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Advanced Feature Flipping/Toggling Across the Server and Client

# fflip

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 out there.

  • Create custom criteria to segment users & features based on your audience.
  • View & edit feature access in one easy place, and not scattered around your code base.
  • System-Agnostic: Support any database, user representation or web framework you can throw at it.
  • Extensible: Supports 3rd-party plugins for your favorite libraries (like our Express integration!)
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: ExampleCriteria, // defined below 
  features: ExampleFeatures  // defined below 
// Get all of a user's enabled features... 
someFreeUser.features = fflip.getFeaturesForUser(someFreeUser);
if(someFreeUser.features.closedBeta === true) {
  console.log('Welcome to the Closed Beta!');
// ... or just check this single feature. 
if (fflip.isFeatureEnabledForUser('closedBeta', someFreeUser) === true) {
  console.log('Welcome to the Closed Beta!');

Criteria are the rules that define access to different features. Each criteria takes a user object and some data as arguments, and returns true/false if the user matches that criteria. You will use these criteria to restrict/allow features for different subsets of your userbase.

var ExampleCriteria = [
    id: 'isPaidUser', // required 
    check: function(user, isPaid) { // required 
      return user.isPaid == isPaid;
    id: 'percentageOfUsers',
    check: function(user, percent) {
      return ( % 100 < percent * 100);
    id: 'allowUserIDs',
    check: function(user, allowedIDs) {
      return allowedIDs.indexOf( > -1;

Features represent some special behaviors in your application. They also define a set of criteria to test users against for each feature. When you ask fflip if a feature is enabled for some user, it will check that user against each rule/criteria, and return "true" if the user passes.

Features are described in the following way:

var ExampleFeatures = [
    id: 'closedBeta', // required 
    // if `criteria` is in an object, ALL criteria in that set must evaluate to true to enable for user 
    criteria: {isPaidUser: true, percentageOfUsers: 0.50}
    id: 'newFeatureRollout',
    // if `criteria` is in an array, ANY ONE set of criteria must evaluate to true to enable for user 
    criteria: [{isPaidUser: true}, {percentageOfUsers: 0.50}]
    id: 'experimentalFeature',
    name: 'An Experimental Feature', // user-defined properties are optional but can be used to add important metadata 
    description: 'Experimental feature still in development, useful for internal development', // user-defined 
    owner: 'Fred K. Schott <>', // user-defined 
    enabled: false, // sets the feature on or off for all users, required unless `criteria` is present instead 

The value present for each rule is passed in as the data argument to it's criteria function. This allows you to write more general, flexible, reusable rules.

Rule sets & lists can be nested and combined. It can help to think of criteria sets as a group of AND operators, and lists as a set of OR operators.

If you'd like to allow wider access to your feature while still preventing a specific group of users, you can use the $veto property. If the $veto property is present on a member of a criteria list (array), and that member evaluates to false, the entire list will evaluate to false regardless of it's other members.

  // Enabled if user is paid OR in the lucky 50% group of other users currently using a modern browser 
  criteria: [{isPaidUser: true}, {percentageOfUsers: 0.50, usingModernBrowser: true}]
  // Enabled if user is paid OR in the lucky 50% group of other users, BUT ONLY if using a modern browser 
  criteria: [{isPaidUser: true}, {percentageOfUsers: 0.50}, {usingModernBrowser: true, $veto: true}]
  • .config(options) -> void: Configure fflip (see below)
  • .isFeatureEnabledForUser(featureName, user) -> boolean: Return true/false if featureName is enabled for user
  • .getFeaturesForUser(user) -> Object: Return object of true/false for all features for user
  • .reload() -> void: Force a reload (if loading features dynamically)

Configure fflip using any of the following options:

  criteria: {}, // Criteria Array 
  features: {}, // Features Array | 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/Format `featuresArr` if needed (format described above) */
  return featuresArr;
// Load Features Asynchronously 
var getFeaturesAsync = function(callback) {
  var collection = db.collection('features');
  collection.find().toArray(function(err, featuresArr) {
    /* Handle err
     * Process/Format `featuresArr` if needed (format described above) */
  criteria: ExampleCriteriaObject,
  features: getFeaturesAsync, // or: getFeaturesSync 
  reload: 60 // update available features every 60 seconds 

As mentioned, fflip's goal is to be flexible enough to integrate with any web framework, database, or ORM. The following integrations are known to exist:

If you're interested in creating an integration, don't hesitate to reach out or create an issue if some functionality is missing. And if you've created an integration, please add it to the list above!

Original logo designed by Luboš Volkov