@featureflip/browser

@featureflip/browser-sdk

Framework-agnostic browser SDK for evaluating Featureflip feature flags.

Installation

npm install @featureflip/browser-sdk

Quick Start

import { FeatureflipClient } from '@featureflip/browser-sdk';

const client = new FeatureflipClient({
  clientKey: 'your-client-sdk-key',
});

await client.initialize();

const showBanner = client.boolVariation('show-banner', false);

API Reference

Constructor

new FeatureflipClient(config: FeatureflipClientConfig)

Configuration Options

Option	Type	Default	Description
clientKey	string	(required)	Client SDK key from your project settings
baseUrl	string	https://eval.featureflip.io	Evaluation API base URL
context	Record<string, unknown>	{}	Initial evaluation context (user attributes)
streaming	boolean	true	Enable SSE streaming for real-time updates
initTimeout	number	10000	Timeout in ms for the initial evaluate request

Methods

initialize(): Promise<void>

Fetches all flag values from the server. Must be called before reading variations. Opens an SSE streaming connection if streaming is enabled.

boolVariation(key: string, defaultValue: boolean): boolean

Returns a boolean flag value, or defaultValue if the flag is missing or not a boolean.

stringVariation(key: string, defaultValue: string): string

Returns a string flag value, or defaultValue if the flag is missing or not a string.

numberVariation(key: string, defaultValue: number): number

Returns a number flag value, or defaultValue if the flag is missing or not a number.

jsonVariation<T>(key: string, defaultValue: T): T

Returns a flag value cast to T, or defaultValue if the flag is missing.

identify(context: Record<string, unknown>): Promise<void>

Re-evaluates all flags with a new context (e.g., after login). Emits change events for any flags whose values changed.

await client.identify({ user_id: '123', plan: 'pro' });

on(event: EventType, handler: EventHandler): void

Subscribe to events.

• 'ready' -- fired after initialize() completes
• 'change' -- fired when flag values change (receives a FlagChanges object)
• 'error' -- fired on streaming or network errors

client.on('change', (changes) => {
  console.log('Flags changed:', changes);
});

off(event: EventType, handler: EventHandler): void

Unsubscribe from events.

close(): void

Closes the SSE streaming connection and cleans up resources.

Testing

Use FeatureflipClient.forTesting() to create a client with predetermined flag values -- no network calls.

const client = FeatureflipClient.forTesting({
  'show-banner': true,
  'button-color': 'blue',
});

client.boolVariation('show-banner', false); // true
client.stringVariation('button-color', 'red'); // 'blue'