How are feature flags used in Javascript applications?
Feature flags, also known as feature toggles, are conditional statements in code that enable or switch off specific functionalities without changing the codebase or redeploying the application. This mechanism allows developers to control the visibility, availability, or activation of features dynamically, either by configuration files or through dedicated management tools.
Implementing feature flags in JavaScript requires adherence to best practices to ensure maintainability, performance, and effective management:
- Naming conventions: Establish clear, consistent naming conventions for feature flags (e.g., feature.new-dashboard, experiment.red-button). This improves readability and makes flags easy to find and understand.
- Clean up obsolete flags: Regularly remove temporary feature flags (e.g., release flags, experiment flags) once they are no longer needed. This prevents “flag debt” and reduces code complexity. Automate this cleanup where possible.
- Performance considerations: Minimize the performance impact of feature flags. Evaluate flags efficiently, potentially using server-side local evaluation or bootstrapping flags on the client for immediate availability.
- Avoid dependencies: Design feature flags to be independent where possible. Avoid creating complex dependencies between flags, as this can make management and debugging more challenging.
- Targeted rollouts: Use feature flags to enable progressive rollouts, allowing new features to be released to a subset of users before a full release. This helps in gathering feedback and mitigating risks.
- Error handling and rollback: Implement robust error handling for feature flag evaluations. Plan for rollback strategies in case a feature flag needs to be switched off or a feature causes issues in production.
- Security and access control: Control access to feature flag management based on roles and policies. Ensure only authorized personnel can modify flag states. Avoid using feature flags for managing sensitive information like secrets or credentials.
- Logging and monitoring: Set up logging to track changes to feature flags and their evaluations. Monitor the impact of flags on application performance and user experience.
- Documentation: Document the purpose, usage, and lifecycle of each feature flag, especially for long-lived or operational flags. This aids in understanding and maintenance.
- Centralized management: Use a dedicated feature flag management platform or a centralized system to control flags. This provides a single source of truth and simplifies management, especially in larger applications.
Benefits of feature flags in JavaScript
Feature flags offer several advantages when integrated into JavaScript applications, particularly for web development teams seeking better control over frontend behavior and release strategies. Here are the key benefits:
- Safe rollouts: Developers can release features to a small user segment before a full rollout, helping identify issues without affecting all users.
- Instant rollbacks: If a feature causes problems, it can be switched off immediately without needing a redeploy, reducing downtime and response time.
- A/B testing and experimentation: Feature flags allow for easy implementation of A/B tests by serving different code paths to different users.
- Reduced merge conflicts: Multiple features can be developed in parallel and conditionally activated, which minimizes long-lived feature branches and merge conflicts.
- User-specific features: Flags can be used to activate features for specified user roles, locations, or accounts, allowing personalized experiences and phased access.
- Improved CI/CD workflows: Features can be merged and deployed continuously, but not activated until they’re ready.
- Enhanced collaboration: Teams outside engineering, like product or marketing, can manage feature activation via flag management tools.
10 best practices for implementing feature flags in JavaScript
1. Naming conventions
In JavaScript applications, naming clarity is essential since flags often interact directly with UI logic, component states, and route conditions. A flag like isBetaDashboardEnabled instantly tells developers that it controls access to a beta version of the dashboard. JavaScript codebases commonly use camelCase, so keeping flag names consistent with project style helps reduce friction during implementation.
Avoid overly generic names or abbreviations, as these can become ambiguous when read in conditionals inside JSX, templating logic, or event handlers. For front-end teams working in frameworks like React, Angular, or Vue, flag names often become part of component props or context values.
Using predictable, descriptive names helps prevent bugs and improves code readability. Additionally, when flags are passed between components or stored in state, clarity in naming avoids confusion during refactoring or debugging.
2. Clean up obsolete flags
In JavaScript apps, lingering feature flags can result in bloated components and unnecessary complexity in rendering logic. Flags may live in conditions that control entire sections of JSX, style toggles, or route visibility. When a flag outlives its purpose, it often leaves behind dead branches that make components harder to maintain and test.
To avoid this, treat every feature flag as a temporary construct and assign ownership and expiration plans from the start. Include comments with flags indicating their expected lifespan, usage context, and removal criteria. In frontend code, it’s also important to review tests that use these flags—obsolete flags may create false positives in unit or integration tests. Use linters or custom tooling to identify flags that haven’t changed state in production for a long time.
3. Performance considerations
Evaluating feature flags in a JavaScript application can introduce delays, especially if the flag values are fetched asynchronously at runtime. This is a common issue in SPAs where rendering depends on user-specific flags retrieved from an API or a flag service.
To minimize performance hits, fetch and cache flags early—ideally during application initialization—and store them in a global state, such as Redux, context API, or a reactive store. Avoid evaluating flags repeatedly inside component lifecycle hooks or render functions. Instead, evaluate them once and pass the result as props.
Also, be careful with flag evaluation in animations, event listeners, or scroll handlers, where frequent checks can impact performance. Some flag platforms offer pre-processing on the server or edge, which can embed resolved flags into HTML templates or initial data payloads.
4. Avoid dependencies
In JavaScript UIs, flags often control component visibility, routing, or behavior changes. When flags become dependent on each other, it creates logic chains that are hard to follow and harder to test. For example, if flagA and flagB are both required to show a component, and one depends on a backend state while the other is set locally, the UI may exhibit inconsistent behavior depending on load order or user state.
To keep things clean, define flags to represent single, independent decisions. If multiple flags are needed together, create a composite flag server-side or evaluate them once and combine results before rendering.
In React, for example , avoid deeply nesting flag-based conditional components. Instead, use abstraction—wrap the logic in a useFeature hook or a utility function that keeps UI code readable and encapsulates complexity.
5. Targeted rollouts
Targeted rollouts in JavaScript applications often involve tailoring the user interface based on attributes like user ID, role, region, or device type. Using SDKs that support these attributes allows frontend code to fetch personalized flags and render accordingly. For example, in React apps, a hook like useFeatureFlag(‘newSidebar’) can return a value determined by backend targeting rules, allowing the app to dynamically adjust its layout or available actions.
Granular rollouts help test frontend features in real-world environments without exposing them to all users. For example, show a new checkout UI only to logged-in users from selected countries or to users in a beta program. In SPAs, these flags can be loaded once and stored in context or global state, reducing repeated evaluation.
6. Error handling and rollback
Even with frontend feature flags, things can break—rendering bugs, missing translations, or state mismatches are common issues in JavaScript apps when toggling features. To prepare for this, wrap flagged logic in try-catch blocks or fallback components. Use lazy loading to defer flagged content and load it only when needed.
A rollback should be as simple as flipping the flag off. For this to work smoothly, ensure the legacy code paths remain operational and tested. In JavaScript, it’s common to keep both new and old components available behind conditional rendering, allowing seamless switching. For example, a flag might toggle between NewSearchBar and LegacySearchBar, and disabling the flag should revert immediately to the old version without side effects or reloading the page.
7. Security and access control
JavaScript runs in the client, making it inherently insecure for enforcing permissions. A feature flag in the browser can be inspected or overridden via the dev console, exposing features meant to be restricted. For example, toggling admin views or premium features based on a client-side flag without backend verification creates a serious security risk.
To mitigate this, ensure all sensitive checks are performed server-side and that flags only influence presentation. For example, even if the UI shows a download button based on a flag, the backend must verify whether the user is authorized before serving the file. Never expose internal feature states or logic in client-delivered JavaScript unless it’s safe for public visibility.
8. Logging and monitoring
In a JavaScript environment, logging when and how flags are evaluated helps track user experiences and debug issues. Log key details such as the flag name, its evaluated value, the user ID or session, and the context. This information is useful when users report UI bugs or inconsistencies and helps identify whether a bug is tied to a given rollout.
Integrate these logs with browser-based monitoring tools like Sentry, Datadog, or a custom telemetry system. Also consider tracking flag changes as analytics events—this allows product and engineering teams to correlate feature releases with changes in conversion rates, errors, or usage patterns. In frontend code, this can be done by sending events from lifecycle hooks or flag evaluation utilities, ideally batching logs to avoid spamming endpoints.
9. Proper documentation
In JavaScript projects, feature flags can quickly spread across multiple files, especially in component-driven frameworks. Documenting each flag with its purpose, scope, and removal plan prevents confusion when reading or updating the code. Inline comments are helpful, but also maintain a central registry (e.g., a JSON or Markdown file) that lists all flags, their owners, and expiration criteria.
For large applications, integrate documentation into the flag management tool if it supports metadata. Make this documentation accessible to both developers and product managers so that everyone knows which features are under control.
10. Use a dedicated feature flag management platform
In JavaScript, especially with SPAs and dynamic UIs, feature flag state often needs to be evaluated in real time based on user identity or session attributes. Dedicated platforms offer JavaScript SDKs that allow users to fetch and evaluate flags directly in the browser. These platforms handle caching, targeting, and remote updates efficiently, reducing the need to hardcode logic or fetch flags manually from backend APIs.
A dedicated service abstracts complexity away from frontend developers, letting them focus on rendering based on simple boolean or variant-based flags. Integration with JavaScript frameworks is usually straightforward—flag values can be passed through React context, Vue reactive data, or Angular services. This simplifies implementation and ensures that changes to feature rollout strategy don’t require frontend redeployment.
Related content: Read our guide to feature flags best practices
Help us continuously improve
Please let us know if you have any feedback about this page.
