Purpose of installing Twitter Bootstrap through npm?

If you NPM those modules you can serve them using static redirect.

First install the packages:

npm install jquery
npm install bootstrap

Then on the server.js:

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

// prepare server
app.use('/api', api); // redirect API calls
app.use('/', express.static(__dirname + '/www')); // redirect root
app.use('/js', express.static(__dirname + '/node_modules/bootstrap/dist/js')); // redirect bootstrap JS
app.use('/js', express.static(__dirname + '/node_modules/jquery/dist')); // redirect JS jQuery
app.use('/css', express.static(__dirname + '/node_modules/bootstrap/dist/css')); // redirect CSS bootstrap

Then, finally, at the .html:

<link rel="stylesheet" href="/css/bootstrap.min.css">
<script src="/js/jquery.min.js"></script>
<script src="/js/bootstrap.min.js"></script>

I would not serve pages directly from the folder where your server.js file is (which is usually the same as node_modules) as proposed by timetowonder, that way people can access your server.js file.

Of course you can simply download and copy & paste on your folder, but with NPM you can simply update when needed... easier, I think.

1. The point of using CDN is that it is faster, first of all, because it is a distributed network, but secondly, because the static files are being cached by the browsers and chances are high that, for example, the CDN's jquery library that your site uses had already been downloaded by the user's browser, and therefore the file had been cached, and therefore no unnecessary download is taking place. That being said, it is still a good idea to provide a fallback.

   Now, the point of bootstrap's npm package

   is that it provides bootstrap's javascript file as a module. As has been mentioned above, this makes it possible to require it using browserify, which is the most likely use case and, as I understand it, the main reason for bootstrap being published on npm.

2. How to use it

   Imagine the following project structure:

   project
   |-- node_modules
   |-- public
   |   |-- css
   |   |-- img
   |   |-- js
   |   |-- index.html
   |-- package.json
   
   

In your index.html you can reference both css and js files like this:

<link rel="stylesheet" href="../node_modules/bootstrap/dist/css/bootstrap.min.css">
<script src="../node_modules/bootstrap/dist/js/bootstrap.min.js"></script>

Which is the simplest way, and correct for the .css files. But it is much better to include the bootstrap.js file like this somewhere in your public/js/*.js files:

var bootstrap = require('bootstrap');

And you include this code only in those javascript files where you actually need bootstrap.js. Browserify takes care of including this file for you.

Now, the drawback is that you now have your front-end files as node_modules dependencies, and the node_modules folder is usually not checked in with git. I think this is the most controversial part, with many opinions and solutions.

---

UPDATE March 2017

Almost two years have passed since I wrote this answer and an update is in place.

Now the generally accepted way is to use a bundler like webpack (or another bundler of choice) to bundle all your assets in a build step.

Firstly, it allows you to use commonjs syntax just like browserify, so to include bootstrap js code in your project you do the same:

const bootstrap = require('bootstrap');

As for the css files, webpack has so called "loaders". They allow you write this in your js code:

require('bootstrap/dist/css/bootstrap.css');

and the css files will be "magically" included in your build. They will be dynamically added as <style /> tags when your app runs, but you can configure webpack to export them as a separate css file. You can read more about that in webpack's documentation.

In conclusion.

1. You should "bundle" your app code with a bundler
2. You shouldn't commit neither node_modules nor the dynamically built files to git. You can add a build script to npm which should be used to deploy files on server. Anyway, this can be done in different ways depending on your preferred build process.