Using require without export

Question

I have this code (which works perfectly well) which I've borrowed from an online resource:

var express = require('express'); var bodyParser = require('body-parser'); var logger = require('morgan');  var app = express();  require('./init/db'); require('./init/cache'); //Bring in Redis  //Define Routes var userRoutes = require('./routes/user');  module.exports = app; 

The bit I don't understand is "require" when used in this way? Here is the file it brings in:

//db.js var mongoose = require('mongoose'); var dbURI = <theURI>;  mongoose.connect(dbURI);  // CONNECTION EVENTS mongoose.connection.on('connected', function() {   console.log('Mongoose connected successfully'); }); 

It's the same with my Redis connection:

//cache.js var redis  = require("redis");  var redisClient  = redis.createClient(process.env.CACHE_PORT, process.env.CACHE_URL); redisClient.auth(process.env.CACHE_PASS);  redisClient.on("ready", function () {   console.log("Cache is connected"); }); 

but as you can see there is no module.exports anywhere in the db.js or cache.js files! When I google this to understand how it works the examples always talk about module.exports and require together.

Questions

1. Could someone explain how require works when used on its own like this?

2. How can I make the cache/Redis connection available so that it can be used in my userRoutes file using something like: var userRoutes = require('./routes/user')(redis);

Answer 1

Could someone explain what is happening with this code? In other words, how does require work when not used with export.

We almost always see require() being used with module.exports, but you don't have to. When you don't export anything, the code in the imported module will still run, but you can't bind the import to a variable and interact with it.

Consider the following Foo.js module :

var foo = {};  foo.greet = function(){     console.log('Hello from Foo!'); }  foo.greet(); 

I can import this module in my main file, like so :

require('./foo'); 

If I run this main file, the code inside the Foo.js module will run, and Hello from Foo! will be printed to the console.

However, I can't interact with the foo object directly. The following code will not work :

require('./foo'); foo.greet(); //ReferenceError, foo is not defined 

I can bind the module import to a variable, but even this will not work :

var foo = require('./foo'); foo.greet(); //TypeError, foo.greet is not a function 

To get it to work, I need to export the foo object from my module, using the module.exports that you are familiar with.

This demonstrates that you don't need to export anything from your modules, just like you don't need to bind the imported module to a variable when you are requiring it. The difference is that you won't be able to interact with the code in the imported module, if you don't export what you don't want to make visible in that module.

In the code in your question, importing Redis works because that module is self-contained, you don't need to interact with it in your code. You only need to import the code so that it can run (require the main Redis module and create the client)