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JavaScript Client SDK

The Flagbase JavaScript Client-side SDK is a lightweight library that enables your web applications to use feature flags easily and efficiently.


  • Efficient and fast feature flag evaluations
  • Client-side API for accessing and manipulating flags, traits, and identifiers
  • Event-driven architecture with support for custom event listeners
  • Extensible, easy-to-use API for flag management


Using npm:

npm install @flagbase/js-client-sdk

Using yarn:

yarn add @flagbase/js-client-sdk


First, import the SDK:

import Client from "@flagbase/js-client-sdk";

Then, initialize the client:

const client = Client("your-client-sdk-key", {
identifier: "your-identifier",
traits: {
"your-trait-key": "your-trait-value",

Now, you can use the SDK to evaluate feature flags and manage user traits. Evaluating Feature Flags

const variation = client.variation("your-flag-key", "control");

if (variation === "treatment") {
// Execute the feature flag enabled code
} else {
// Execute the feature flag disabled code

Managing User Traits

Get user traits:

const allTraits = client.getAllTraits();
const specificTrait = client.getTrait("your-trait-key");

Set user traits:

client.setTrait("your-trait-key", "new-trait-value");

Listening to Events

client.on("CONTEXT_CHANGE", (eventMessage, eventContext) => {
console.log("Event:", eventMessage, eventContext);

API Reference Client

  • Client(clientKey: string, identity: Identity, opts?: ClientOptions): IClient


  • variation(flagKey: string, defaultVariationKey: string): Flag["variationKey"]
  • getIdentifier(): Identity["identifier"]
  • setIdentifier(identifier: string): void
  • getAllTraits(): Identity["traits"]
  • getTrait(traitKey: string): string | number
  • setTrait(traitKey: string, traitValue: string | number): void
  • getAllFlags(): Flagset
  • getInternalData(): InternalData
  • on(eventName: EventType, listenerFn: ListenerFn): void
  • off(eventName: EventType, listenerFn?: ListenerFn): void
  • clear(): void
  • destroy(): void