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unleash.unleash/docs/sdks/proxy-javascript.md

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---
id: proxy-javascript
title: JavaScript Proxy SDK
---
In this guide we explain how to use feature toggles in a Single Page App via [The Unleash Proxy](./unleash-proxy). You can also checkout the source code for the [JavaScript Proxy SDK](https://github.com/unleash-hosted/unleash-proxy-client-js).
### Introduction
For single-page apps we have a tiny proxy-client in JavaScript, without any external dependencies, except from browser APIs. This client will store toggles relevant for the current user in local-storage and synchronize with the Unleash Proxy in the background. This means we can bootstrap the toggles for a specific use the next time the user visits the web-page.
> We are looking in to also [supporting react-native](https://github.com/Unleash/unleash/issues/785) with this SDK. Reach out if you want to help us validate the implementation.
**Step 1: Install**
```
npm install unleash-proxy-client --save
```
**Step 2: Initialize the SDK**
You need to have a Unleash-hosted instance, and the proxy need to be enabled. In addition you will need a proxy-specific clientKey in order to connect to the Unleash-hosted Proxy.
```js
import { UnleashClient } from 'unleash-proxy-client';
const unleash = new UnleashClient({
url: 'https://eu.unleash-hosted.com/hosted/proxy',
clientKey: 'your-proxy-key',
appName: 'my-webapp'
});
// Used to set the context fields, shared with the Unleash Proxy
unleash.updateContext({userId: '1233'});
// Start the background polling
unleash.start();
```
**Step 3: Check if feature toggle is enabled**
```js
unleash.isEnabled('proxy.demo');
```
...or get toggle variant:
```js
const variant = unleash.getVariant('proxy.demo');
if(variant.name === '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.
```js
unleash.on('update', () => {
const myToggle = unleash.isEnabled('proxy.demo');
//do something useful
});
```