Node JS Feature Flags

Node JS Feature Flag Resources/Solutions

  • LaunchDarkly Node.js Feature Flag SDK – LaunchDarkly
    • An installable feature flag software development kit for Node.js apps.  This SDK harnesses the LaunchDarkly app and allows you to target users and manage rollouts.  Feature flags are evaluated in microseconds.
    • Main Website
    • GitHub Repo
  • Flipit – Darren M.
    • “A module that allows you to use feature flags (also known as feature flipping) in Node.js. You can enable/disable features programmatically or via an external configuration file. Any changes to the configuration file will update the module without requiring a restart.”
    • GitHub Repo
  • NPM Feature Flags – (alpha) Randall Koutnik
    • “PRE-ALPHA. USE WITH CAUTION. This module lets developers set user levels and restrict access to features based on said levels. It also provides a web interface to customize those levels without pushing additional code or restarting the server. Currently, Feature Flags only supports restriction by url.”
    • GitHub Repo
  • FFlip – Fred Schott
    • “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”
    • GitHub Repo
  • Unleash – Unleash
    • “Unleash is a feature toggle system, that gives you a great overview over all feature toggles across all your applications and services.

      The main motivation for doing feature toggling is to decouple the process for deploying code to production and releasing new features. This helps reducing risk, and allow us to easily manage which features to enable”

    • GitHub Repo


LaunchDarkly Node.js SDK Reference

This reference guide documents all of the methods available in LaunchDarkly’s Node.js SDK, and explains in detail how these methods work. If you want to dig even deeper, LaunchDarkly’s SDKs are open source– head to LaunchDarkly’s Node.js SDK repository on GitHub to look under the hood.

Getting started

If you haven’t taken a look at our Quickstart guide yet, we recommend starting there to see how install our SDK into your Node.js application.

Once the SDK is installed, you’ll want to create a single, shared instance of the LaunchDarkly client:

ld_client = LaunchDarkly.init("YOUR_SDK_KEY");

LDClient must be a singleton

It’s important to make this a singleton– internally, the client instance maintains internal state that allows us to serve feature flags without making any remote requests. Be sure that you’re not instantiating a new client with every request.

The client will emit a ready event when it has been initialized and can serve feature flags:

ld_client.once(`ready`, function() {

Customizing your client

You can also pass custom parameters to the client by creating a custom configuration object:

var config = {"timeout": 3};
ld_client = LaunchDarkly.init("YOUR_SDK_KEY", config);

Here, we’ve customized the client timeout parameters. The complete list of customizable parameters is as follows:

  • timeout: Must be a number, in seconds, and controls the request timeout to LaunchDarkly
  • capacity: Must be a number, and controls the maximum size of the event buffer. LaunchDarkly sends events asynchronously, and buffers them for efficiency.
  • logger: Configures a logger for warnings and errors generated by the SDK.
  • flush_interval: Must be a number, in seconds, and controls how long LaunchDarkly buffers events before sending them back to our server. If your server generates many events per second, we suggest decreasing the flush_interval and / or increasing capacity to meet your needs.
  • proxy_host: Must be a string. Allows you to specify a host for an optional HTTP proxy.
  • proxy_port: Must be a number. Allows you to specify a port for an optional HTTP proxy. Both the host and port must be specified to enable proxy support
  • proxy_auth: Must be a string. Allows you to specify basic authentication parameters for an optional HTTP proxy. Usually of the form username:password.
  • offline: Must be a boolean. Whether the client should be initialized in offline mode. In offline mode, default values are returned for all flags and no remote network requests are made.
  • stream: Must be a boolean. Whether streaming or polling should be used to receive flag updates.
  • use_ldd: Must be a boolean. If true, the SDK will rely on LDD for feature updates. See Deployment optionsfor more details.


Feature flag targeting and rollouts are all determined by the user you pass to your variation calls. In our Node.JS SDK, users are simply JSON objects. Here’s an example:

var user = {
  "key": "aa0ceb",
  "firstName": "Ernestina",
  "lastName": "Evans",
  "email": "",
  "custom": {
    "groups": ["Google", "Microsoft"]

Let’s walk through this snippet. The most important attribute is the user key– in this case we’ve used the hash"aa0ceb". The user key is the only mandatory user attribute. The key should also uniquely identify each user. You can use a primary key, an e-mail address, or a hash, as long as the same user always has the same key. We recommend using a hash if possible.

All of the other attributes (like firstName, email, and the custom attributes) are optional. The attributes you specify will automatically appear on our dashboard, meaning that you can start segmenting and targeting users with these attributes.

Besides the key, LaunchDarkly supports the following attributes at the “top level”. Remember, all of these are optional:

  • ip: Must be an IP address. If you provide an IP, LaunchDarkly will use a geolocation service to automatically infer a country for the user (unless you’ve already specified one).
  • firstName: Must be a string. If you provide a first name, you can search for users on the Users page by name.
  • lastName: Must be a string. If you provide a last name, you can search for users on the Users page by name.
  • country: Must be a string representing the country associated with the user.
  • email: Must be a string representing the user’s e-mail address. If an avatar URL is not provided, we’ll useGravatar to try to display an avatar for the user on the Users page.
  • avatar: Must be an absolute URL to an avatar image for the user.
  • name: Must be a string. You can search for users on the User page by name
  • anonymous: Must be a boolean. See the section below on anonymous users for more details.

In addition to built-in attributes, you can pass us any of your own user data by passing custom attributes, like thegroups attribute in the example above.

A note on types

Most of our built-in attributes (like names and e-mail addresses) expect string values. Custom attribute values can be strings, booleans (like true or false), numbers, or lists of strings, booleans or numbers.

If you enter a custom value on our dashboard that looks like a number or a boolean, it’ll be interpreted that way.

Custom attributes are one of the most powerful features of LaunchDarkly. They let you target users according to any data that you want to send to us– organizations, groups, account plans– anything you pass to us becomes available instantly on our dashboard.

Anonymous users

You can also distinguish logged-in users from anonymous users in the SDK, as follows:

var user = {"key":"aa0ceb", "anonymous": true};

You will still need to generate a unique key for anonymous users– session IDs or UUIDs work best for this.

Anonymous users work just like regular users, except that they won’t appear on your Users page in LaunchDarkly. You also can’t search for anonymous users on your Features page, and you can’t search or autocomplete by anonymous user keys. This is actually a good thing– it keeps anonymous users from polluting your Users page!


The variation method determines which variation of a feature flag a user receives.

ld_client.variation("your.feature.key", user, false, function(err, show_feature) {
  if (show_feature) {
      # application code to show the feature
  else {
      # the code to run if the feature is off 

variation calls take the feature flag key, an LDUser, and a default value.

The default value will only be returned if an error is encountered– for example, if the feature flag key doesn’t exist or the user doesn’t have a key specified.

The variation call will automatically create a user in LaunchDarkly if a user with that user key doesn’t exist already. There’s no need to create users ahead of time (but if you do need to, take a look at Identify).

All flags

The allFlags method produces a map of feature flag keys to their values for a specific user.

This method can be useful for passing feature flags to your front-end. In particular, it can be used to provide bootstrap flag settings for our JavaScript SDK.

ld_client.all_flags(user, function(err, flags) {
  # a map of flag keys to values


The track method allows you to record actions your users take on your site. This lets you record events that take place on your server. In LaunchDarkly, you can tie these events to goals in A/B tests. Here’s a simple example:

ld_client.track("Signed up", user);


The identify creates or updates users on LaunchDarkly, making them available for targeting and autocomplete on the dashboard. In most cases, you won’t need to call identify— the variation call will automatically create users on the dashboard for you. identify can be useful if you want to pre-populate your dashboard before launching any features.


Secure mode hash

The secure_mode_hash method computes an HMAC signature of a user signed with the client’s SDK key. If you’re using our JavaScript SDK for client-side flags, this method generates the signature you need for secure mode.



Internally, the LaunchDarkly SDK keeps an event buffer for track and identify calls. These are flushed periodically in a background thread. In some situations (for example, if you’re testing out the SDK in a REPL), you may want to manually call flush to process events immediately.


Note that the flush interval is configurable– if you need to change the interval, you can do so when configuring your client instance.

Offline mode

In some situations, you might want to stop making remote calls to LaunchDarkly and fall back to default values for your feature flags. For example, if your software is both cloud-hosted and distributed to customers to run on premise, it might make sense to fall back to defaults when running on premise. You can do this by setting offlinemode in the client’s config object.

var config = {"offline": true};
ld_client = LaunchDarkly.init("YOUR_SDK_KEY", config);
ld_client.variation("any.feature.flag", user, false, cb) // cb will always be invoked with the default value (false)


Close safely shuts down the client instance and releases all resources associated with the client. In most long-running applications, you should not have to call close.