A framework for organizing and loading code by feature with built-in rollout support.
Last updated 8 years ago by chrisjpowers .
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Successfully organizing your source code as your app grows can be quite a feat. That's why I wrote Feat.

Feat is a tool for modularly organizing your Node.js code by feature rathan than taxonomy. Rails-style model, controller, view, etc. directories bloat uncontrollably over time, so Feat puts all the code that pertains to a given feature in its own directory. Every file that has to do with the given feature is colocated, making it easy find your source code. Your file structure grows horizontally rather than vertically, leveraging feature names to add an extra layer of context to the code it contains.

Additionally, Feat allows you to easily toggle your features on and off without having to restart your server! This gives you built-in dynamic feature rollout functionality and can even help you A/B test your features.

Disclaimer: Feat's API is still in flux, so breaking changes may occur.


Install Feat as a global npm package so that you can use the feat command anywhere:

npm install -g feat

On the command line, cd to a Node.js project and use feat add to add a feature:

cd /path/to/myproject
feat add first-feature

This will add a ./features/first-feature directory to your project. It also will add ./features.json where it will persist your current features configuration. You can use this command to add additional features, or simply add your own folders to the ./features directory.



First you need to add feat.middleware() to your Connect-compatible server:

var feat = require("feat"),
    express = require("express"),
    app = express.createServer();


Feature Servers

If you look in ./features/first-feature/index.js you will see that by default it creates an Express server and exports it as exports.server. When you boot your app up, Feat will wire up the servers of all enabled features, leaving out the disabled features.

Here's a trivial example of a feature's server:

// ./features/first-feature/index.js
var feat = require("feat"),
    express = require("express");

var server = exports.server = express.createServer();

server.get("/foo", function(req, res) {

Enabling/Disabling Features

If you start your server and visit /foo you will see the server respond with "bar" because new features are turned on by default. To disable a feature, use feat disable:

feat disable first-feature

Now if you visit /foo you should should not see a response. To turn the feature back on, use feat enable:

feat enable first-feature

NOTE: Features can be enabled and disabled "hot" without restarting your server!

Feature Events

While it should be a goal to keep features as detached from one another as possible, clearly there will points of interaction that will have to be setup when a feature is turned on and torn down when a feature is turned off. It is a good practice to have features offer services to other features to isolate and control the interactions.

For example, let's say that we have a navigation feature that manages the links in our site's nav. If our first-feature needs to display a link in the navigation, it should only do so when it is active. If it is deactivated, it needs to remove its link from the navigation.

The feat object is an EventEmitter that emits events when features are turned on and off. Here is how we could handle our navigation:

// ./features/first-feature/index.js
var feat = require("feat"),
    express = require("express"),
    nav = require("../navigation");

var server = exports.server = express.createServer();

server.get("/foo", function(req, res) {

feat.on("first-feature:on", function() {
  nav.add("/foo", "Foo");

feat.on("first-feature:off", function() {

Assuming that the navigation feature exported the add and remove functions, we would be able to use them in this way to ensure our nav is only present when this feature is turned on.

Running an Isolated Feature Server

As an app grows in size, it can be useful in development to run a server with your feature but without the rest. Feat supports this with the start command:

feat start first-feature

Without a feature name, feat start runs whatever command you have for npm start in your package.json file.

Running Isolated Feature Specs

Similarly, running specs for a single feature is supported:

feat test first-feature

Feat assumes your feature directory has either a spec or test directory with an optional spec-helper.js or test-helper.js file and tests that use a *test.js or *spec.js naming convention. NOTE: Coffeescript files will also work automatically.

feat test runs your feature's server at localhost:8228 by default, adding {ENV: "test"} to process.env. You can specify a custom port if needbe by setting the PORT environment variable when you start feat test.

Running feat test without a feature name will run your server with npm test and load all your features' tests.



While enabling/disabling features from the command line works, it may be nice to have a GUI for easily managing feature state in development and testing. Feat ships with a GUI middleware that you can add to your server stack:

var feat = require("feat"),
    express = require("express"),
    app = express.createServer();

if(process.env.ENV === "development") {

There is currently no authentication built into the GUI, so it's important that you do not deploy it to a production environment. One interesting possibility is to create a "gui" feature that uses this middleware, that way you are able to turn it on and off on the fly!


Right now this organization strategy is experimental and I haven't actually built out a project with it yet to validate whether or not it's effective. I am actively looking for feedback and ideas about what works, what doesn't and what is missing!

  • Add code examples
  • Build some apps with Feat to validate/invalidate the approach.
  • Explore using Feat for graduated feature rollouts.
  • Explore using Feat to A/B test features.
  • Evaluate feat test and feat server -- are they useful?
  • Evaluate on-the-fly feature switching -- is it worth the cost?
  • Improve the GUI plugin
  • Use something better than fs.watchFile to watch for changes to features.json

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