client SDK

JavaScript feature flags

featureflow-client runs in the browser. It fetches already-evaluated values for one user and caches them in localStorage. Version 2.x targets modern browsers. React apps should use the React client. Node servers should use featureflow-node-sdk.

JavaScript docs · GitHub · featureflow-client

Set it up with your coding agent

Paste this into Claude Code, Cursor or any coding agent. It installs the JavaScript SDK, creates the client and evaluates your first flag.

Or set it up by hand

Install

npm install --save featureflow-client

<script crossorigin="anonymous" src="https://unpkg.com/featureflow-client@2/dist/featureflow.umd.min.js"></script>

Initialise

import Featureflow from 'featureflow-client';

const user = {
  id: 'user-123',
  attributes: { tier: 'gold', country: 'australia', roles: ['beta'] },
};

const featureflow = await Featureflow.init('sdk-js-env-YOUR_KEY', user);

Evaluate a flag

const checkout = featureflow.evaluate('new-checkout');
if (checkout.isOn()) {
  showNewCheckout();
} else {
  showLegacyCheckout();
}

Kill switch

evaluate('new-checkout').isOn() is the check. A flag that has not loaded reads as off, so make off the safe state. defaultFeatures only applies with offline: true, which is useful in tests. Create the client once. A second init on the same page sends duplicate impression events.

The operational side is the kill switch. Turning the flag off does not require a redeploy.

Gradual rollout

Percentage rollouts bucket on the user id. If you omit the user entirely, each page load gets a new random anonymous id. Pass a user object with an empty id to get a persistent anonymous id, stored in localStorage and in an ff-anonymous-id cookie unless useCookies is false. Use your account id for signed-in users via updateUser(). evaluate() does not take a user. There is one user per client.

See progressive rollout for the dashboard control. Release pipelines and approval workflows, if the change itself needs a review, are on the Business plan at $399 a month. That price is on the pricing page.

Targeting

Put attributes on the user you pass to init or updateUser. Rules in the dashboard match them. await featureflow.updateUser() with no user switches to a new anonymous user. From version 2.2.0, jsonValue() reads a variant's JSON config or returns undefined.

Frequently asked questions

Other SDKs

• All SDKs
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