Feature Flags in Bun (The New JavaScript Runtime)

Bun is making waves in the JavaScript ecosystem. This speedy new toolkit promises faster startup times, native TypeScript support, and improved performance compared to Node.js. But as Bun gains adoption, developers require modern tooling to match its capabilities. Feature flags are one such tool, and are perfect for Bun applications.

Coming up, we'll learn what Bun is and how it compares to Node.js, then dive into using ConfigCat feature flags in Bun projects. Finally, we'll discuss how to safely roll out new features, A/B test them, and quickly fix problems in production, all with the help of feature flags.

What is Bun? (Bun vs. Node.js)

Bun was created as a JavaScript runtime replacement for Node.js. With added features like bundling, testing, and package management, it later became a toolkit for developing both JavaScript and TypeScript applications with better performance.

Here is a comparison between them:

Aspect	Bun	Node.js
Startup Time	About 10 times faster	Slower to start up
TypeScript Support	Built in, no setup needed	Requires extra tools (ts-node, tsc, etc.)
Package Manager	Comes with one	Comes with npm (bundled with Node.js); can also use yarn, pnpm, etc.
How Fast It Runs	Very optimized	Mature and stable
Available Libraries	Growing	Huge selection (npm ecosystem)
Works Like Node.js	Mostly compatible	Industry standard

Why Use Feature Flags in Bun Applications?

Bun's speed and near-instant startup time make it an excellent choice for applications where behavior may need to change frequently. Feature flags complement this perfectly by allowing you to toggle features, run experiments, or adjust application behavior without a redeployment or app restart.

For command-line tools, Bun's fast startup means users get immediate responses. For serverless functions, faster cold starts translate to lower costs and better performance. However, raw speed alone isn't enough; software teams need to roll out changes safely, test new ideas with small user groups, and roll back quickly when something goes wrong.

This is exactly what feature flags provide. When combined with ConfigCat's feature management and remote configuration capabilities, Bun applications gain:

• Real-time control over features and configuration
• Safe, gradual rollouts instead of risky big releases
• Instant kill switches for problematic code
• Dynamic A/B testing without code changes
• Flexible behavior for CLI tools, servers, and serverless environments

In a nutshell, Bun gives you speed. Feature flags give you control. Together, they enable fast, reliable, and modern software delivery.

How to Use Feature Flags in Bun with ConfigCat

To help you understand how to integrate ConfigCat feature flags in a Bun application or environment, here's a step-by-step guide:

Step 1: Create a Bun Project and Install the Bun SDK

First, open a terminal and install bun:

npm install -g bun

With Bun installed, create a new directory and initialize a Bun project using the following commands. When prompted, select Blank as the project template to use.

mkdir bun-feature-flags
cd bun-feature-flags
bun init

Then, proceed to install the ConfigCat Bun SDK:

bun add @configcat/sdk

Bun's package manager is really fast. The whole process of installing and setting it up takes a few seconds.

Step 2: Initialize the ConfigCat Client

Create a reusable ConfigCat service so you can import and use it to evaluate feature flags throughout your application. In your Bun project, create a new file named configcat.service.ts, then paste the code below into the file.

import * as configcat from "@configcat/sdk/bun";
import {
  PollingMode,
  type IConfigCatClient,
  type SettingValue,
  type User,
} from "@configcat/sdk/bun";

export class ConfigCatService {
  private readonly client?: IConfigCatClient;

  private readonly SDK_KEY =
    "YOUR-CONFIGCAT-SDK-KEY"; //Add your ConfigCat SDK key here.
  private defaultUser?: User;
  private readonly POLL_INTERVAL_SECONDS = 30;

  constructor() {
    try {
      this.client = configcat.getClient(this.SDK_KEY, PollingMode.AutoPoll, {
        pollIntervalSeconds: this.POLL_INTERVAL_SECONDS,
      });
    } catch (error) {
      console.error("ConfigCat initialization failed:", error);
    }
  }

  // Get the value of a setting value
  getValue<T extends SettingValue>(key: string, defaultValue: T, user?: User) {
    return this.client?.getValueAsync(
      key,
      defaultValue,
      user ?? this.defaultUser,
    );
  }

  getAllValues(user?: User) {
    return this.client?.getAllValuesAsync(
      user ?? this.defaultUser,
    );
  }

  dispose(): void {
    this.client?.dispose();
  }

  [Symbol.dispose](): void {
    console.log("Disposing ConfigCat Service");
    this.dispose();
  }
}

info

Replace #YOUR-CONFIGCAT-SDK-KEY# with your actual SDK key from ConfigCat.

By using AutoPoll, the SDK periodically fetches your latest feature flag data every 60 seconds, so your flags remain in sync without requiring manual updates.

Step 3: Evaluating Feature Flags

To evaluate feature flags, import the ConfigCat service into your Bun entry file (index.ts) and use it as shown below.

import { ConfigCatService } from "./configcat.service";

async function createApp() {
    using configCatService = new ConfigCatService();

    const isMyFeatureEnabled = await configCatService.getValue("myFeatureFlag", false);

    console.log("The feature flag value is: ", isMyFeatureEnabled);

    return;
}

createApp();

With this setup, you can evaluate feature flags anywhere in your application.

Real-World Use Cases for Bun Feature Flags

To build on what we previously learned, let's look at some practical use cases for using feature flags in a Bun environment or codebase.

Progressive Feature Rollout

Imagine you're building a new API endpoint and want to test it with 10% of users first, then gradually increase to 100%.

Because ConfigCat's percentage targeting evaluates users deterministically, you can pass a User Object containing a unique identifier as the third parameter so users consistently fall into the rollout group.

import { ConfigCatService } from "./configcat.service";

async function handleRequest(userId: string, request: Request) {
    using configCatService = new ConfigCatService();

    const newApiEnabled = await configCatService
    .getValue(
        "new-api-endpoint",
        false,
        { identifier: userId } // User Object
    );

    if (newApiEnabled) {
        return handleWithNewAPI(request);
    } else {
        return handleWithLegacyAPI(request);
    }
}

In ConfigCat's dashboard, you can set the new-api-endpoint flag to target a percentage of users. As you gain confidence, you can increase the percentage without touching the actual code.

A/B Testing in Bun CLI Tools

Bun also excels at CLI tools. Here's a way to run an A/B test experiment to evaluate two different user experiences:

import { parseArgs } from "util";
import { ConfigCatService } from "./configcat.service";

async function main() {
    using configCatService = new ConfigCatService();

    const { values } = parseArgs({
        options: {
            user: {
                type: "string",
                short: "u",
            },
        },
    });

    const userId = values.user || "anonymous";

    const experimentVariant = await configCatService.getValue(
        "ui-experiment-variant",
        "control",
        { identifier: userId }
    );

    if (experimentVariant === "variant-b") {
        console.log("User sees experience B");
        // Run variant B logic
    } else {
        console.log("User sees control experience");
        // Run control (variant A) logic
    }
}
main().catch(console.error);

Serverless Functions with Kill Switches

The start-up performance of Bun makes it ideal for serverless environments. Feature flags provide instant kill switches:

import { ConfigCatService } from "./configcat.service";

export async function handler(event: any) {
    using configCatService = new ConfigCatService();

    // Check if the feature is experiencing issues
    const maintenanceMode = await configCatService.getValue(
        "maintenance-mode",
        false
    );

    if (maintenanceMode) {
        return {
            statusCode: 503,
            body: JSON.stringify({
                error: "Service temporarily unavailable",
            }),
        };
    }

    // Proceed with normal logic
    return {
        statusCode: 200,
        body: JSON.stringify({ message: "Success" }),
    };
}

If your service degrades, you can toggle the flag off in ConfigCat without a code redeployment.

Advanced ConfigCat Features in Bun

There are several advanced ConfigCat features for Bun that we can look into. Some of them are:

Targeting Rules

ConfigCat allows you to define targeting rules for your feature flags, ensuring features are delivered to the right users based on attributes such as identifier, country, email, and more. Here's a simple example of how to implement it:

// Sample user object
const userObject = {
    identifier: "[email protected]",
    email: "[email protected]",
    country: "US",
    custom: {
        subscription: "premium",
        joinDate: "2023-01-15",
    },
};

const premiumFeature = await configCatService.getValue(
    "premium-analytics",
    false,
    userObject
);

In ConfigCat's dashboard, create a targeting rule: "If subscription equals premium, then Serve value: true". Now the flag will ensure that only users with a premium subscription see the feature.

Getting All Flag Values

For cases of dashboards or reporting, you can fetch all flags at once using the code below.

const allValues = await configCatService.getAllValues({
    identifier: "user123",
});

allValues.forEach((item) => {
    console.log(`${item.settingKey}: ${item.settingValue}`);
});

Snapshots for Synchronous Evaluation

Bun supports synchronous operations for flags already cached. The code below shows the implementation for that.

const snapshot = client.snapshot();
const value = snapshot.getValue("my-flag", false, userObject);
console.log(value);

This is perfect for synchronous code paths where async/await isn't practical.

Best Practices for Feature Flags in Bun

To use feature flags in the best and most effective way, here are some best practices to look into.

Use Semantic Flag Names

Name flags clearly to indicate their purpose:

• new-checkout-flow (better than words like feature-x, new-feature)
• premium-analytics-dashboard (better than words like flag-123, test-flag)

Implement Proper Logging

Feature flags can be difficult to debug without visibility into how values are evaluated. ConfigCat includes a built-in logger that helps you inspect flag evaluations and targeting decisions.

You can enable SDK logging when creating the client:

import * as configcat from "@configcat/sdk/bun";

const configCatClient = configcat.getClient(
    process.env.CONFIGCAT_SDK_KEY,
    configcat.PollingMode.AutoPoll,
    {
        logger: configcat.createConsoleLogger(configcat.LogLevel.Info),
    }
);

With LogLevel.Info, the SDK logs:

• Feature flag evaluations
• Targeting rule matches
• Returned values

This visibility makes it much easier to diagnose rollout issues and verify targeting behavior during development and testing.

When to Add Application-Level Logging

While ConfigCat's logger is useful for diagnostics, you may still want to log business events for analytics, monitoring, or auditing.

const enabled = await configCatClient.getValueAsync("my-flag", false, {
    identifier: userId,
});

if (enabled) {
    auditLogger.info("Feature enabled", {
        feature: "my-flag",
        userId,
    });
}

This type of logging is helpful for:

• Tracking feature adoption
• Auditing behavior
• Product analytics
• Monitoring gradual rollouts

Cache Results When Possible

For performance-critical code, cache the flag value during the request.

async function handleRequest(userId: string) {
    const flags = {
        newUI: await configCatService.getValue("new-ui", false, { identifier: userId }),
        analytics: await configCatService.getValue("advanced-analytics", false, {
            identifier: userId,
        }),
    };

  // Use flags.newUI and flags.analytics throughout the request
  // No additional flag evaluations needed
}

Clean Up Old Flags

As features become permanent or experiments conclude, remove the corresponding flags to keep your codebase and dashboard clean.

Unused flags, often called zombie flags, increase technical debt, complicate maintenance, and may trigger legacy logic unexpectedly.

ConfigCat helps you manage stale flags by:

• Identifying inactive flags in the Zombie Flags dashboard
• Sending automated zombie flag email reports
• Providing an API for cleanup automation

If you enable Code References, ConfigCat can show where a flag is used across your repositories, making it safer to remove unused flags.

You can go a step further by scanning your codebase with the ConfigCat CLI:

configcat scan . --print

This detects flag usage, highlights references to deleted flags, and helps prevent breaking changes during cleanup.

Regularly removing stale flags keeps your feature flag system maintainable and prevents long-term technical debt.

Document Flag Purpose

In ConfigCat, you can add descriptions like:

• When the flag was created
• What feature or experiment does it controls
• The rollout strategy (gradual rollout, A/B test, internal only)
• The expected removal date
• The team or owner responsible

Use tags to organize and filter flags at scale. For example:

• experiment
• release
• ops
• beta
• cleanup-required

Tags make it easier to track flags by purpose, environment, or lifecycle stage.

You can also include descriptive hints in the flag description to guide future developers, such as:

"Temporary rollout flag for the new checkout flow. Remove after full rollout."

Clear documentation reduces onboarding friction, prevents misuse, and makes flag cleanup safer and faster.

Conclusion

Feature flags are no longer just a nice-to-have; they're essential for modern software delivery. Combined with Bun's performance and simplicity, they create a powerful foundation for building reliable, scalable applications.

Whether you're building CLI tools, serverless functions, or backend services, integrating ConfigCat feature flags in Bun takes minutes and pays dividends immediately.

Start with a simple kill switch. Progress to A/B testing. Eventually, you'll wonder how you ever shipped software without feature flags.

Ready to try it? Grab your Bun SDK key from ConfigCat and deploy your first feature flag today. Your users and your deployment pipeline will thank you.

You can also check out ConfigCat on Facebook, X, LinkedIn, and GitHub.