Feature Flags with JavaScript Proxies
Tags: javascript, webdev, programming, opensource
Published: 2023-09-12 18:20:20

[JavaScript Proxies](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) are objects that can intercept and customize the behavior of fundamental operations on other objects, such as getting and setting properties, invoking functions, and much more.

With JavaScript Proxies, you can create "virtual" objects that behave like real objects. This is achieved by intercepting calls to certain methods and functions and allowing you to customize the behavior of those calls.

```js
const targetObject = {
  name: 'Alice',
  age: 28
};

const handler = {
  get: function(target, property) {
    console.log(`Getting ${property}`);
    return target[property];
  },
  set: function(target, property, value) {
    console.log(`Setting ${property} to ${value}`);
    target[property] = value;
  }
};

const proxyObject = new Proxy(targetObject, handler);

console.log(proxyObject.name); // Logs "Getting name" and "Alice"
proxyObject.age = 29; // Logs "Setting age to 29"
```

Proxies can be used to implement feature flags in JavaScript in a flexible and powerful way. Here are some of the benefits of using proxies for feature flags:

1. **Dynamic feature flag evaluation**: Using proxies, you can dynamically evaluate whether or not a particular feature flag is enabled or disabled. This allows you to change the behavior of your application in real-time, without having to restart the application or update the code.

2. **Customizable flag behavior**: Proxies allow you to customize the behavior of a feature flag based on the context of the application. For example, you could enable a feature flag for a particular user or group of users, or enable a feature flag only in a particular environment.

3. **Separation of concerns**: By using proxies to implement feature flags, you can separate the concerns of feature flag management and application logic. This allows you to more easily manage your feature flags and update them without having to modify the application code.

4. **Testing and experimentation**: Using proxies, you can experiment with different feature flag configurations and test the impact of those changes on your application. This allows you to make data-driven decisions about feature flag behavior and optimize the user experience.

Examples of how you can use JavaScript proxies to implement feature flags:

### 1. Dynamic flag evaluation

In this example, we use a proxy to dynamically evaluate the `newFeatureEnabled` feature flag based on the current date. The `get` method of the proxy intercepts calls to the `newFeatureEnabled` property and returns `true` if the current date is after September 1, 2023. This allows us to enable the new feature dynamically without having to restart the application or update the code.

```js
const featureFlags = new Proxy({
  newFeatureEnabled: false
}, {
  get(target, property) {
    if (property === 'newFeatureEnabled') {
      const date = new Date();
      return date >= new Date('2023-09-01');
    }
    return target[property];
  }
});

if (featureFlags.newFeatureEnabled) {
  // Code for new feature goes here
}
```

### 2. Customizable flag behavior

In this example, we use a proxy to customize the behavior of the `newFeatureEnabled` feature flag based on the current user. The `get` method of the proxy intercepts calls to the `newFeatureEnabled` property and returns `true` if the current user has access to the new feature. This allows us to enable the new feature for certain users or groups of users.

```js
const featureFlags = new Proxy({
  newFeatureEnabled: false
}, {
  get(target, property) {
    if (property === 'newFeatureEnabled') {
      const user = getCurrentUser();
      return user.hasAccessToNewFeature;
    }
    return target[property];
  }
});

if (featureFlags.newFeatureEnabled) {
  // Code for new feature goes here
}
```

### 3. Separation of concerns

In this example, we use a proxy to separate the concerns of feature flag management and application logic. The `get` method of the proxy intercepts calls to any property and fetches the corresponding feature flag value from the server. This allows us to update the feature flag values without having to modify the application code.

```js
const featureFlags = new Proxy({}, {
  get(target, property) {
    // Fetch feature flag value from server
    const value = fetchFeatureFlagValue(property);
    return value;
  }
});

if (featureFlags.newFeatureEnabled) {
  // Code for new feature goes here
}
```

### 4. Testing and experimentation

In this example, we use a proxy to experiment with the `newFeatureEnabled` feature flag. The `get` method of the proxy intercepts calls to the `newFeatureEnabled` property and returns `true` for users in the experimental group. This allows us to test the impact of the new feature on a small group of users before rolling it out to everyone.

```js
const featureFlags = new Proxy({
  newFeatureEnabled: false
}, {
  get(target, property) {
    if (property === 'newFeatureEnabled') {
      const experimentGroup = getCurrentExperimentGroup();
      if (experimentGroup === 'experimentalGroup') {
        return true;
      }
    }
    return target[property];
  }
});

if (featureFlags.newFeatureEnabled) {
  // Code for new feature goes here
}
```

### Conclusion

Overall, using proxies for feature flags can provide a more flexible and powerful way to manage feature flag behavior in your JavaScript application. By separating the concerns of feature flag management and application logic, you can more easily manage your feature flags and optimize the user experience of your application.

-----

#### Basestack Platform

**[Basestack](https://basestack.co/)** is an open-source stack designed specifically for developers and startups. It offers a suite of tools, including Feature Flags, with additional tools such as Feedback, Forms, and more on the horizon. Our goal is to empower you in building great products.

![Basestack Feature Flags Demo](https://i.imgur.com/Oq3YxXR.gif)

- [Website](https://basestack.co/)
- [Docs](https://docs.basestack.co/)
- [Github](https://github.com/basestack-co/basestack)
- [Twitter](https://twitter.com/basestack_co)