MongoDB Connection Error

I'm working on this video: https://teamtreehouse.com/library/default-values-in-mongoose

Background: Last week, I purchased an iMac and migrated from a Windows 10 machine. On Win10, I didn't have this error. Now that I've migrated to Mac, I'm getting this error.

Initially, I thought I was missing NPM packages, so I reinstalled all of them. Then I thought it might have been my code, which likely isn't the issue because I didn't have the issue the week before using the same exact files.

'use strict';

var mongoose = require('mongoose');

mongoose.connect('mongodb://localhost:27017/sandbox');

var db = mongoose.connection;
db.on("error", function(err) {
  console.error("Connection Error:", err);
});

db.once("open", function() {
  console.log("Database connection successful");
  // All database communication goes here

  var Schema = mongoose.Schema;
  var AnimalSchema = new Schema({
    type:   {type: String, default: "goldfish"},
    color:  {type: String, default: "small"},
    size:   {type: String, default: "golden"},
    mass:   {type: Number, default: 0.007},
    name:   {type: String, default: "Angela"}
  });
  var Animal = mongoose.model("Animal", AnimalSchema);

  var elephant = new Animal({
    type: "elephant",
    size: "big",
    color: "gray",
    mass: 6000,
    name: "Lawrence"
  });

  var animal = new Animal({}); // goldfish

  Animal.remove({}, function() {
    if (err) console.error("Save Failed", err);
    elephant.save(function(err) {
      if (err) console.error("Save Failed", err);
      animal.save(function(err) {
        if (err) console.error("Save Failed", err);
        db.close(function() {
          console.log("db connection closed");
        });
      });
    });
  });

});

When I run "node mongoose_sandbox.js" this is the error I receive in the console. I don't know how to use console syntax here.

(node:20191) DeprecationWarning: open() is deprecated in mongoose >= 4.11.0, use openUri() instead, or set the useMongoClient option if using connect() or createConnection(). See http://mongoosejs.com/docs/4.x/docs/connections.html#use-mongo-client

Connection Error: { MongoError: failed to connect to server [localhost:27017] on first connect [MongoError: connect ECONNREFUSED 127.0.0.1:27017] at Pool.<anonymous> (/Applications/MAMP/htdocs/treehouse_rest_api_with_express/node_modules/mongodb-core/lib/topologies/server.js:336:35) at Pool.emit (events.js:198:13) at Connection.<anonymous> (/Applications/MAMP/htdocs/treehouse_rest_api_with_express/node_modules/mongodb-core/lib/connection/pool.js:280:12) at Object.onceWrapper (events.js:286:20) at Connection.emit (events.js:198:13) at Socket.<anonymous> (/Applications/MAMP/htdocs/treehouse_rest_api_with_express/node_modules/mongodb-core/lib/connection/connection.js:189:49) at Object.onceWrapper (events.js:286:20) at Socket.emit (events.js:198:13) at emitErrorNT (internal/streams/destroy.js:91:8) at emitErrorAndCloseNT (internal/streams/destroy.js:59:3) at process._tickCallback (internal/process/next_tick.js:63:19) name: 'MongoError', message: 'failed to connect to server [localhost:27017] on first connect [MongoError: connect ECONNREFUSED 127.0.0.1:27017]' }