> ## Documentation Index
> Fetch the complete documentation index at: https://docs2.growthbook.io/llms.txt
> Use this file to discover all available pages before exploring further.
# Feature Flags with Express.js and GrowthBook
> This guide shows how to integrate GrowthBook with Express.js to manage server-side feature flags.
This guide walks through how to add GrowthBook feature flags to an [Express.js](https://expressjs.com/) application. It assumes you are starting from scratch, so if you already have an Express.js application, you can skip to step 2.
## 1. Create an Express.js app
First, install Express.js
```bash npm theme={null}
npm install express
```
```bash Yarn theme={null}
yarn add express
```
```bash pnpm theme={null}
pnpm add express
```
```bash bun theme={null}
bun add express
```
Then, create an `index.js` file with a simple hello world route
```js theme={null}
const express = require('express')
const app = express()
app.get('/', (req, res) => {
res.send('Hello World!')
})
// Listen on port 3000
const port = 3000
app.listen(port, () => {
console.log(`Example app listening on port ${port}`)
})
```
Finally, run the app with:
```bash theme={null}
node index.js
```
Visit `http://localhost:3000` and you should see a "Hello World!" response!
## 2. Create a GrowthBook Account
GrowthBook can be [self-hosted](/self-host), but for simplicity, we will use GrowthBook Cloud in this guide, which is free for simple projects like this.
Go to [https://app.growthbook.io](https://app.growthbook.io) and create a new account if you don't have one yet.
Once you are logged in, create an **SDK Connection** and select Node.js as the language. This will generate a unique **Client Key** for you. Keep note of this key as we will use it in the next step.
## 3. Integrate the GrowthBook JavaScript SDK into the Express.js app
Create the file `.env` if it doesn't exist yet and add the generated key there:
```bash theme={null}
GROWTHBOOK_API_HOST=https://cdn.growthbook.io
GROWTHBOOK_CLIENT_KEY=
```
Now let's install the GrowthBook JavaScript SDK
```bash npm theme={null}
npm install @growthbook/growthbook
```
```bash Yarn theme={null}
yarn add @growthbook/growthbook
```
```bash pnpm theme={null}
pnpm add @growthbook/growthbook
```
```bash bun theme={null}
bun add @growthbook/growthbook
```
Lets now modify the top of `index.js` file and create a **GrowthBookClient** instance:
```js theme={null}
const express = require('express')
const app = express()
const { GrowthBookClient } = require("@growthbook/growthbook")
// GrowthBookClient instance
const client = new GrowthBookClient({
apiHost: process.env.GROWTHBOOK_API_HOST,
clientKey: process.env.GROWTHBOOK_CLIENT_KEY
});
// Initialize it
client.init().then((status) => {
console.log("GrowthBook initialized", status);
});
```
Lets also add a middleware to `index.js` that creates a user-scoped instance for every request. Make sure to place this above the route handlers:
```js theme={null}
app.use((req, res, next) => {
// Attributes about the current user/request
const userContext = {
attributes: {
// In a real app this would come from a cookie or session
// We would also add more attributes like country, etc
id: "123",
}
}
// Make this available to all subsequent route handlers
req.growthbook = client.createScopedInstance(userContext);
next();
});
```
Lets re-start the Node process and make sure you see the "GrowthBook initialized" message in the console and the status is successful. Since we are using a `.env` file, we need to modify the command to tell Node.js to load it.
```bash theme={null}
node --env-file=.env index.js
```
## 4. Create a Feature in GrowthBook
Back in the GrowthBook application, we can create a new feature. For this tutorial, we'll make a simple on/off feature flag that determines whether or not we show the message in Spanish.
The key we chose (spanish-greeting) is what we will reference when using the GrowthBook SDK.
We can now edit the route in `index.js` to use this flag:
```js theme={null}
app.get('/', (req, res) => {
let message = "Hello World!";
if (req.growthbook.isOn("spanish-greeting")) {
message = "Hola Mundo!";
}
res.send(message);
})
```
Now, if you restart the Node process and refresh the page in the browser, you will still see the original "Hello World!" because when we created the feature, we set it to be Off by default.
## 5. Target Specific Users
Now we can add rules to the feature to turn it on for specific users.
In the `userContext` we added, we hard-coded an id of "123". We can now use this id to create a rule in GrowthBook to turn on the feature for this user.
On the feature page, click the "Add Rule" button and select "Force Value" as the rule type. Then add targeting by attribute and save the rule. It should look something like this:
Rules start out in a draft state in GrowthBook. You need to publish the draft to make it live.
After publishing, restart the Node process once more and refresh the page. You should now see the message in Spanish. ¡Qué bueno!
Try changing the id in `userContext` to something else like `456`, restart Node, and the response will now switch back to English.
## Conclusion and Next Steps
In this tutorial, you learned how to use a simple feature flag in an Express app and target individual users. But this barely scratches the surface of what you can do with GrowthBook.
Here are a few next steps you can take:
* Use more [advanced targeting](/features/targeting)
* Run [A/B tests](/feature-flag-experiments)
View the [full Node.js docs](/lib/node) for more information on all of the options available in the GrowthBook SDK, including streaming updates, persistent caching, and more.