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Node.js SDK reference (server-side)

Read time: 7 minutes
Last edited: Feb 10, 2024
Recent major versions

Version 9 of the Node.js (server-side) SDK supports migration feature flags. These are temporary flags used to migrate data or systems while keeping your application available and disruption free. To learn more about upgrading, read Node.js (server-side) 8.x to 9.0 migration guide.

Version 8 of the Node.js (server-side) SDK has been re-written in TypeScript, and the GitHub repository and package names have been updated. To learn more about upgrading, read Node.js (server-side) 7.x to 8.0 migration guide.

Version 7 of the Node.js (server-side) SDK replaces users with contexts. A context is a generalized way of referring to the people, services, machines, or other resources that encounter feature flags in your product. Contexts replace another data object in LaunchDarkly: "users." To learn more about upgrading, read Node.js (server-side) 6.x to 7.0 migration guide and Best practices for upgrading users to contexts.

Code samples on this page are from these several most recent SDK versions where they differ.


This topic documents how to get started with the server-side Node.js SDK. It also links to reference information for all of the features the server-side Node.js SDK supports.

SDK quick links

LaunchDarkly's SDKs are open source. In addition to this reference guide, we provide source, API reference documentation, and sample applications:

SDK API documentationSDK API docs
GitHub repositorynode-server-sdk
Sample applicationsNode.js (server-side)
Node.js (server-side), TypeScript
Node.js (server-side) with bootstrapping
OpenFeature Node.js (server-side)
Published modulenpm
For use in server-side applications only

This SDK is intended for use in multi-user Node.js server applications. If you want to set up LaunchDarkly in JavaScript in a browser environment, read the JavaScript SDK reference. If you're creating a client-side Node application, read the Node.js SDK reference (client-side). If you're creating a desktop application in Electron, read the Electron SDK reference.

To learn more about LaunchDarkly's different SDK types, read Client-side, server-side, and edge SDKs.

The sample code snippets for this SDK are available in both JavaScript and TypeScript, where the sample code differs. To learn more, read Using LaunchDarkly with TypeScript.

Getting started

After you complete the Getting Started process, follow these instructions to start using the LaunchDarkly SDK in your Node.js application.

First, install the LaunchDarkly SDK as a dependency in your application using your application's dependency manager.

Here's how:

npm install @launchdarkly/node-server-sdk
# In earlier versions, the package name was launchdarkly-node-server-sdk or ldclient-node

Next, import the LaunchDarkly client in your application code:

const ld = require('@launchdarkly/node-server-sdk');
The Node.js (server-side) SDK uses an SDK key

The Node.js (server-side) SDK uses an SDK key. Your environment's SDK key is available in the Projects tab of your Account settings page. To learn more about key types, read Keys.

After you install and import the SDK, create a single, shared instance of LDClient. Specify your SDK key here to authorize your application to connect to a particular environment within LaunchDarkly.

Here's how:

const client = ld.init('sdk-key-123abc');
LDClient must be a singleton

It's important to make LDClient a singleton for each LaunchDarkly project. The client instance maintains internal state that allows us to serve feature flags without making any remote requests. Do not instantiate a new client with every request.

If you have multiple LaunchDarkly projects, you can create one LDClient for each. In this situation, the clients operate independently. For example, they do not share a single connection to LaunchDarkly.

The client emits a ready event when you initialize it and it can serve feature flags.

Using client, you can check which variation a particular context will receive for a given feature flag. The ready event is only emitted once, when you first initialize the client. In a production application, place your client.variation code so that it is invoked as needed.

Here is an example:

const context = {
"kind": 'user',
"key": 'user-key-123abc',
"name": 'Sandy'
client.once('ready', () => {
client.variation('flag-key-123abc', context, false,
(err, showFeature) => {
if (showFeature) {
// application code to show the feature
} else {
// the code to run if the feature is off

Promises and async

All asynchronous SDK methods which accept a callback also return a Promise. This means that if your application uses promises to manage asynchronous operations, interacting with the SDK should be convenient. Because the async/await syntax is based on Promises, these methods also work with await.

Here is an example:

// Using the .then() method to add a continuation handler for a Promise
client.variation('flag-key-123abc', context, false).then((value) => {
// application code
// Using "await" instead, within an async function
const value = await client.variation('flag-key-123abc', context, false);

There is also an alternative to the ready event:

// Using .then() and .catch() to add success and error handlers to a Promise
client.waitForInitialization().then((client) => {
// initialization complete
}).catch((err) => {
// initialization failed
// Using "await" instead, within an async function
try {
await client.waitForInitialization();
// Initialization complete
} catch (err) {
// Initialization failed

allFlagsState and flush also return a Promise.

Shut down the client

Shut down the client when your application terminates. To learn more, read Shutting down.

Unlike other LaunchDarkly SDKs, the Node.js (server-side) SDK does not automatically send pending analytics events to LaunchDarkly when it shuts down. To send analytics events, you first need to call flush.

Supported features

This SDK supports the following features: