Can't connect to database

I get the following console error after requiring the database in my app.js file:

/Users/Nate/Documents/Sites/mean-todo/node_modules/mongodb/lib/server.js:242 process.nextTick(function() { throw err; }) ^ Error: getaddrinfo ENOTFOUND localhost localhost:27017 at errnoException (dns.js:26:10) at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:77:26)

I've followed the instructions on this video three times and each time I get the same response. I used Homebrew to install MongoDB on my Macbook, and am able to run both mongod and nodemon. After I installed Mongoose, I created the database.js file. The following is the my database.js file:

'use strict';

var mongoose = require('mongoose');

mongoose.connect('mongodb://localhost/mean-todo', function(err){
  if(err){
    console.log('Could not connect to MongoDB!');
  }else{
    console.log('Successfully connected to MongoDB!');
  }
});

But again, as soon as I require it in my app.js file (require('./database');), it throws the error. I tried adding ":27017" after "localhost" (I saw that in a SO forum), but that didn't work, either.