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https://github.com/unleash/unleash-client-nextjs

Unleash SDK for Next.js
https://github.com/unleash/unleash-client-nextjs

ab-testing abtesting continuous-delivery continuous-deployment experimentation feature-flags hacktoberfest javascript nextjs node node-js sdk server-rendering server-side-rendering typescript unleash vercel

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Unleash SDK for Next.js

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README 

        
# Unleash Next.js SDK



This package allows easy integration of [Unleash](https://github.com/unleash/unleash) feature flags in a [Next.js](https://nextjs.org/) application.



## Setup



### Installation



To install, simply run:



```sh

npm install @unleash/nextjs

# or

yarn add @unleash/nextjs

# or

pnpm add @unleash/nextjs

```



There is an [`./example`](./example) project that you can [deploy to Vercel](https://vercel.com/new/clone?repository-url=https%3A%2F%2Fgithub.com%2FUnleash%2Funleash-client-nextjs%2Ftree%2Fmain%2Fexample) or [edit in CodeSandbox](https://codesandbox.io/s/github/Unleash/unleash-client-nextjs/tree/main/example).



### Environment variables



This package will attempt to load configuration from

[Next.js Environment variables](https://nextjs.org/docs/basic-features/environment-variables).



When using Unleash **client-side**, with `` or `getFrontendFlags()` configure:



- `NEXT_PUBLIC_UNLEASH_FRONTEND_API_URL`. URL should end with `/api/frontend` or `/proxy`

- `NEXT_PUBLIC_UNLEASH_FRONTEND_API_TOKEN` [client-side API token](https://docs.getunleash.io/reference/api-tokens-and-client-keys#front-end-tokens)

  if you're using the [front-end API](https://docs.getunleash.io/reference/front-end-api),

  or a [proxy client key](https://docs.getunleash.io/reference/api-tokens-and-client-keys#proxy-client-keys)

  if you're using a [proxy](https://docs.getunleash.io/reference/unleash-proxy)

- Using [Edge](https://docs.getunleash.io/reference/unleash-edge) is the same as using the frontend API, so you'll need a FRONTEND_API_TOKEN, and your URL should end with `/api/frontend`.



If using **server-side** (SSR, SSG, API), using `getDefinitions()` and `evaluateFlags()`, set:



- `UNLEASH_SERVER_API_URL` of you instance. URL should end with `/api`

- `UNLEASH_SERVER_API_TOKEN` [server-side API client token](https://docs.getunleash.io/reference/api-tokens-and-client-keys#client-tokens)



#### Detailed explanation



| Prefixable     | Variable                     | Default                                                   |

| -------------- | ---------------------------- | --------------------------------------------------------- |

| `NEXT_PUBLIC_` | `UNLEASH_SERVER_API_URL`     | `http://localhost:4242/api`                               |

| `NEXT_PUBLIC_` | `UNLEASH_FRONTEND_API_URL`   | `<(NEXT_PUBLIC_)UNLEASH_SERVER_API_URL>/frontend`         |

| **No**         | `UNLEASH_SERVER_API_TOKEN`   | `default:development.unleash-insecure-api-token`          |

| **No**         | `UNLEASH_SERVER_INSTANCE_ID` | undefined                                                 |

| `NEXT_PUBLIC_` | `UNLEASH_FRONTEND_API_TOKEN` | `default:development.unleash-insecure-frontend-api-token` |

| `NEXT_PUBLIC_` | `UNLEASH_APP_NAME`           | `nextjs`                                                  |



If you plan to use configuration in the browser, add `NEXT_PUBLIC_` prefix.

If both are defined and available, private variable takes priority.

You can use both to have different values on client-side and server-side.



---



💡 **Usage with GitLab's feature flags**: To use this SDK with [GitLab Feature Flags](https://docs.gitlab.com/ee/operations/feature_flags.html), use `UNLEASH_SERVER_INSTANCE_ID` instead of `UNLEASH_SERVER_API_TOKEN` to [authorize with GitLab's service](https://docs.gitlab.com/ee/operations/feature_flags.html#get-access-credentials).



---



# Usage



## A). **App router**



This package is ready for server-side use with [App Router](https://nextjs.org/docs/app/building-your-application/routing).



Refer to [`./example/README.md#App-router`](https://github.com/Unleash/unleash-client-nextjs/tree/main/example#app-router) for an implementation example.



```tsx

import { cookies } from "next/headers";

import { evaluateFlags, flagsClient, getDefinitions } from "@unleash/nextjs";



const getFlag = async () => {

  const cookieStore = cookies();

  const sessionId =

    cookieStore.get("unleash-session-id")?.value ||

    `${Math.floor(Math.random() * 1_000_000_000)}`;



  const definitions = await getDefinitions({

    fetchOptions: {

      next: { revalidate: 15 }, // Cache layer like Unleash Proxy!

    },

  });



  const { toggles } = evaluateFlags(definitions, {

    sessionId,

  });

  const flags = flagsClient(toggles);



  return flags.isEnabled("nextjs-example");

};



export default async function Page() {

  const isEnabled = await getFlag();



  return (

    


      Feature flag is{" "}

      

        {isEnabled ? "ENABLED" : "DISABLED"}

      

      .

    


  );

}

```



## B). Middleware



It's possible to run this SDK in Next.js Edge Middleware. This is a great use case for A/B testing, where you can transparently redirect users to different pages based on a feature flag. Target pages can be statically generated, improving performance.



Refer to [`./example/README.md#Middleware`](https://github.com/Unleash/unleash-client-nextjs/tree/main/example#middleware) for an implementation example.



## C). Client-side only - simple use case and for development purposes (CSR)



Fastest way to get started is to connect frontend directly to Unleash.

You can find out more about direct [Front-end API access](https://docs.getunleash.io/reference/front-end-api) in our documentation,

including a guide on how to [setup a client-side SDK key](https://docs.getunleash.io/how-to/how-to-create-api-tokens).



Important: Hooks and provider are only available in `@unleash/nextjs/client`.



```tsx

import type { AppProps } from "next/app";

import { FlagProvider } from "@unleash/nextjs/client";



export default function App({ Component, pageProps }: AppProps) {

  return (

    

      

    

  );

}

```



With `` in place you can now use hooks like: `useFlag`, `useVariant`, or `useFlagsStatus` to block rendering until flags are ready.



```jsx

import { useFlag } from "@unleash/nextjs/client";



const YourComponent = () => {

  const isEnabled = useFlag("nextjs-example");



  return <>{isEnabled ? "ENABLED" : "DISABLED"}>;

};

```



Optionally, you can configure `FlagProvider` with the `config` prop. It will take priority over environment variables.



```jsx

", // replaces NEXT_PUBLIC_UNLEASH_FRONTEND_API_TOKEN

    appName: "nextjs", // replaces NEXT_PUBLIC_UNLEASH_APP_NAME



    refreshInterval: 15, // additional client configuration

    // see https://github.com/Unleash/unleash-proxy-client-js#available-options

  }}

>

```



If you only plan to use [Unleash client-side React SDK](https://github.com/Unleash/proxy-client-react) now also works with Next.js. Check documentation there for more examples.



## D). Static Site Generation, optimized performance (SSG)



With same access as in the client-side example above you can resolve Unleash feature flags when building static pages.



```tsx

import {

  flagsClient,

  getDefinitions,

  evaluateFlags,

  getFrontendFlags,

  type IVariant,

} from "@unleash/nextjs";

import type { GetStaticProps, NextPage } from "next";



type Data = {

  isEnabled: boolean;

  variant: IVariant;

};



const ExamplePage: NextPage = ({ isEnabled, variant }) => (

  <>

    Flag status: {isEnabled ? "ENABLED" : "DISABLED"}

    


    Variant: {variant.name}

  >

);



export const getStaticProps: GetStaticProps = async (_ctx) => {

  /* Using server-side SDK: */

  const definitions = await getDefinitions();

  const context = {}; // optional, see https://docs.getunleash.io/reference/unleash-context

  const { toggles } = evaluateFlags(definitions, context);



  /* Or with the proxy/front-end API */

  // const { toggles } = await getFrontendFlags({ context });



  const flags = flagsClient(toggles);



  return {

    props: {

      isEnabled: flags.isEnabled("nextjs-example"),

      variant: flags.getVariant("nextjs-example"),

    },

  };

};



export default ExamplePage;

```



The same approach will work for [ISR (Incremental Static Regeneration)](https://nextjs.org/docs/basic-features/data-fetching/incremental-static-regeneration).



Both `getDefinitions()` and `getFrontendFlags()` can take arguments overriding URL, token and other request parameters.



## E). Server Side Rendering (SSR)



```tsx

import {

  flagsClient,

  evaluateFlags,

  getDefinitions,

  type IVariant,

} from "@unleash/nextjs";

import type { GetServerSideProps, NextPage } from "next";



type Data = {

  isEnabled: boolean;

};



const ExamplePage: NextPage = ({ isEnabled }) => (

  <>Flag status: {isEnabled ? "ENABLED" : "DISABLED"}>

);



export const getServerSideProps: GetServerSideProps = async (ctx) => {

  const sessionId =

    ctx.req.cookies["unleash-session-id"] ||

    `${Math.floor(Math.random() * 1_000_000_000)}`;

  ctx.res.setHeader("set-cookie", `unleash-session-id=${sessionId}; path=/;`);



  const context = {

    sessionId, // needed for stickiness

    // userId: "123" // etc

  };



  const definitions = await getDefinitions(); // Uses UNLEASH_SERVER_API_URL

  const { toggles } = evaluateFlags(definitions, context);



  const flags = flagsClient(toggles); // instantiates a static (non-syncing) unleash-proxy-client



  return {

    props: {

      isEnabled: flags.isEnabled("nextjs-example")

    },

  };

};



export default ExamplePage;

```



## F). Bootstrapping / rehydration



You can bootstrap Unleash React SDK to have values loaded from the start.

Initial value can be customized server-side.



```tsx

import App, { AppContext, type AppProps } from "next/app";

import {

  FlagProvider,

  getFrontendFlags,

  type IMutableContext,

  type IToggle,

} from "@unleash/nextjs";



type Data = {

  toggles: IToggle[];

  context: IMutableContext;

};



export default function CustomApp({

  Component,

  pageProps,

  toggles,

  context,

}: AppProps & Data) {

  return (

    

      

    

  );

}



CustomApp.getInitialProps = async (ctx: AppContext) => {

  const context = {

    userId: "123",

  };



  const { toggles } = await getFrontendFlags(); // use Unleash Proxy



  return {

    ...(await App.getInitialProps(ctx)),

    bootstrap: toggles,

    context, // pass context along so client can refetch correct values

  };

};

```



# Server-side flags and metrics (experimental since 1.5.0-beta.3)



Next.js applications using Server-Side Rendering (SSR) are often deployed in serverless or short-lived environments, such as Vercel. This creates challenges for server-side metrics reporting.



Typically, Unleash backend SDKs (like the [Node.js SDK](https://github.com/Unleash/unleash-client-node) run in long-lived processes, allowing them to cache metrics locally and send them to the Unleash API or Edge API at scheduled intervals.



However, in some short-lived serverless environments where Next.js is commonly hosted (e.g., Vercel), there is no persistent in-memory cache across multiple requests. As a result, metrics must be reported on each request.



To address this, the SDK provides a `sendMetrics` function that can be called wherever needed, but it should be executed after feature flag checks `client.isEnabled()` or variant checks `client.getVariant()`.



We also recommend setting up the [Edge API](https://github.com/Unleash/unleash-edge)  in front of your Unleash API. This helps protect your Unleash API from excessive traffic caused by per-request metrics reporting.



```tsx

const enabled = flags.isEnabled("nextjs-example");



await flags.sendMetrics();

```



## No `setInterval` support (e.g. Vercel)



If your runtime does not allow `setInterval` calls then you can report metrics on each request as shown below. Consider using Unleash Edge in this scenario.



### Page router

```tsx

import { evaluateFlags, flagsClient, getDefinitions } from "@unleash/nextjs";

import {GetServerSideProps, NextPage} from "next";



type Data = {

  isEnabled: boolean;

};



export const getServerSideProps: GetServerSideProps = async () => {

  const definitions = await getDefinitions({

    fetchOptions: {

      next: { revalidate: 15 }, // Cache layer like Unleash Proxy!

    },

  });

  const context = {};

  const { toggles } = evaluateFlags(definitions, context);

  const flags = flagsClient(toggles);



  const enabled = flags.isEnabled("nextjs-example");



  // await client.sendMetrics().catch(() => {}); // blocking metrics

  flags.sendMetrics().catch(() => {}); // non-blocking metrics



  return {

    props: { isEnabled: enabled },

  };

};



const ExamplePage: NextPage = ({ isEnabled }) => (

        <>Flag status: {isEnabled ? "ENABLED" : "DISABLED"}>

);



export default ExamplePage;

```



### App router



```tsx

import {evaluateFlags, flagsClient, getDefinitions,} from "@unleash/nextjs";



export default async function Page() {

   const definitions = await getDefinitions({

     fetchOptions: {

       next: { revalidate: 15 }, // Cache layer like Unleash Proxy!

     },

   });

   const context = {};

   const { toggles } = evaluateFlags(definitions, context);

   const flags = flagsClient(toggles);



   const enabled = flags.isEnabled("nextjs-example");



   // await client.sendMetrics().catch(() => {}); // blocking metrics

   flags.sendMetrics().catch(() => {}); // non-blocking metrics



    return  <>Flag status: {enabled ? "ENABLED" : "DISABLED"}>

}

```



## Next.js middleware



In `middleware.ts` you can use [`event.waitUntil()`](https://nextjs.org/docs/app/building-your-application/routing/middleware#waituntil-and-nextfetchevent) to reliably send metrics without delaying the response.



```tsx

export async function middleware(req: NextRequest, event: NextFetchEvent) {

    // ...

    const evaluated = evaluateFlags(definitions, context);

    const flags = flagsClient(evaluated.toggles)

    const isEnabled = flags.isEnabled("example-flag")



    event.waitUntil(flags.sendMetrics())

    // ...

}

```



## `setInterval` support (e.g. long-running Node process or AWS lambda)



If your Next application resolves flags only in SSR mode and `setInterval` is supported then you may also consider using [Node.js SDK](https://github.com/Unleash/unleash-client-node) instead, which is less taxing on metrics reporting.

Check [this blog post](https://docs.getunleash.io/feature-flag-tutorials/serverless/lambda) for more information on short-running lambdas that still support `setInterval`.



# ⚗️ CLI (experimental)



You can use `unleash [action] [options]` in your `package.json` `scripts` section, or with:



```sh

npx @unleash/nextjs

```



For the CLI to work, the following [environment variables](#environment-variables) must be configured with appropriate values:



- `UNLEASH_SERVER_API_URL`

- `UNLEASH_SERVER_API_TOKEN`



The CLI will attempt to read environment values from any `.env` files if they're present. You can also set the variables directly when invoking the interface, as in the [CLI usage example](#example).



## Usage



- `get-definitions ` Download feature flags definitions for bootstrapping (offline use) of server-side SDK.

- `generate-types [options] ` Generate types and typed functions from feature flags defined in an Unleash instance.

  It will also generate strictly typed versions of `useFlag`, `useVariant`, `useFlags` and `flagsClient` (unless `--typesOnly` is used).

  - `-t, --typesOnly` don't include typed versions of functions exported from `@unleash/nextjs` (default: false)

  - `-b, --bootstrap ` load definitions from a file instead of fetching definitions - work offline

- `-V` Output the version number



## Example



Try it now



```sh

UNLEASH_SERVER_API_URL=https://app.unleash-hosted.com/demo/api \

UNLEASH_SERVER_API_TOKEN=test-server:default.8a090f30679be7254af997864d66b86e44dcfc5291916adff72a0fb5 \

npx @unleash/nextjs generate-types ./unleash.ts

```



# Known limitation



- When used server-side, this SDK does not support the "Hostname" and "IP" strategies. Use custom context fields and constraints instead.