Unleash Next.js SDK

This package allows easy integration of Unleash feature flags in a Next.js application.

Setup

Installation

To install, simply run:

npm install @unleash/nextjs
# or
yarn add @unleash/nextjs
# or
pnpm add @unleash/nextjs

There is an ./example project that you can deploy to Vercel or edit in CodeSandbox.

Environment variables

This package will attempt to load configuration from Next.js Environment variables.

When using Unleash client-side, with <FlagProvider /> 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 if you're using the front-end API, or a proxy client key if you're using a proxy
• Using 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

Detailed explanation

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, use UNLEASH_SERVER_INSTANCE_ID instead of UNLEASH_SERVER_API_TOKEN to authorize with GitLab's service.

Usage

A). App router

This package is ready for server-side use with App Router.

Refer to ./example/README.md#App-router for an implementation example.

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 (
    <p>
      Feature flag is{" "}
      <strong>
        <code>{isEnabled ? "ENABLED" : "DISABLED"}</code>
      </strong>
      .
    </p>
  );
}

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 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 in our documentation, including a guide on how to setup a client-side SDK key.

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

import type { AppProps } from "next/app";
import { FlagProvider } from "@unleash/nextjs/client";

export default function App({ Component, pageProps }: AppProps) {
  return (
    <FlagProvider>
      <Component {...pageProps} />
    </FlagProvider>
  );
}

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

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.

<FlagProvider
  config={{
    url: "http://localhost:4242/api/frontend", // replaces NEXT_PUBLIC_UNLEASH_FRONTEND_API_URL
    clientKey: "<Frontend_API_token>", // 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 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.

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<Data> = ({ isEnabled, variant }) => (
  <>
    Flag status: {isEnabled ? "ENABLED" : "DISABLED"}
    <br />
    Variant: {variant.name}
  </>
);

export const getStaticProps: GetStaticProps<Data> = 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).

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

E). Server Side Rendering (SSR)

import {
  flagsClient,
  evaluateFlags,
  getDefinitions,
  type IVariant,
} from "@unleash/nextjs";
import type { GetServerSideProps, NextPage } from "next";

type Data = {
  isEnabled: boolean;
};

const ExamplePage: NextPage<Data> = ({ isEnabled }) => (
  <>Flag status: {isEnabled ? "ENABLED" : "DISABLED"}</>
);

export const getServerSideProps: GetServerSideProps<Data> = 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.

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 (
    <FlagProvider
      config={{
        bootstrap: toggles,
        context,
      }}
    >
      <Component {...pageProps} />
    </FlagProvider>
  );
}

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 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 in front of your Unleash API. This helps protect your Unleash API from excessive traffic caused by per-request metrics reporting.

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

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

type Data = {
  isEnabled: boolean;
};

export const getServerSideProps: GetServerSideProps<Data> = 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<Data> = ({ isEnabled }) => (
        <>Flag status: {isEnabled ? "ENABLED" : "DISABLED"}</>
);

export default ExamplePage;

App router

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() to reliably send metrics without delaying the response.

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 instead, which is less taxing on metrics reporting. Check this blog post 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:

npx @unleash/nextjs

For the CLI to work, the following 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.

Usage

• get-definitions <outputFile.json> Download feature flags definitions for bootstrapping (offline use) of server-side SDK.
• generate-types [options] <outputFile.ts> 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 <sourceFile.json> load definitions from a file instead of fetching definitions - work offline
• -V Output the version number

Example

Try it now

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.