+## Running unit tests
+
+Before we get into the test and walk through it, let install mocha CLI by ``npm install -g mocha``. It will support us on running our tests just typing ``mocha twitter-spec.js``. The following is an image that illustrates the test result.
+
+![image that ilustrates unit tests running](http://i1368.photobucket.com/albums/ag182/igorribeirolima/running%20unit%20tests_zpshqazy5po.png)
+
+Take a look on this [video](http://showterm.io/3c970843502e140bcfabd#slow) and see step by step in detail everything discussed so far.
+
+## Conclusion 
+
+As you can see it's not painful on overcoming hurdles like *(i)* injecting mocks for other modules, *(ii)* leaking private variables or *(iii)* overriding variables within the module. That's it folks. Hope you catch the idea on how simple and fun is to do dependency injection for unit testing. Thanks for reading.

package.json

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+{
+  "name": "twitter-rest-api",
+  "version": "0.0.0",
+  "dependencies": {
+    "connect": "2.9.0",
+    "express": "~3.4.7",
+    "twit": "~1.1.11",
+    "async": "~0.2.9",
+    "moment": "~2.5.1",
+    "rewire": "2.3.1"
+  },
+  "engines": {
+    "node": ">=0.10.25"
+  }
+}

server.js

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+var express      = require('express'),
+    tweets       = require('./twitter.js'),
+    response     = function(req, res, err, data) {
+      if (req.query.callback) {
+        res.jsonp(err || data);
+      } else {
+        res.json(err || data);
+      }
+    };
+
+var app = express()
+  .use(express.static(__dirname + './'))
+  .get('/twitter/timeline/:user', function(req, res) {
+    tweets(req.params.user, function(err, data) {
+      response(req, res, err, data);
+    });
+  })
+  .listen(process.env.PORT || 5000);

twitter-spec-mock-data.js

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+var rewire  = require('rewire'),
+    assert  = require('assert'),
+    twitter = rewire('./twitter.js'),
+    mocks   = require('./twitter-spec-mock-data.js');
+
+describe('twitter module', function(){
+  describe('simplify function', function(){
+    var simplify;
+   
+    before(function() {
+      simplify = twitter.__get__('simplify');
+    });
+
+    it('should be defined', function(){
+      assert.ok(simplify);
+    });
+
+    describe('simplify a tweet', function(){
+      var tweet, mock;
+     
+      before(function() {
+        mock = mocks[0];
+        tweet = simplify(mock);
+      });
+
+      it('should have 4 properties', function() {
+        assert.equal( Object.keys(tweet).length, 4 );
+      });
+
+      it('should have date property', function() {
+        assert.ok(tweet.date);
+      });
+
+      it('should have id property', function() {
+        assert.ok(tweet.id);
+      });
+
+      it('should have user property', function() {
+        assert.ok(tweet.user);
+      });
+
+      it('should have id property within user', function() {
+        assert.ok(tweet.user.id);
+      });
+
+      it('should have tweet property', function() {
+        assert.ok(tweet.tweet);
+      });
+     
+      describe('format dates as `MMMM Do YYYY, h:mm:ss a`', function() {
+        var revert;
+
+        describe('English format', function() {
+          before(function() {
+            revert = twitter.__set__('process.env.MomentLang', 'en');
+            tweet = simplify(mock);
+          });
+
+          it('should be `March 6th 2015, 2:29:13 am`', function() {
+            assert.equal(tweet.date, 'March 6th 2015, 2:29:13 am');
+          });
+
+          after(function(){
+            revert();
+          });
+        });
+
+        describe('Brazilian format', function() {
+          before(function() {
+            revert = twitter.__set__('process.env.MomentLang', 'pt-br');
+            tweet = simplify(mock);
+          });
+
+          it('should be `Março 6º 2015, 2:29:13 am`', function() {
+            assert.equal(tweet.date, 'Março 6º 2015, 2:29:13 am');
+          });
+         
+          after(function(){
+            revert();
+          });
+          
+        });
+      });
+
+    });
+
+  });
+  
+  describe('retrieve timeline feed', function() {
+    var revert;
+    before(function() {
+      revert = twitter.__set__("T.get", function( api, query, callback ) {
+        callback( null, mocks);
+      });
+    });
+
+    describe('igorribeirolima timeline', function() {
+      var tweets;
+      before(function(done){
+        twitter('igorribeirolima', function(err, data) {
+          tweets = data;
+          done();
+        });
+      });
+
+      it('should have 19 tweets', function() {
+        assert.equal(tweets.length, 19);
+      });
+
+    });
+
+    after(function() {
+      revert();
+    });
+
+   });
+});

twitter-spec.js

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+var Twit   = require('twit'),
+    async  = require('async'),
+    moment = require('moment'),
+    T      = new Twit({
+      consumer_key: process.env.TwitterConsumerKey || '...',
+      consumer_secret: process.env.TwitterConsumerSecret || '...',
+      access_token: process.env.TwitterAccessToken || '...',
+      access_token_secret: process.env.TwitterAccessTokenSecret || '...'
+    }),
+    
+    mapReducingTweets = function(tweet, callback) {
+      callback(null, simplify(tweet));
+    },
+
+    simplify = function(tweet) {
+      var date = moment(tweet.created_at, "ddd MMM DD HH:mm:ss zz YYYY");
+      date.lang( process.env.MomentLang );
+      return {
+        date: date.format('MMMM Do YYYY, h:mm:ss a'),
+        id: tweet.id,
+        user: {
+          id: tweet.user.id
+        },
+        tweet: tweet.text
+      };
+    };
+
+module.exports = function(username, callback) {
+  T.get("statuses/user_timeline", {
+    screen_name: username,
+    count: 25
+  }, function(err, tweets) {
+    if (err) callback(err);
+    else async.map(tweets, mapReducingTweets, function(err, simplified_tweets) {
+      callback(null, simplified_tweets);
+    });
+  })
+};