Advanced Feature Flipping/Toggling Across the Server & Client

npm install fflip
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Build Status


Follow @FredKSchott for development news and updates!

Working on an experimental new design? Starting a closed beta? Rolling out a new feature over the next few weeks? Fa-fa-fa-flip it! fflip gives you complete control over releasing new functionality to your users based on thier user id, join date, membership status, and whatever else you can think of. fflip's goal is to be the most powerful and extendable feature flipping/toggling module out there.

  • Describes custom criteria and features using easy-to-read JSON
  • Delivers features down to the client for additional client-side feature flipping
  • Includes Express Middleware for easy integration with Express applications
  • Everything-Agnostic: Supports any database, user representation or framework you can throw at it
npm install fflip --save

Getting Started

Below is a simple example that uses fflip to deliver a closed beta to a fraction of users:

// Include fflip
var fflip = require('fflip');

  criteria: ExampleCriteriaObject, // defined below
  features: ExampleFeaturesObject  // defined below

// Get a User's Enabled Features
var Features = fflip.userFeatures(someFreeUser);
if(Features.closedBeta) {
  console.log('Welcome to the Closed Beta!');


Criteria are the rules that features can test users against. Each rule takes a user and a data argument to test against, and returns true/false if the user matches that criteria. The data argument can be any type, as long as you handle it correctly in the function you describe.

var ExampleCriteriaObject = {
  isPaidUser: function(user, isPaid) {
    return user.isPaid == isPaid;
  percentageOfUsers: function(user, percent) {
    return (user.id % 100 < percent * 100);
  allowUserIDs: function(user, idArr) {
    for(var id in idArr) {
      if(user.id == idArr[id]) 
        return true;
    return false;


Features contain sets of criteria to test users against. The value associated with the criteria is passed in as the data argument of the criteria function. A user will have a featured enabled if they match all listed criteria, otherwise the feature is disabled. Features can include other optional properties for context. Features are described as follows:

var ExampleFeaturesObject = {
  paidFeature: {
    criteria: {
      isPaidUser: true
  closedBeta: {
    name: "A Closed Beta",
    criteria: {
      allowUserIDs: [20,30,80,181]
  newFeatureRollout: {
    name: "A New Feature Rollout",
    description: "Rollout of that new feature over the next month",
    owner: "FredKSchott", // Remember: These are all optional, only criteria is required 
    criteria: {
      isPaidUser: false,
      percentageOfUsers: 0.50


void   config(options)                   // Configure fflip (see below)
Object userFeatures(user)                // Return object of true/false for all features for user
Bool   userHasFeature(user, featureName) // Return true/false if featureName is enabled for user
void   reload()                          // Force a reload (if loading features dynamically)
void   __express(app)                    // Connect with an Express app (see below)

Configure fflip using any of the following options:

  criteria: {}, // Criteria Object
  features: {}, // Features Object | Function (see below)
  reload: 30,   // Interval for refreshing features, in seconds

Loading Features Dynamically

fflip also accepts functions for loading features. If fflip is passed a funciton with no arguments it will call the function and accept the return value. To load asyncronously, pass a function that sends a features object to a callback. fflip will recieve the callback and set the data accordingly. In both cases, fflip will save the function and call it again every X seconds, as set by the reload parameter.

// Load Criteria Syncronously
var getFeaturesSync = function() {
  var collection = db.collection('features');
  var featuresArr = collection.find().toArray();
  /* Proccess featuresArr -> featuresObj (format described above) */
  return featuresObj;

// Load Features Asyncronously
var getFeaturesAsync = function(fflip_callback) {
  var collection = db.collection('features');
  collection.find().toArray(function(err, featuresArr) {
    /* Handle err
     * Proccess featuresArr -> featuresObj (format described above) */

  features: getFeaturesAsync, // or: getFeaturesSync
  reload: 60 /* Call the function again and update every 60 secondss */

Express Integration

fflip provides easy integration with the popular web framework Express.
Just call fflip.__express(app) wherever you set up your express application to enable the following:

A route for manually flipping on/off features

If you have cookies enabled, you can visit /fflip/:name/:action to manually override a feature to always return true/false for your own session. Just replace ':name' with the Feature name and ':action' with 1 to enable, 0 to disable, or -1 to reset (remove the cookie override). This override is stored in the user's cookie.


A fflip object is attached to the request, and includes the following funciontality:

req.fflip = {
  flags: Any override flags set by the fflip cookie
  features: A user's fflip features object. Empty until setForUser() is called.
  setForUser(user): Given a user, attaches the features object to the request (at req.fflip.features)
  has(featureName): Given a feature name, returns the feature boolean, undefined if feature doesn't exist, or null if setForUser() has't been called

Automatically deliver Features to the client

The fflip Express middleware wraps res.render() to always the req.fflip.features object as a Features template variable. To deliver this down to the client, just make sure your template contains the code <script>var Features = <%= Features %></script>.

Special Thanks

Original logo designed by LuboŇ° Volkov

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