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Overview

The @flagsync/react-sdk integrates into React applications for client-side feature management and event tracking—ideal for single-user contexts like browser environments.
Requires React 16.3+ due to its dependency on the React Context API.

npm

Latest version available on npm

GitHub

Explore the source code on GitHub

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Installation

Install the SDK with your preferred package manager: 
npm install @flagsync/react-sdk
npm install -D @flagsync/cli

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Quickstart

A basic example of using the SDK in a React application: 
import { useFlag, FlagSyncProvider } from '@flagsync/react-sdk';

function MyApp() {
  return (
    <FlagSyncProvider config={{
      sdkKey: 'your-sdk-key',
      core: {
        key: 'user-123'
      }
    }}>
      <MyComponent />
    </FlagSyncProvider>
  );
}

function MyComponent() {
  const { value, isReady, isReadyFromStore } = useFlag('my-first-kill-switch', false);

  if (!isReady) {
    return <div>Loading...</div>;
  }

  return <div>Feature is {value ? 'enabled' : 'disabled'}</div>;
}

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Initialization

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Get Your SDK Key

Find your client-side SDK key in your workspace settings—safe for web/mobile apps (keep server-side keys private).

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Initialize the SDK

Wrap your application with FlagSyncProvider and provide the required configuration: 
import { FlagSyncProvider } from '@flagsync/react-sdk';

function App() {
  return (
    <FlagSyncProvider
      config={{
        sdkKey: 'your-sdk-key',
        core: {
          key: 'user-123'
        }
        // other configuration options...
      }}
    >
      {/* Your app components */}
    </FlagSyncProvider>
  );
}
Ensure the key in FsUserContext is unique and persistent for accurate MAU tracking and consistent flag evaluations. See User Context Best Practices for details.

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Wait for Readiness

Prevent flickering by ensuring the SDK is ready before your app renders. Use the waitForReady prop in FlagSyncProvider to delay rendering until the SDK is initialized, or check readiness manually with hooks or render prop components:
1

Using waitForReady

Add waitForReady to FlagSyncProvider to delay rendering until the SDK is ready, avoiding flicker during initialization.
<FlagSyncProvider waitForReady config={config}>
  <MyApp />
</FlagSyncProvider>
2

Using hooks or render prop components

Check isReady with hooks like useFlag, useFlags, or useFlagSyncClient to control rendering
const { isReady, isReadyFromStore } = useFlag('my-first-kill-switch', false);
Identical properties are provided in <FlagSyncFlag />, and <FlagSyncFlags />.

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SDK Not Ready

If the SDK isn’t ready, it returns the defaultValue or control: 
// value: false (defaultValue) isReady: false
const { value, isReady } = useFlag('flag-one', false); 

// value: "control", isReady: false
const { value, isReady } = useFlag('flag-two');

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SDK Ready

Once ready, hooks return the server-evaluated value: 
// value: Server-evaluated value, isReady: true
const { value, isReady } = useFlag('flag-one');
  • Client-side SDKs can bootstrap via LocalStorage or an initial flag set—values apply until the SDK is ready.
  • See Flag Evaluation: Overview.

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Usage

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Evaluate Flags

Evaluates a feature flag for user context. Applies targeting rules, rollouts, and defaults values. 
// Get a single flag value
const { value } = useFlag('my-first-kill-switch', defaultValue);

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FlagSync CLI

When using the CLI, flag values are automatically inferred from the generated types: 
useFlag('layout');              // Type: 'v1' | 'v2' | 'v3'
useFlag('killswitch');          // Type: boolean
useFlag('price-discount');      // Type: 0.1 | 0.2
useFlag('price-discount', 0.5); // ❌ TS Error: Invalid default value
useFlag('invalid-flag');        // ❌ TS Error: Invalid flag key
Without generated types, you must manually specify the type: 
useFlag<'v1' | 'v2'>('layout');            // Type: 'v1' | 'v2'
useFlag<number>('price-discount');         // Type: number
useFlag<boolean>('enable-feature', false); // Type: boolean (defaultValue must be true or false)
useFlag('no-type')                         // Type: unknown
An Impression is automatically logged either of the flag hooks are called.

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Track Events

Submit user actions with the useTrack hook, which supports two usage patterns:
1

With a pre-filled event key

Bind the hook to a specific event key for repeated tracking:
const track = useTrack('page-visit'); // Bound to eventKey

useEffect(() => track())
2

With a dynamic event key

Use a generic track function to specify the event key at call time:
const track = useTrack();

<Button onClick={() => {
  track('purchase-event', null, { productId: 'prod-123' })
}}>
  Buy Now
</Button>

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SDK Event Listeners

The SDK emits these events for SDK lifecycle management:
  • SDK_UPDATE: Emitted when flags are updated
  • SDK_READY: Emitted when the SDK is ready
  • SDK_READY_FROM_STORE: Emitted when flags are loaded from storage
  • ERROR: Emitted when an error occurs during initialization
Access these events using the useFlagSyncClient hook: 
import {useFlagSyncClient} from '@flagsync/react-sdk'

const client = useFlagSyncClient();

useEffect(() => {
  const onUpdate = () => {
    console.log('Flags updated:', client.lastUpdated);
  };

  client.on('SDK_UPDATE', onUpdate);
  return () => client.off('SDK_UPDATE', onUpdate);
}, [client]);
SDK_UPDATE does not fire if syncing is disabled.

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Configuration

Configure the SDK with the FsConfig interface: 
interface FsConfig {
  sdkKey: string;                     // Required: Your SDK key
  core: {
    key: string;                      // Required: User identifier
    attributes?: Record<string, any>; // Optional: Custom user attributes
  };
  bootstrap?: Record<string, any>;    // Optional: Initial flag values
  storage?: {
    type?: 'memory' | 'localstorage'; // Optional: Storage type
    prefix?: string;                  // Optional: Storage key prefix
  };
  sync?: {
    type?: 'stream' | 'poll' | 'off'; // Optional: Sync strategy
    pollRate?: number;                // Optional: Polling interval in seconds
  };
  tracking?: {
    impressions?: {
      maxQueueSize: number;           // Required: Max impressions queue size
      pushRate: number;               // Required: Impressions push rate
    };
    events?: {
      maxQueueSize: number;           // Required: Max events queue size
      pushRate: number;               // Required: Events push rate
    };
  };
  urls?: {
    sdk?: string;                     // Optional: SDK endpoint URL
  };
  logger?: Partial<ILogger>;          // Optional: Custom logger
  logLevel?: LogLevel;                // Optional: Logging level
  metadata?: Record<string, any>;     // Optional: Additional metadata
}

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Custom Attributes

Define user attributes for targeting in Flags: User Segments. 
<FlagSyncProvider
    config={{
      sdkKey: 'your-sdk-key',
      core: {
        key: 'user-123',
        attributes: {
          plan: 'premium',
          country: 'US',
          userType: 'enterprise'
        }
      }
    }}
>
  {/* Your app */}
</FlagSyncProvider>
Ensure the key in FsUserContext is unique and persistent for accurate MAU tracking and consistent flag evaluations. See User Context Best Practices for details.

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Flag Syncing

Configure flag update strategies with the sync object: stream, poll, or off.
By default, flag updates propagate in milliseconds via server-side events (SSE), ensuring the latest values are used in evaluations.
  • Components using flag hooks re-render automatically—no event listeners needed.
  • To disable this behavior, set sync to off, requiring a page refresh to fetch updated flags.
1

Stream (Default)

Stream updates via SSE—flag updates are reevaluated on the server and sent to the client:
const config: FsConfig = {
  sdkKey: 'your-sdk-key',
  core: { key: 'user-123' },
  sync: {
    type: 'stream'
  }
};
2

Polling

Poll the server on an interval:
const config: FsConfig = {
  sdkKey: 'your-sdk-key',
  core: { key: 'user-123' },
  sync: {
    type: 'poll',
    pollRate: 60
  }
};
3

Off

Disable syncing:
const config: FsConfig = {
  sdkKey: 'your-sdk-key',
  core: { key: 'user-123' },
  sync: {
    type: 'off'
  }
};

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Bootstrapping

Initialize the SDK with a set of bootstrap flags: 
<FlagSyncProvider
    config={{
      sdkKey: 'your-sdk-key',
      core: { key: 'user-123' },
      bootstrap: {
        'feature-1': true,
        'feature-2': false
      }
    }}
>
  {/* Your app */}
</FlagSyncProvider>
Bootstrapped values apply before the SDK is ready—see Flag Evaluation: Overview.

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Storage

Choose between memory and LocalStorage storage types:
1

Memory (Default)

const config: FsConfig = {
  sdkKey: 'your-sdk-key',
  core: { key: 'user-123' },
  storage: {
    type: 'memory'
  }
};
2

LocalStorage

const config: FsConfig = {
  sdkKey: 'your-sdk-key',
  core: { key: 'user-123' },
  storage: {
    type: 'localstorage'
    prefix: 'flagsync' // Optional: Custom prefix for storage keys
  }
};
Use the useFlagSyncClient to get the client.isReadyFromStore property.

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Best Practices

  • Use isReady checks to handle loading states. See Wait for Readiness.
  • Select a sync strategy (stream/poll/off) based on your application’s needs.
  • Use useTrack for consistent event tracking in components.
  • Add user attributes for targeted feature rollouts.
  • Consider bootstrapping for faster initial renders.

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Environment Variables

Set the following environment variable:
  • FLAGSYNC_SDK_KEY: Your client-side FlagSync SDK key (required)