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    JavaScript SDK reference

    Read time: 6 minutes
    Last edited: May 12, 2023
    Version 3 of the JavaScript 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."

    Code samples on this page are from the two most recent SDK versions where they differ. To learn more about upgrading, read JavaScript SDK 2.x to 3.0 migration guide and Best practices for upgrading users to contexts.


    This topic documents how to get started with the client-side JavaScript 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:

    SDK API documentationSDK API docs
    GitHub repositoryjs-client-sdk
    Sample applicationJavaScript
    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. It is intended for client-side (browser) feature flags only. 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.

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

    Do you need information about Svelte, Angular, or other frameworks?

    LaunchDarkly does not offer SDKs for all languages or frameworks. If you're searching for information about using Svelte, Preact, or Angular with LaunchDarkly, you may be able to use the JavaScript SDK instead.

    To request support for a specific language or framework, contact Support.

    This SDK does two things:

    • Makes feature flags available to your client-side (front-end) JavaScript code.
    • Sends click, page view, and custom events from your front-end for A/B tests and analytics.

    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.

    Browser support

    The LaunchDarkly client-side JavaScript SDK can be used in all major browsers. However, not all browsers have built-in support for the standard APIs that it uses. Those APIs are Promise, EventSource, and querySelectorAll. The SDK always requires Promise, but the other two are optional depending on which SDK features you use.

    The standard solution for ensuring that you will get the same functionality even in browsers that do not have native support for these features is to use polyfills. For a detailed description, and links to information about which browsers may require this, read JS SDK requirements and polyfills.

    Additionally, the JavaScript SDK versions 3.0.0 and 3.1.0 use optional chaining. If you encounter an error related to optional chaining during transpiling, bundling, or running tests, updating to version 3.1.1 should resolve the error.

    Do Not Track and ad blocking software

    The JavaScript SDK respects the Do Not Track events header. If an end user has Do Not Track enabled in their browser, the SDK does not send analytics events for flag evaluations or goals. In addition, ad-blocking software may block analytics events from being sent. This does not impact feature flag evaluations. To learn more about the events SDKs send to LaunchDarkly, read Analytics events.

    Getting started

    After you complete the Getting started process, follow these instructions to start using the LaunchDarkly JavaScript SDK in your JavaScript code.

    The first step is to make the JavaScript SDK available as a dependency. There are two ways to make the JavaScript SDK available: as a module with a package manager, or with a script tag.

    Making the SDK available with a package manager

    In most cases, making the JavaScript SDK available to your application or site requires running one of the following in your project:

    npm install launchdarkly-js-client-sdk

    If you are using a package manager, and combining dependencies with your code using a tool such as Webpack, there are various ways to import the JavaScript SDK into your code.

    Here are some examples in commonly used frameworks:

    // Using require()
    const LDClient = require('launchdarkly-js-client-sdk');
    // Using ES2015 imports
    import * as LDClient from 'launchdarkly-js-client-sdk';
    // Using TypeScript imports
    import * as LDClient from 'launchdarkly-js-client-sdk';

    In earlier versions of the SDK, the package was named ldclient-js instead of launchdarkly-js-client-sdk.

    Making the SDK available with a script tag

    To load the JavaScript SDK as a script tag, include one of the following in the <head> tag of your site on any pages where you need feature flags or want to track metrics for Experimentation.

    Do not use script tags from unpkg or jsDelivr in production

    The script tag in the self-hosted example below references a script which is deployed alongside other JavaScript resources in your application. We recommend that you use this method in production.

    Do not use script tags with sources from unpkg and jsDelivr in production environments. These introduce a critical dependency on a third-party service. The unpkg and jsDelivr scripts are intended to be used only for ease of development and getting started.

    In production environments, we strongly recommend that you self-host the JavaScript SDK alongside your other JavaScript resources.

    If you are working in a development environment, you can load the SDK from unpkg or jsDelivr using the example code below. Replace the <EXAMPLE-VERSION> with your desired version. To learn more, read Releases in the JavaScript SDK GitHub repository. We recommend pinning to an exact SDK version if you are using a third-party hosting service.

    Here is an example of code to include in the <head> tag of your site:

    <!-- recommended for production environments -->
    <script src="path/to/ldclient.min.js"></script>

    Working with the client

    After you install the dependency, initialize the LaunchDarkly client. Then, use an event or promise to determine when the client is ready. Next, subscribe to updates. Finally, shut down the client when your application terminates.

    The following sections describe these steps in more detail:

    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 context will receive the fallback 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.

    Initialize the client

    To create a client instance, you need your LaunchDarkly environment's client-side ID.

    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.

    In practice, you should templatize your client-side ID so that you can use the same initialization code when you switch between development, QA, and production environments.

    Feature flag targeting and rollouts are determined by the end user viewing the page. You must pass a context to the SDK during initialization before requesting any feature flags with variation. If you fail to pass a valid context to the SDK during initialization, you will receive a 400 error.

    Here is an example showing how to initialize the client:

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

    Initializing the client makes a remote request to LaunchDarkly. Depending on your network conditions, it may take a couple hundred milliseconds before the SDK emits the ready event. If you require feature flag values before rendering the page, we recommend bootstrapping the client. If you bootstrap the client, it will emit the ready event immediately. To learn more, read Bootstrapping.

    To learn about best practices when using other default flag value methods, read Eliminating flicker when using default flag values.

    Use events to determine when the client is ready

    To find out when the client is ready, you can use one of two mechanisms: events or promises.

    The client object can emit JavaScript events. It emits a ready event when it receives initial flag values from LaunchDarkly. You can listen for this event to determine when the client is ready to evaluate flags.

    Here's how:

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

    Use promises to determine when the client is ready

    To find out when the client is ready, you can use one of two mechanisms: events or promises.

    The SDK has two methods that return a promise for initialization: waitUntilReady() and waitForInitialization(). The behavior of waitUntilReady() is equivalent to the ready event. The promise resolves when the client receives its initial flag data. As with all promises, you can either use .then() to provide a callback, or use await if you are writing asynchronous code.

    Here is an example:

    client.waitUntilReady().then(() => {
    // initialization succeeded,
    // flag values are now available through the client
    // or, with await:
    await client.waitUntilReady();
    // initialization succeeded,
    // flag values are now available through the client

    The other method that returns a promise, waitForInitialization(), is similar to waitUntilReady() except that it also tells you if initialization fails by rejecting the promise. This method never rejects the waitUntilReady() promise.

    Here is an example:

    client.waitForInitialization().then(() => {
    // initialization succeeded, flag values are now available
    }).catch(err => {
    // initialization failed
    // or, with await:
    try {
    await client.waitForInitialization();
    // initialization succeeded, flag values are now available
    } catch (err) {
    // initialization failed

    The SDK only decides initialization has failed if it receives an error response indicating that the environment ID is invalid. If it has trouble connecting to LaunchDarkly, it will keep retrying until it succeeds.

    Subscribe to updates

    The SDK does not subscribe to streaming real-time updates automatically when you initialize it.

    If you set the streaming configuration option to true, the client will always attempt to maintain a streaming connection. If you leave the streaming value undefined, which is the default, the client will open a streaming connection if you subscribe to change or change:flag-key events. In other words, subscribing to the SDK's change event by calling .on('change'). causes the SDK to open a streaming connection to LaunchDarkly. This is the only way to receive real-time updates. To learn more, read streaming.

    If you do enable streaming, you will also need EventSource. If you also enable the SDK's useReport configuration option, you will need LaunchDarkly's EventSource polyfill. To learn more, read EventSource.

    Shut down the client

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

    Supported features

    This SDK supports the following features: