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    Node.js SDK reference (client-side)

    Read time: 3 minutes
    Last edited: Jan 27, 2023
    Version 3 of the Node.js 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.

    You can upgrade your SDK at any time. However, the ability to target by context, or review context instances that have encountered flags in your application, is only available for customers in the contexts Early Access Program (EAP). If you want access this feature, join the EAP. If you are not part of the EAP, your application can send user contexts to LaunchDarkly and they will appear on the Users list.

    Code samples on this page are from the two most recent SDK versions where they differ. To learn more about upgrading, read Node.js SDK 2.x to 3.0 migration guide.

    Overview

    This topic documents how to get started with the client-side Node.js SDK, and links to reference information on all of the supported features.

    SDK quick links

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

    ResourceLocation
    SDK API documentationSDK API docs
    GitHub repositorynode-client-sdk
    Sample applicationNode.js (client-side)
    Published modulenpm
    For use in mobile, desktop, and embedded client applications only

    This SDK is intended for use in single-user mobile, desktop, and embedded applications. If you have a Node.js application and want to set up LaunchDarkly on the server-side, read the server-side Node.js SDK reference. If you are using Electron, there is an Electron SDK more specifically designed for that environment.

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

    This SDK is closely related to the browser JavaScript SDK and has almost exactly the same API, but does not have any browser-specific functionality and adds several features specific to Node.

    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 code.

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

    Here's how:

    npm install launchdarkly-node-client-sdk

    Next, import the LaunchDarkly client in your application code:

    const LaunchDarkly = require('launchdarkly-node-client-sdk');

    After you install and import the SDK, create a single, shared instance of LDClient. To create a client instance, you need your environment's client-side ID. This authorizes your application to connect to a particular environment within LaunchDarkly.

    To find and copy your LaunchDarkly client-side ID:

    1. Navigate to the Account settings page.
    2. Click the Projects tab.
    3. Click the name of your project. The Environments tab appears.
    4. Click the environment's client-side ID to copy it to your clipboard.
    Never embed a server-side SDK key into a client-side application

    Client-side IDs are not secret and you can expose them in your client-side code without risk. However, never embed a server-side SDK key into a client-side application.

    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 LaunchDarkly 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.

    Feature flag targeting and rollouts are determined by the active user. You must pass a user context to the SDK during initialization before requesting any feature flags with variation. Failure to pass a valid user context to the SDK during initialization will result in an error.

    Here's how to initialize the client:

    const context = {
    kind: 'user',
    key: 'user-key-123abc'
    };
    const client = LaunchDarkly.initialize('client-side-id-123abc', context);

    The client emits a ready event when you have initialized it. You can also use the waitForInitialization() method, which returns a Promise. After you have initialized it, you can safely call variation to access your feature flags. The SDK emits the ready event only once, when the client first initializes. In a production application, your calls to client.variation would normally not be inside of this event handler.

    To emit a ready event:

    client.on('ready', () => {
    // initialization succeeded, flag values are now available
    const flagValue = client.variation('flag-key-123abc', false);
    // etc.
    });
    Streaming updates

    The SDK does not subscribe to streaming real-time updates automatically when you initialize it. Setting the streaming option to true in the client configuration causes the SDK to open a streaming connection to LaunchDarkly and receive live feature flag updates. You can also specify an event handler with client.on('change') to be notified immediately when a flag changes.

    Making feature flags available to this SDK

    You must make feature flags available to client-side SDKs before the SDK can evaluate those flags. If an SDK tries to evaluate a feature flag that is not available, the user will receive the default value for that flag.

    To make a flag available to this SDK, check the SDKs using Client-side ID checkbox during flag creation, or on the flag's Settings tab. To make all of a project's flags available to this SDK by default, check the SDKs using Client-side ID checkbox in your project Settings.

    Shutting down

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

    Supported features

    This SDK supports the following features: