Feature Flags, Toggles & A/B Testing

Everything you need for turbocharging your modern product development

Your first feature

Let’s get started on your first feature using a feature flag

Step 1 - Create a new Feature Flag

Start by creating a new Feature Flag. Pick a name related to the product feature you’re building. As an example, if you are building a new mobile registration UI flow, “Mobile Registration” is a good name.

It’s also a good practice to write in a good description that is easy for people other than you to understand what this feature is about.

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Once you create a new Feature Flag, you will see a screen like this: Screen Shot 2021-06-14 at 5 02 47 PM

The Feature Flag is enabled by default (meaning calls to this flag will evaluate the rules and conditions you specify next). You can disable flags by clicking on the toggle button (this stops the flag from evaluating rules and conditions and instead just returns false). Let’s leave it on for now.

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Step 2 - Create a new Rule

By default, your new Feature Flag will not pass any rule and hence will be returning false for all checks on the client side. In order to turn on this feature, you will need to target this feature to a specific set of folks. You do that by clicking on Add Rule

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You can choose from a list of targeting criteria like User ID, Country, Browser, Operating System, etc.

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Let’s go ahead and choose Operating System is Any of and pick Android and iOS for our Mobile only targeting.

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Now hit the Add Rule button and you should now see the new rule in the Feature Flag details page.

Go ahead and hit Save Changes on the top right to commit these changes. New rules and conditions are staged at first, and the changes are only commited when you finalize them by clicking “Save Changes”

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You can now test this feature flag by configuring the User object in the “Test Flag” section.

Step 3 - Adding additional rules

Let’s add one more rule to our Feature Flag, one that allows us to test the new feature - say, for dogfooding purposes.

This time, let’s pick Email Contains any of and choose a domain. Like this:

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Hit Add Rule and don’t forget to Save Changes.

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Step 4 - Create a new Client API key

Navigate to the API Keys section of the Statsig console. Here you would see different types of API keys you can generate.

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Server Secret Keys are used only for server side integrations. These keys are secrets that you want to safe-guard and that means not using it on clients.

Client SDK Keys are used in product apps or websites to access Statsig features. It’s okay to embed these keys in apps or website.

For the purposes of this exercise, let’s go ahead and create a new Client SDK Key.

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Copy this newly created SDK key. We’re going to use this in the next step.

Step 5 - Using your new Flag via the Statsig SDK

Now that we have configured this feature flag, let’s go ahead and implement the feature behind this feature flag. For this exercise, let’s pick a website that uses our JS SDK.

Statsig supports many different SDKs. Please checkout Getting Started for the full list of Client side SDKs

Statsig is available from jsdelivr, an open source CDN. So let’s open the Console Window of a new Chrome browser window for the next set of steps.

The current version at this writing is 2.0.3, but using this link https://cdn.jsdelivr.net/npm/statsig-js/dist/statsig-prod-web-sdk.min.js will get you the latest SDK version that’s available.

Copy and paste the following code in your console:

var scrpt = document.createElement("script");
scrpt.src =


Step 6 - Check flags using the SDK

Copy and paste the following code in your console:

await statsig.initialize("YOUR_SDK_KEY", {});


Now that we have initialized Statsig, we can go ahead and make a call to our newly created Feature Flag. Copy and paste the following code in the console.


You should get a response ‘false’ back from the server.


This is because we are not running this code on iOS or Android. And the way we configured our Feature Flag, the feature is only available on mobile.

Step 7 - Change the Environment to Mobile

Chrome DevTools allows us to change our Device type to mobile. Let’s use that feature to simulate the request coming from a mobile device. First, go ahead and toggle the Mobile Device toolbar to ON by clicking on this icon at the top left corner:


Since the environment has changed, we need to call updateUser on Statsig. Copy and paste the following code in the console.

await statsig.updateUser({});

Now, let’s rerun the flag check again.


This time, you should see true, meaning that this feature is now available on this ‘mobile’ device!


Step 7 - Let’s try a different check

Let’s toggle the mobile device to off, so we’re now back to Desktop device. This time let’s try a different check - one that uses the user email as the condition. When we update the user this time, we will prodive an employee email address.

Type the following code in your console:

await statsig.updateUser({ email: "vijaye@statsig.com" });


And let’s try the same checkGate once more


This time, you should get back a true indicating that this feature is available for this user.


Step 8 - How to use these flags in production

The psuedocode for feature flags would look something like this:

if (statsig.checkGate("mobile_registration")) {
  // Show mobile registration UI
} else {
  // Use the default path

Happy Feature Gating!