Feature flags in JavaScript

Feature flags are a great way to control the rollout of features in applications, letting you switch features on or off without redeploying code. This guide covers the use of feature flags, detailing their advantages, how to implement them, best practices for effective management, and additional considerations for maintaining a robust feature flagging system in JavaScript applications.

Understanding feature flags

Feature flags, also known as feature toggles, serve as switchable conditions in code, deciding whether certain features are active. They allow for a more controlled and strategic approach to feature release, supporting practices like canary releases and A/B testing.

Feature flags come in various forms:

• Release toggles: Control the release of a feature to users.
• Experiment toggles: Allow for A/B testing and controlled experiments.
• Ops toggles: Manage operational aspects like infrastructure feature controls.
• Permission toggles: Toggle features based on user permissions.

A feature flag’s lifecycle typically progresses from development to testing, to release, and finally, removal once the feature is stable and permanently active.

Benefits of using feature flags

Feature flags enable teams to test new features in production environments with actual users, thereby reducing the risk and detecting issues early. They facilitate trunk-based development, where all developers commit to a single branch, and features are toggled off until ready. Additionally, feature flags allow for strategic technical debt management, where short-term compromises can be toggled off and revisited at a later time.

How to implement feature flags

Implementation strategies for feature flags can range from simple to sophisticated, including gradual rollouts where a feature is enabled for a percentage of users, which is increased over time. For microservices architectures, feature flags can be distributed via a centralized service or managed locally, depending on the level of control and granularity required.

// A simple flag with a gradual rollout capability
const userPercentage = getUserPercentage(); // hypothetical function to get user percentage
const featureFlags = {
  newUI: {
    isEnabled: process.env.NEW_UI === 'true',
    rolloutPercentage: 50 // Feature is enabled for 50% of users
  },
  // ...
};

if (featureFlags.newUI.isEnabled && userPercentage < featureFlags.newUI.rolloutPercentage) {
  // Code for the new UI
}

Client-side vs server-side flags

While client-side flags provide immediate control and are ideal for UI elements, they pose security risks if not handled correctly. Sensitive feature flags should be managed server-side and synchronized with the client as needed.

Feature flag services and libraries

The choice between different services like LaunchDarkly, Unleash, Split.io, and Flagsmith often comes down to factors like ease of integration, feature set, and the specific needs of a project, such as support for A/B testing or advanced user segmentation. You can also use Basedash to implement your own feature flag system using a SQL database.

Best practices for using feature flags

Maintain a consistent naming convention for flags to avoid confusion, especially as the number of flags grows. Handle flag dependencies carefully to ensure that enabling or disabling one flag does not unexpectedly affect another feature.

Testing with feature flags

Developers should design tests to account for both active and inactive states of feature flags, maintaining separate test cases for each state. Consider the impact of feature flags on the principles of test-driven development (TDD), and ensure that TDD practices are adapted to accommodate feature flagging.

Feature flag patterns

Demonstrate successful feature flag patterns, like canary releases, with real-world examples, while also discussing anti-patterns to avoid, such as using flags for too many variations of a feature, which can lead to complexity and technical debt.

Monitoring and analytics

Set up robust monitoring and logging for feature flag toggles to quickly identify and react to issues. Analytics can be used to determine the usage and impact of features behind flags, informing future product decisions.

// Example logging when a feature flag state changes
const logger = createLogger(); // Hypothetical logger setup

function setFeatureFlag(flagName, isEnabled) {
  const oldValue = featureFlags[flagName];
  featureFlags[flagName] = isEnabled;
  logger.info(`Feature flag "${flagName}" changed from ${oldValue} to ${isEnabled}`);
}

Security considerations

Security for feature flags encompasses everything from access controls to prevent unauthorized use, to encrypting flag data as it travels across networks or when stored.

Maintaining feature flags

Technical debt from feature flags should be managed actively. This includes policies for deprecating flags, safely removing them, and regularly auditing the flag inventory to keep it lean.