Building with Feature Flags 1

A frontend feature flag looks easy because the visible decision is often small: show one component or another. The harder questions are about when the browser learns the flag value, what happens before it arrives, what information is sent for targeting, and what a user can discover by inspecting the application.

The basic browser pattern

browser starts
   ↓
feature SDK initializes
   ↓
configuration becomes available
   ↓
flag is evaluated
   ↓
render the selected experience

This pattern applies whether the application uses React, Angular, Vue, Svelte, or plain JavaScript. The framework changes the syntax. The architectural questions remain the same.

Avoid the flash of the wrong experience

If the page renders before the flag client is ready, a user may briefly see the old experience and then watch it switch to the new one. The reverse can happen too.

For important experiences, decide explicitly how startup should work. Common choices include waiting for flag initialization, using a server supplied bootstrap value, rendering a neutral loading state, or using a cached value from a previous session.

There is no universal answer. A small cosmetic experiment can tolerate more flexibility than checkout, authentication, or a workflow where the user could lose entered data.

Client side flags are visible

A browser is not a trusted execution environment. Users can inspect JavaScript, network requests, local storage and flag configuration delivered to the client.

That means a frontend flag must never become the only authorization check.

UI flag says admin feature is hidden
             ≠
user is not authorized

The backend must still enforce access to protected data and operations.

Keep targeting context intentional

Frontend evaluations often use context such as account ID, plan, locale, application version, anonymous visitor ID, or authenticated user ID. Send only the attributes needed for the decision.

This matters for privacy as well as maintainability. A feature platform should not become an accidental copy of the user profile.

Stable bucketing matters

A 10 percent rollout should normally give the same user the same experience across page loads. Feature platforms achieve this through deterministic bucketing based on a stable targeting key.

If an anonymous visitor receives one variation and then changes identity after sign in, think through whether that transition is acceptable. Experiments especially need consistent exposure rules or their measurements become harder to trust.

Frameworks should not define the architecture

React hooks, Angular services and Vue composables are useful integration surfaces, but the application should still have a clear boundary around feature decisions.

page or component
      ↓
feature decision layer
      ↓
OpenFeature or vendor SDK
      ↓
provider

This makes testing easier and keeps feature platform details from spreading through every component.

What to test

  • the default state before configuration is ready
  • the enabled and disabled experiences
  • targeting for authenticated and anonymous users
  • behavior when the provider cannot refresh
  • access control on the backend regardless of the frontend flag
  • analytics exposure so an experiment is measured only when the user actually sees it

The best frontend integration makes the feature decision visible in the code without making the entire UI dependent on the mechanics of one feature flag vendor.

Feature Flags series

← What Is OpenFeature?   Feature Flags in Backend Services →