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JavaScript Proxy SDK

This guide shows you how to use feature toggles in a single-page app with the Unleash Proxy and the Unleash front-end API. You can also check out the source code for the JavaScript Proxy SDK on GitHub for more details around the SDK.


The JavaScript proxy client is a tiny Unleash client written in JavaScript without any external dependencies (except from browser APIs). This client stores toggles relevant for the current user in localStorage and synchronizes with Unleash (the proxy or the Unleash front-end API) in the background. Because toggles are stored in the user's browser, the client can use them to bootstrap itself the next time the user visits the same web page.

We are looking in to also supporting react-native with this SDK. Reach out if you want to help us validate the implementation.

How to use the JavaScript Proxy SDK

Step 1: Install

npm install unleash-proxy-client

Step 2: Initialize the SDK

import { UnleashClient } from 'unleash-proxy-client';

const unleash = new UnleashClient({
url: '',
clientKey: 'your-client-key',
appName: 'my-webapp',

// Use `updateContext` to set Unleash context fields.
unleash.updateContext({ userId: '1233' });

// Start the background polling

Option A: Connecting to the Unleash proxy


To connect to an Unleash proxy, you need to have an instance of the proxy running.

Add the proxy's URL and a proxy client key. The configuration section of the Unleash proxy docs contain more info on how to configure client keys for your proxy.

Option B: Connecting directly to Unleash

Use the url to your Unleash instance's front-end API (<unleash-url>/api/frontend) as the url parameter. For the clientKey parameter, use a FRONTEND token generated from your Unleash instance. Refer to the how to create API tokens guide for the necessary steps.

You might also need to set up cross-origin resource sharing (CORS) for your instance. Visit the CORS section of the front-end API guide for more information on setting up CORS.

Step 3: Check if feature toggle is enabled


...or get toggle variant:

const variant = unleash.getVariant('proxy.demo');
if ( === 'blue') {
// something with variant blue...

Listen for updates via the EventEmitter

The client is also an event emitter. This means that your code can subscribe to updates from the client. This is a neat way to update a single page app when toggle state updates.

unleash.on('update', () => {
const myToggle = unleash.isEnabled('proxy.demo');
//do something useful