# Feature Flags (Feature Toggles)

Feature Flags (also known as Feature Toggles, Feature Flippers, or Feature Controls) are a software development technique that allows you to turn specific functionality on or off during runtime, without deploying new code.

## 1. Decoupling Deployment from Release

Traditionally, deploying code meant releasing features to users. Feature flags break this coupling.

*   **Deployment:** Moving code to production servers.
*   **Release:** Making features visible/available to users.

With flags, you can deploy unfinished code to production (hidden behind a flag) and "release" it later by flipping a switch.

## 2. Types of Toggles

Not all flags are the same. They differ in longevity and dynamism.

### Release Toggles
Used to hide incomplete features for Trunk-Based Development.
*   **Lifespan:** Short (days/weeks).
*   **Dynamism:** Static (often just a config file change).

### Experiment Toggles (A/B Testing)
Used to perform multivariate or A/B testing.
*   **Lifespan:** Medium (weeks/months).
*   **Dynamism:** Dynamic (different users see different values).

### Ops Toggles (Kill Switches)
Used to control operational aspects of the system. For example, a flag to disable a heavy "Recommendations" widget if the database is under high load.
*   **Lifespan:** Long/Permanent.
*   **Dynamism:** Dynamic (flipped by operators in response to incidents).

### Permission Toggles
Used to change the features or product experience that certain users receive (e.g., Premium vs. Free users).
*   **Lifespan:** Permanent.
*   **Dynamism:** Dynamic (based on user attributes).

## 3. Implementation

### Simple (If/Else)
At its core, a feature flag is just an `if` statement.

```javascript
const flags = {
  newCheckout: false
};

function renderCheckout() {
  if (flags.newCheckout) {
    return <NewCheckout />;
  } else {
    return <OldCheckout />;
  }
}
```

### Advanced (Contextual)
Real-world flags often depend on context (User ID, Region, Browser).

```javascript
if (featureManager.isEnabled("new-checkout", currentUser)) {
    // Show new checkout
}
```

## 4. Managing Technical Debt

Feature flags are essentially **Technical Debt** by design. They introduce conditional logic and dead code paths.

*   **Cleanup:** Once a feature is fully released and stable, the flag **must** be removed. If not, the code becomes a nightmare of nested `if` statements.
*   **Flag Retirement:** Make removing the flag a task in your backlog immediately after the release.

[[programming/continuous-deployment]]
[[programming/git-version-control]]
[[programming/canary-deployment]]
[[programming/software-testing-basics]]