1. 1. Purpose
  2. 2. Setting up
  3. 3. Writing some code
  4. 4. Seeing it work

Purpose

Example of how to separate route code in Express.js using npm link.

Setting up

We can use the express generator to get up and running quickly with Express. Install it with npm install express-generator -g if you don’t already have it:

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tmp$ express todo-server
tmp$ cd todo-server
todo-server$ npm install

It might be worth mentioning that at the time of writing, this installs express at version approx 4.9.0 or ~4.9.0.

At this point we could npm start to start our server. As defined in package.json, we’re basically running node ./bin/www.

We now want to start working on a separate module which will contain the logic behind the /items route in our application:

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todo-server$ mkdir ../todo-items
todo-server$ cd ../todo-items
todo-items$ npm init
todo-items$ npm install --save express

Writing some code

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todo-items$ vim index.js
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var express = require('express');
var router = express.Router();

var items = require('./items.json');

router.get('/', function(req, res) {
  res.send(items);
});

module.exports = router;
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todo-items$ vim items.json
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{
    "task A": true,
    "task B": false
}

We’re just defining a GET on ‘/‘. This means that, whichever route our todo-items module gets loaded on in todo-server, making a GET request on it will give us back our items.json data.

That’s great but todo-server doesn’t have todo-items installed so how’s it going to use it? We could publish todo-items on npm or host it in a Git repo on Github for example.

As you can see, we’re just experimenting with todo-items for now. Maybe later we’ll use a proper database. Maybe we’re not sure which database to go with. Point is, we don’t want to publish or host todo-items as it’s still early days, and besides, it would be better to avoid having to stay re-installing the module for every single change we make.

Enter npm link:

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todo-items$ npm link
/Users/justin/.nvm/v0.10.33/lib/node_modules/todo-items -> /Users/justin/tmp/todo-items

todo-items$ cd ../todo-server
todo-server$ npm link todo-items
/Users/justin/tmp/todo-server/node_modules/todo-items -> /Users/justin/.nvm/v0.10.33/lib/node_modules/todo-items -> /Users/justin/tmp/todo-items

Executing it from todo-items we get a link to this directory from our global node_modules. npm link todo-items in todo-server gets us a link in todo-server‘s node_modules to the link in our global node_modules, effectively linking back to our todo-items directory containing our implementation for that module.

End result - if you list the contents of todo-server‘s node_modules you’ll see we have our todo-items in there, albeit a link and not an actual directory structure as our other dependencies are installed as. Now, since the todo-items installation in our todo-server is just a link, any updates we make in todo-items will automatically take effect in our todo-server project.

Seeing it work

Simply require and use:

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todo-server$ vim app.js
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var items = require('todo-items');
app.use('/items', items);

Now we can npm start, hit http://localhost:3000/items in a browser, and get our items.json back.