# Feature Flag

URL: https://softwaredictionary.org/terms/feature-flag
Category: DevOps & Cloud
Last updated: 2026-09-30

In short: A feature flag is a switch in code that turns a feature on or off at runtime, letting teams deploy code without releasing it to every user at once.

## What is a feature flag?

A feature flag, also called a feature toggle, is a conditional check in the code that decides whether a piece of functionality is active. The value of the flag comes from outside the code, such as a configuration file, a database, or a dedicated flag service, so it can change while the application is running. This separates deploying code, which puts it on the servers, from releasing a feature, which makes it visible to users.

At its simplest, a flag is an `if` statement around the new code path. More advanced systems evaluate flags on every request using targeting rules, for example turning a feature on for internal staff, for users in one country, or for a random 5% of accounts, and increasing that share over time. If the feature causes errors, anyone with access can switch it off in seconds, which is often faster and safer than a rollback.

Teams use flags for gradual rollouts, beta programs, kill switches for risky dependencies, and A/B testing, and flags make trunk-based development practical because unfinished work can be merged to the main branch while hidden behind a flag. Think of a flag like the light switches in a newly wired house: the electrician installs all the wiring in advance, and you decide later which rooms to light up. The main cost is complexity, so old flags should be removed once a feature is fully launched, or they pile up as technical debt.

Feature flags are often confused with canary deployments and environment variables. A canary deployment routes a share of traffic to a new version of the whole application at the infrastructure level, while a feature flag switches individual features inside one running version, often per user. An environment variable is usually read once at startup and is the same for every request, whereas a flag can be changed live and can give different users different answers.

## Key takeaways

- A feature flag turns functionality on or off at runtime without a new deployment.
- It separates deploying code from releasing a feature to users.
- Targeting rules can enable a feature for specific users, groups, or a percentage of traffic.
- A flag doubles as a kill switch when a new feature misbehaves in production.
- Stale flags add complexity and should be removed after a full launch.

## Example: Hiding a new checkout flow behind a flag

```javascript
// The flag values come from a flag service or config, not from the code
const flags = await loadFlags({ userId: user.id, country: user.country });

if (flags.isEnabled("new-checkout")) {
  renderNewCheckout(cart); // only users the flag targets see this
} else {
  renderLegacyCheckout(cart); // everyone else keeps the current flow
}
```

## Frequently asked questions

**What is the difference between a feature flag and a feature branch?**

A feature branch keeps unfinished work in a separate Git branch until it is merged. A feature flag lets unfinished code be merged into the main branch and deployed while it stays switched off, which avoids long-lived branches and painful merges.

**Are feature flags technical debt?**

They become technical debt when they stay in the code after a feature is fully launched. Many teams give each release flag an owner and an expiry date, then delete the flag and the old code path once the rollout is finished.

**Can feature flags be used for A/B testing?**

Yes. A flag can assign users to different variants of a feature, and an analytics system then compares how each group behaves. A/B testing is one use of feature flags, alongside gradual rollouts, kill switches, and beta access.

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Software Dictionary: https://softwaredictionary.org/ · https://softwaredictionary.org/llms.txt