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A simple singleton key-function store for registering feature toggles and variant values.


You use npm right?

npm install flip-js


Flip is a singleton, so everytime you require('flip') it will return the same instance. It's inspired by routes in express and template helpers in handlebars if you're familiar with either of those.

You'll want to register features somewhere in your server setup.

var flip = require('flip');
flip.register('my-feature', function (subject) {
    return subject.isAdmin;

Then somewhere in your route or request handler you can check if the feature should be enabled.

var flip = require('flip');
module.exports = function (req, res) {
    var templateContext = {
        'my-feature-enabled': flip.check('my-feature', req.user)

You can also run checks asynchronously by providing a callback to the check function.

var flip = require('flip');
module.exports = function (req, res) {
    flip.check('my-feature', req.user, function (myFeature) {
        var templateContext = {
            'my-feature-enabled': myFeature


Flip comes with a few baked check functions.

Blessed List

Most times you just have a list of keys (users for example) that should have the feature enabled. In this case you can provide a list of the blessed users and be on your way.

// Register a feature and provide a list of blessed users 
flip.register('my-feature', flip.bundled.blessedList(['drk', 'astro']));
// Then provide the user in the check to see if the feature should be enabled 
flip.check('my-feature', 'drk'); // true 
flip.check('my-feature', 'bogus'); // false 


Another common use case is a/b testing or providing a variant per session. Here you can provide a hash of variants where the values are list of keys (users for example) for each variant.

var variants = {
    awesome: ['astro'],
    favorite: ['drk']
flip.register('my-variant', flip.bundled.variant(variants));
// Check for variant for a given user 
flip.check('my-variant', 'astro'); // 'awesome' 
flip.check('my-variant', 'drk'); // 'favorite' 
flip.check('my-variant', 'bogus'); // false (no variant has been assigned!) 

In the last example — 'bogus' — no variant has been assigned. You can handle the logic here for assigning a variant for 'bogus' and then register the variant again. In this case the variants are stored in memory which isn't extremely useful, but no reason you couldn't use another store like redis.


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