Unit testing in node.js and mocking modules

If your module has a constructor that takes the app and request as constructor arguments (in other words dependency injection), you can just provided a stubbed/mocked implementation of request in your test.
If not: I know you can override in tests what RequireJS returns when requiring something, so I would assume the same would be possible in node.js. So if you can change the configuration of the module loader in a test, you can configure it to return a stubbed/mocked implementation of request.
Personally, I'd go the dependency injection route. Having to configure your module loader in tests is a pain in the backside...

I recently did this. Forgot where I read about it but it's pretty easy.

Here's the basics of my code.

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

var MyServer = function(options) {
  var app = express();
  app.get(/^\/foo.html/, sendFoo);
  app.get(/^\/bar.html/, sendBar);
  var server = options.server || http.createServer(app);
  server.listen(options.port);

  function sendFoo(req, res) {
     res.writeHead(200);
     res.write("foo");
     res.end();
  }

  function sendBar(req, res) {
     res.writeHead(200);
     res.write("bar");
     res.end();
  }

  this.close = function() {
     server.close();
  }
};

As you can see my server either users http.createServer or the one I pass in. Maybe I should always pass it in? But in any case. I made a mock server

var events = require('events');

var MockHTTPServer = function() {
  var eventEmitter = new events.EventEmitter();
  var self = this;

  this.once = eventEmitter.once.bind(eventEmitter);

  this.listen = function() {
    eventEmitter.emit('listening', self, 0);
  };

  this.close = function() {
  };
};

And a MockResponse

var MockResponse = function(callback) {
  this.headers = {};
  this.statusCode = -1;
  this.body = undefined;

  this.setHeader = function(key, value) {
    this.headers[key] = value;
  }.bind(this);

  this.writeHead = function(statusCode, headers) {
    this.statusCode = statusCode;
    Object.keys(headers).forEach(function(key) {
      this.headers[key] = headers[key];
    }.bind(this));
  }.bind(this);

  this.write = function(data, encoding) {
    this.body = data.toString(encoding);
  }.bind(this);

  this.end = function() {
    var fn = callback;
    callback = undefined;
    fn(this);
  }.bind(this);
};

Note this is not a complete response mock. I just tried it in my tests and only added the functions that were called.

To actually test I made a test server

var Promise = require('promise');

var TestServer = function(callback) {
  var server = new MyServer({
    server: new MockHTTPServer,
  }, callback);

  this.close = function() {
    server.close();
  };

  var request = function(req, callback) {
    var res = new MockResponse(callback);
    server.handleRequest(req, res);
  };

  var getP = function(url) {
    return new Promise(function(resolve, reject) {
      request({ url: url, method: 'GET'}, resolve);
    });
  };

  this.getP = getP;
  this.request = request;
};

Which, using mocha, I can test like this

describe('test server', function() {

  var server;

  before(function(done) {
    server = new TestServer(done);
  });

  describe('test', function() {
    it('serves bar', function(done) {
      server.getP("http://localhost/bar.html").then(function(res) {
        res.body.should.containEql("bar");
      }).then(done, done);
    });

    it('serves foo', function(done) {
      server.getP("http://localhost/foo.html").then(function(res) {
        res.body.should.containEql("foo");
      }).then(done, done);
    });
  });

  after(function(done) {
    server.close();
    done();
  });

});

You can see I just make up request objects on the fly