Freecodecamp error en la conexión MongoDB and Mongoose

Mi código era así:

mongoose.connect(MONGO_URI, { useNewUrlParser: true, useUnifiedTopology: true });

y lanzaba el error:

ReferenceError: MONGO_URI is not defined
    at Object.<anonymous> (/home/runner/boilerplate-mongomongoose/myApp.js:6:18)

 

cuando debería ser así:

myApp.js

 /** # MONGOOSE SETUP #
/*  ================== */

/** 1) Install & Set up mongoose */
const mongoose = require('mongoose');
mongoose.connect(process.env.MONGO_URI, { useNewUrlParser: true, useUnifiedTopology: true });