Feature flags with OpenFeature and Node.js

What are feature flags?

Feature flags (a.k.a. feature toggles) are a technique that lets you turn features on or off at runtime without deploying new code. They separate code deployment from feature release.

Use cases

• A/B Testing: Targeting different user groups with different variants of a feature or UI to gather data and measure which version performs better. (Experiment Toggles)
• Enabling trunk-based development: Allows teams to ship in-progress features into production while keeping them off. (Release Toggles)
• Kill Switches: Disable a broken piece of code in production wrapped by a feature flag without deploying new code. (Ops Toggles)
• Access Control: Dynamically grant users access to a certain feature based on their tier or subscription plan. (Permission Toggles)

What is OpenFeature?

OpenFeature is an open specification that provides a vendor-agnostic, community-driven API for feature flagging. In other words, it allows you to plug into different feature flagging providers that implement this specification. You are not locked into a single vendor-specific API.

Here’s how to get started with OpenFeature.

Note: This example uses ConfigCat as the provider, but you can swap in any OpenFeature-compliant provider without changing your application code. To follow along with ConfigCat specifically, sign up for a free account, create a boolean flag named isSignUpEnabled set to true, and export your SDK key as CONFIGCAT_SDK_KEY.

Installation

npm install --save @openfeature/server-sdk @openfeature/config-cat-provider

Usage

import { OpenFeature } from "@openfeature/server-sdk";
import { ConfigCatProvider } from "@openfeature/config-cat-provider";
import { PollingMode } from "@configcat/sdk";

const provider = ConfigCatProvider.create(
  process.env.CONFIGCAT_SDK_KEY,
  PollingMode.AutoPoll,
);

await OpenFeature.setProviderAndWait(provider);

const client = OpenFeature.getClient();
const isSignUpEnabled = await client.getBooleanValue("isSignUpEnabled", false);
console.log("isSignUpEnabled", isSignUpEnabled); // logs: true, since we set the flag's value to true in the dashboard

A quick walkthrough of what’s happening:

• ConfigCatProvider.create connects OpenFeature to ConfigCat using your SDK key, and PollingMode.AutoPoll tells the provider to automatically refresh flag values in the background at a set interval, rather than fetching on every request.
• OpenFeature.setProviderAndWait registers the provider globally and waits until it’s fully initialized before continuing, so your first flag evaluation doesn’t run against stale or missing data.
• getBooleanValue("isSignUpEnabled", false) evaluates the isSignUpEnabled flag and returns its current boolean value. The second argument, false, is the default value returned if the flag can’t be evaluated (for example, if the provider fails to connect).

From here, you can toggle isSignUpEnabled from your ConfigCat dashboard at any time without redeploying your app. To go further, check out the OpenFeature docs for other providers, or explore boolean, string, number, and object flag types beyond the basic example above.