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New JavaScript Client SDK (Beta)

Getting Started

The following will outline how to get up and running with Statsig for JavaScript Precomputed Evaluations.

Create an Account

To work with the SDK, you will need a Statsig account. If you don't yet have an account, go ahead and sign up for a free account now.

You could skip this for now, but you will need an SDK key and some gates/experiments to use with the SDK in just a minute.


You can install the Statsig SDK via npm, yarn or jsdelivr:

npm install @statsig/js-client

Initialize the SDK

After installation, you will need to initialize the SDK using a Client SDK key from the "API Keys" tab on the Statsig console.

These Client SDK Keys are intended to be embedded in client side applications. If need be, you can invalidate or create new SDK Keys for other applications/SDK integrations.


Do NOT embed your Server Secret Key in client side applications

In addition to the SDK key, you should also pass in a StatsigUser for feature gate targeting and experimentation grouping purposes.

The 3rd parameter is optional and allows you to pass in a StatsigOptions to customize the SDK.

  • In advanced use cases, you may want to Prefetch or Bootstrap (Provide) values for initialization. See Using EvaluationsDataAdapter to learn how this can be achieved.

  • It is possible to apply a timeout to async operations. See Foo.

Working with the SDK

Checking a Gate

Now that your SDK is initialized, let's check a Feature Gate. Feature Gates can be used to create logic branches in code that can be rolled out to different users from the Statsig Console. Gates are always CLOSED or OFF (think return false;) by default.


Reading a Dynamic Config

Feature Gates can be very useful for simple on/off switches, with optional but advanced user targeting. However, if you want to be able send a different set of values (strings, numbers, and etc.) to your clients based on specific user attributes, e.g. country, Dynamic Configs can help you with that. The API is very similar to Feature Gates, but you get an entire json object you can configure on the server and you can fetch typed parameters from it. For example:


Getting an Layer/Experiment

Then we have Layers/Experiments, which you can use to run A/B/n experiments. We offer two APIs, but we recommend the use of layers to enable quicker iterations with parameter reuse.


Logging an Event

Now that you have a Feature Gate or an Experiment set up, you may want to track some custom events and see how your new features or different experiment groups affect these events. This is super easy with Statsig - simply call the Log Event API for the event, and you can additionally provide some value and/or an object of metadata to be logged together with the event:


Evaluation Details

When you receive a value, it may be useful to know how the SDK came to that result. For this purpose, on every Statsig type there exists a way to check the EvaluationDetails object.

This object contains the following fields:

  • Reason: This is a string containing the source as well as whether or not the specific config was found.
    • Network:Recognized means the SDK has the latest values and found an entry for the config.
    • Cache:Unrecognized means we are working with cached values, are could not find the given config.
    • For the full list of possible combinations, see the page on Debugging.
  • lcut: Last Config Update Time - This is the unix timestamp for when any configuration in your project changed.
    • lcut works as a version number for you project. If it changes, values are deemed out of date and possibly need to be refetched.
  • receivedAt: This is the unix timestamp of when the SDK received these values. This can be useful in knowing how old your cache is.

Code Examples

Prefer seeing it in practice? Included in the open source repository are some Code Examples. View these for common use cases for the SDK.

Statsig User

You should provide a StatsigUser object whenever possible when initializing the SDK, passing as much information as possible in order to take advantage of advanced gate and config conditions (like country or OS/browser level checks).

Most of the time, the userID field is needed in order to provide a consistent experience for a given user (see logged-out experiments to understand how to correctly run experiments for logged-out users).

If the user is logged out at the SDK init time, you can leave the `userID` out for now, and we will use a stable device ID that we create and store in the local storage for targeting purposes.

Besides userID, we also have email, ip, userAgent, country, locale and appVersion as top-level fields on StatsigUser. In addition, you can pass any key-value pairs in an object/dictionary to the custom field and be able to create targeting based on them.

Once the user logs in or has an update/changed, make sure to call updateUser with the updated userID and/or any other updated user attributes:

Private Attributes

Have sensitive user PII data that should not be logged? No problem, we have a solution for it! On the StatsigUser object we also have a field called privateAttributes, which is a simple object/dictionary that you can use to set private user attributes. Any attribute set in privateAttributes will only be used for evaluation/targeting, and removed from any logs before they are sent to Statsig server.

For example, if you have feature gates that should only pass for users with emails ending in "", but do not want to log your users' email addresses to Statsig, you can simply add the key-value pair { email: "" } to privateAttributes on the user and that's it!

Updating Users

At some point, you will probably need to change the StatsigUser being used by the Statsig client. To do this you can call updateUserSync or updateUserAsync.

In advanced use cases, you may want to Prefetch or Bootstrap (Provide) values for the updating of a user. See Using EvaluationsDataAdapter to learn how this can be achieved.


Prefetching Users

In some usecases, it can be useful to prefetch values for a given user, such that switching to that user later can be done synchronously.

To do this, you would call prefetchData and then later updateUserSync.


Statsig Options

disableLogging boolean

Prevents sending any events over the network.

disableStorage boolean

Prevents writing anything to storage.

Note: caching will not work if storage is disabled

networkConfig object

Allows for fine grained control over which api or urls are hit for specific Statsig network requests

See NetworkConfig object

api string

The API to use for all SDK network requests. You should not need to override this unless you have a custom API that implements the Statsig endpoints.

logEventUrl string

The URL used to flush queued events via a POST request. Takes precedence over NetworkConfig.api.


networkTimeoutMs number

The maximum amount of time (in milliseconds) that any network request can take before timing out.

default: 10,000 ms (10 seconds)

preventAllNetworkTraffic boolean

Intended for testing purposes. Prevents any network requests being made.

networkOverrideFunc function

Overrides the default networking layer used by the Statsig client. By default, the client use fetch, but overriding this you could use axios or raw XMLHttpRequest

default: fetch

initializeUrl string

The URL used to fetch the latest evaluations for a given user. Takes precedence over NetworkConfig.api.


environment StatsigEnvironment

An object you can use to set environment variables that apply to all of your users in the same session.

logLevel LogLevel

How much information is allowed to be printed to the console.

default: LogLevel.Warn

loggingBufferMaxSize number

The maximum number of events to batch before flushing logs to Statsig.

default: 50

loggingIntervalMs number

How often (in milliseconds) to flush logs to Statsig.

default: 10,000 ms (10 seconds)

overrideAdapter OverrideAdapter

An implementor of OverrideAdapter, used to alter evaluations before its returned to the caller of a check api (checkGate/getExperiment etc).

includeCurrentPageUrlWithEvents boolean

(Web only) Should the 'current page' url be included with logged events.

default: true

disableStatsigEncoding boolean

Whether or not Statsig should use raw JSON for network requests where possible.

default: false

disableCompression boolean

Whether or not Statsig should compress JSON bodies for network requests where possible.

default: false

dataAdapter EvaluationsDataAdapter

An implementor of EvaluationsDataAdapter, used to customize the initialization/update flow.

default: StatsigEvaluationsDataAdapter

customUserCacheKeyFunc CustomCacheKeyGenerator

Overrides the default cache key generation. Given the SDKKey and current StatsigUser, return a key to be used for storing values related to that user.

default: Cache key is a hash of the sdkKey + user.userID + user.customIDs.

Shutting Statsig Down

In order to save users' data and battery usage, as well as prevent logged events from being dropped, we keep event logs in client cache and flush periodically. Because of this, some events may not have been sent when your app shuts down.

To make sure all logged events are properly flushed or saved locally, you should tell Statsig to shutdown when your app is closing:



Async timeouts

For the Statsig client calls that hit the network (initializeAsync and updateUserAsync), it is possible to specify a maximum amount of time (in milliseconds) that the method is allowed to take before the promise is resolved.


If this timeout is hit, the promise will resolve and cached values will be used (If any are available). The network request will continue and upon a response, will write values to cache so the will be available for subsequent calls.

Data Adapter

The EvaluationsDataAdapter type outlines how the StatsigClient should fetch and cache data during initialize and update operations. By default, the StatsigClient uses StatsigEvaluationsDataAdapter, a Statsig provided implementor of the EvaluationsDataAdapter type. StatsigEvaluationsDataAdapter provides ways to fetch data synchronously from Local Storage and asynchronously from Statsig's servers. See Using EvaluationsDataAdapter to learn more and see example usage.

Client Event Emitter

It is possible to subscribe to StatsigClientEvents (Not to be confused with StatsigEvent). These events occur at various stages while using the Statsig client. You can subscribe to specific events by specifying the StatsigClientEvent name, or, all events by using the wildcard token '*'.


The full list of events and descriptions can be found here.

Multiple Client Instances

In some usecases, it might be useful to have multiple instances of the Statsig client running.

🔥IMPORTANT: You must use two different SDK keys for this to work. Various functionality of the Statsig client is keyed on the SDK key being used. Using the same key will lead to collisions.



How do I run experiments for logged out users?

See the guide on device level experiments

Does the SDK use the browser local storage or cookies? If so, for what purposes?

The SDK does not use any cookies.

It does use the local storage for feature targeting and experimentation purposes only. Values for feature gates, dynamic configs and experiments are cached in the local storage, which are used as a backup in the event that your website/app cannot reach the Statsig server to fetch the latest values. If any events were logged but could not be sent to Statsig server due to issues like network failure, we also save them in the local storage to be sent again when network restores.