Advanced Feature Flipping/Toggling Across the Server & Client


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 their user id, join date, membership status, and whatever else you can think of. fflip's goal is to be the most powerful and extensible feature flipping/toggling module on. the. planet.

  • 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 additional features like feature flipping via cookie
  • Everything-Agnostic: Supports any database, user representation or framework you can throw at it
npm install fflip --save

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 = {
  isPaidUserfunction(userisPaid) {
    return user.isPaid == isPaid;
  percentageOfUsersfunction(userpercent) {
    return (user.id % 100 < percent * 100);
  allowUserIDsfunction(useridArr) {
    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 or router (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 

fflip also accepts functions for loading features. If fflip is passed a function with no arguments it will call the function and accept the return value. To load asynchronously, pass a function that sends a features object to a callback. fflip will receive 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 Synchronously 
var getFeaturesSync = function() {
  var collection = db.collection('features');
  var featuresArr = collection.find().toArray();
  /* Process featuresArr -> featuresObj (format described above) */
  return featuresObj;
// Load Features Asynchronously 
var getFeaturesAsync = function(fflip_callback) {
  var collection = db.collection('features');
  collection.find().toArray(function(errfeaturesArr) {
    /* Handle err
     * Process featuresArr -> featuresObj (format described above) */
  criteria: ExampleCriteriaObject,
  features: getFeaturesAsync, // or: getFeaturesSync 
  reload: 60 // update available features every 60 seconds 

fflip provides easy integration with the popular web framework Express.
Just call fflip.express() with your Express application or Express 4.0 router to enable the following:

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 as in the user's cookie under the name fflip.

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

req.fflip = {
  setForUser(user): Given a user, attaches the features object to the request (at req.fflip.features). Make sure you do this before calling has()!
  has(featureName): Given a feature name, returns the feature boolean, undefined if feature doesn't exist. Throws an error if setForUser() hasn't been called

NOTE: This will only be populated if you call req.fflip.setForUser beforehand.
The fflip Express middleware includes a Features template variable that contains your user's enabled features. Here is an example of how to use it with Handlebars: {{#if Features.closedBeta}} Welcome to the Beta! {{/if}}

NOTE: This will only be populated if you call req.fflip.setForUser beforehand.
The fflip Express middleware also includes a FeaturesJSON template variable that is the JSON string of your user's enabled features. To deliver this down to the client, just make sure your template something like this: <script>var Features = {{ FeaturesJSON }}; </script>.

If you need a finer-grained Express integration, such as changing the URL for manual overrides, adding security middleware, or applying middleware on a subset of routes, you can use express_middleware and express_route directly.

app.get('/custom_path/:name/:action', fflip.express_route);

Original logo designed by Luboš Volkov