Experiment JavaScript SDK
Official documentation for Amplitude Experiment's Client-side JavaScript SDK implementation.
SDK Resources
Install¶
Install the Experiment JavaScript Client SDK.
npm install --save @amplitude/experiment-js-client
yarn add @amplitude/experiment-js-client
<script src="https://unpkg.com/@amplitude/experiment-js-client@1.9.0/dist/experiment.umd.js"></script>
<script>
// TODO: Replace DEPLOYMENT_KEY with your own deployment key.
// If you're using a 3rd party for analytics, configure an exposure
// tracking provider.
window.experiment = Experiment.Experiment.initializeWithAmplitudeAnalytics(
'DEPLOYMENT_KEY'
);
</script>
Quick Start
The right way to initialize the Experiment SDK depends on whether you use an Amplitude SDK for analytics or a third party (e.g. Segment).
import { Experiment } from '@amplitude/experiment-js-client';
// (1) Initialize the experiment client with Amplitude Analytics.
const experiment = Experiment.initializeWithAmplitudeAnalytics(
'DEPLOYMENT_KEY'
);
// (2) Start the SDK and await the promise result.
await experiment.start();
// (3) Lookup a flag's variant.
const variant = experiment.variant('FLAG_KEY');
if (variant.value === 'on') {
// Flag is on
} else {
// Flag is off
}
import { Experiment } from '@amplitude/experiment-js-client';
// (1) Initialize the experiment client with Amplitude Analytics and
// implement an exposure tracking provider.
const experiment = Experiment.initialize(
'DEPLOYMENT_KEY',
{
exposureTrackingProvider: {
track: (exposure) => {
// TODO: Implement exposure tracking
// analytics.track('$exposure', exposure)
}
}
}
);
// (2) Start the SDK with the user and await the promise result.
const user = {
user_id: 'user@company.com',
device_id: 'abcdefg',
user_properties: {
premium: true,
},
}
await experiment.start(user);
// (3) Lookup a flag's variant.
const variant = experiment.variant('FLAG_KEY');
if (variant.value === 'on') {
// Flag is on
} else {
// Flag is off
}
Not getting the expected variant result for your flag? Make sure your flag is activated, has a deployment set, and has users allocated.
Core functions¶
The following functions make up the core of the Experiment client-side SDK.
Initialize¶
The SDK client should be initialized in your application on startup. The deployment key argument passed into the apiKey
parameter must live within the same project that you are sending analytics events to.
initializeWithAmplitudeAnalytics(apiKey: string, config?: ExperimentConfig): ExperimentClient
initialize(apiKey: string, config?: ExperimentConfig): ExperimentClient
Parameter | Requirement | Description |
---|---|---|
apiKey |
required | The deployment key which authorizes fetch requests and determines which flags should be evaluated for the user. |
config |
optional | The client configuration used to customize SDK client behavior. |
The initializer returns a singleton instance, so subsequent initializations for the same instance name will always return the initial instance. To create multiple instances, use the instanceName
configuration.
import { Experiment } from '@amplitude/experiment-js-client';
const experiment = initializeWithAmplitudeAnalytics('DEPLOYMENT_KEY');
Instance name
If you're using a custom instance name for analytics, you need to set the same value in the instanceName
configuration option in the experiment SDK, or visa versa.
import { Experiment } from '@amplitude/experiment-js-client';
const experiment = Experiment.initialize(
'DEPLOYMENT_KEY',
{
exposureTrackingProvider: {
track: (exposure) => {
// TODO: Implement exposure tracking
// analytics.track('$exposure', exposure)
}
}
}
);
Configuration¶
The SDK client can be configured once on initialization.
Configuration Options
Name |
Description | Default Value |
---|---|---|
debug |
Enable additional debug logging within the SDK. Should be set to false in production builds. | false |
fallbackVariant |
The default variant to fall back if a variant for the provided key doesn't exist. | {} |
initialVariants |
An initial set of variants to access. This field is valuable for bootstrapping the client SDK with values rendered by the server using server-side rendering (SSR). | {} |
serverZone |
Select the Amplitude data center to get flags and variants from, us or eu . |
us |
serverUrl |
The host to fetch remote evaluation variants from. For hitting the EU data center, use serverZone . |
https://api.lab.amplitude.com |
flagsServerUrl |
The host to fetch local evaluation flags from. For hitting the EU data center, use serverZone . |
https://flag.lab.amplitude.com |
fetchTimeoutMillis |
The timeout for fetching variants in milliseconds. | 10000 |
retryFetchOnFailure |
Whether to retry variant fetches in the background if the request doesn't succeed. | true |
automaticExposureTracking |
If true, calling variant() will track an exposure event through the configured exposureTrackingProvider . If no exposure tracking provider is set, this configuration option does nothing. |
true |
fetchOnStart |
If true, always fetch remote evaluation variants on start. If false, never fetch on start. If undefined, dynamically determine whether to fetch on start. | undefined |
pollOnStart |
Poll for local evaluation flag configuration updates once per minute on start. | true |
automaticFetchOnAmplitudeIdentityChange |
Only matters if you use the initializeWithAmplitudeAnalytics initialization function to seamlessly integrate with the Amplitude Analytics SDK. If true any change to the user ID, device ID or user properties from analytics will trigger the experiment SDK to fetch variants and update it's cache. |
false |
userProvider |
An interface used to provide the user object to fetch() when called. See Experiment User for more information. |
null |
exposureTrackingProvider |
Implement and configure this interface to track exposure events through the experiment SDK, either automatically or explicitly. | null |
instanceName |
Custom instance name for experiment SDK instance. The value of this field is case-sensitive. | null |
EU Data Center
If you're using Amplitude's EU data center, configure the serverZone
option on initialization to eu
.
Integrations¶
If you use either Amplitude or Segment Analytics SDKs to track events into Amplitude, you'll want to set up an integration on initialization. Integrations automatically implement provider interfaces to enable a more streamlined developer experience by making it easier to manage user identity and track exposures events.
Amplitude integration (click to open)
The Amplitude Experiment SDK is set up to integrate seamlessly with the Amplitude Analytics SDK.
import * as amplitude from '@amplitude/analytics-browser';
import { Experiment } from '@amplitude/experiment-js-client';
amplitude.init('API_KEY');
const experiment = Experiment.initializeWithAmplitudeAnalytics('DEPLOYMENT_KEY');
Using the integration initializer will automatically configure implementations of the user provider and exposure tracking provider interfaces to pull user data from the Amplitude Analytics SDK and track exposure events.
Supported Versions
All versions of the next-generation Amplitude analytics Browser SDK support this integration.
Legacy Analytics SDK Version | Experiment SDK Version |
---|---|
8.18.1+ |
1.4.1+ |
Segment integration (click to open)
Experiment's integration with Segment Analytics must be configured manually. The Experiment SDK must then be configured on initialization with an instance of the the exposure tracking provider. Make sure this happens after the analytics SDK has been loaded an initialized.
analytics.ready(() => {
const experiment = Experiment.initialize('DEPLOYMENT_KEY', {
exposureTrackingProvider: {
track: (exposure) => {
analytics.track('$exposure', exposure)
}
}
});
});
When starting the SDK, pass the segment anonymous ID and user ID for the device ID and user ID, respectively.
await experiment.start({
user_id: analytics.user().id(),
device_id: analytics.user().analyticsId(),
});
Start¶
Start the SDK by getting flag configurations from the server and fetching remote evaluation variants for the user. The SDK is ready once the returned promise resolves.
start(user?: ExperimentUser): Promise<void>
Parameter | Requirement | Description |
---|---|---|
user |
optional | Explicit user information to pass with the request to fetch variants. This user information is merged with user information provided from integrations via the user provider, preferring properties passed explicitly to fetch() over provided properties. Also sets the user in the SDK for reuse. |
Call start()
when your application is initializing, after user information is available to use to evaluate or fetch variants. The returned promise resolves after loading local evaluation flag configurations and fetching remote evaluation variants.
Configure the behavior of start()
by setting fetchOnStart
in the SDK configuration on initialization to improve performance based on the needs of your application.
- If your application always relies on remote evaluation on startup, set
fetchOnStart
totrue
to load flag configurations and fetch remote evaluation variants simultaneously. - If your application never relies on remote evaluation, set
fetchOnStart
tofalse
to avoid increased startup latency if someone accidentally creates a remote evaluation flag. - If your application relies on remote evaluation, but not right at startup, you may set
fetchOnStart
tofalse
and callfetch()
and await the promise separately.
await experiment.start();
const user = {
user_id: 'user@company.com',
device_id: 'abcdefg',
user_properties: {
premium: true
}
};
await experiment.start(user);
Fetch¶
Fetches variants for a user and store the results in the client for fast access. This function remote evaluates the user for flags associated with the deployment used to initialize the SDK client.
Fetch on user identity change
If you want the most up-to-date variants for the user, it's recommended that you call fetch()
whenever the user state changes in a meaningful way. For example, if the user logs in and receives a user ID, or has a user property set which may effect flag or experiment targeting rules.
In addition, pass new user properties explicitly to fetch()
instead of relying on user enrichment prior to remote evaluation. This is because user properties that are synced remotely through a separate system have no timing guarantees with respect to fetch()
--i.e. a race.
fetch(user?: ExperimentUser, options?: FetchOptions): Promise<Client>
Parameter | Requirement | Description |
---|---|---|
user |
optional | Explicit user information to pass with the request to evaluate. This user information is merged with user information provided from integrations via the user provider, preferring properties passed explicitly to fetch() over provided properties. |
options |
optional | Explicit flag keys to fetch. |
Account level bucketing and analysis support (v1.5.6+)
If your organization has purchased the Accounts add-on you may perform bucketing and analysis on groups rather than users. Reach out to your representative to gain access to this beta feature.
Groups must either be included in the user sent with the fetch request (recommended), or identified with the user via a group identify call from the Group Identify API or via setGroup()
from an analytics SDK.
await fetch({groups: {'org name': ['Amplitude']}});
const user = {
user_id: 'user@company.com',
device_id: 'abcdefg',
user_properties: {
'premium': true,
},
};
await experiment.fetch(user);
If you're using an integration or a custom user provider then you can fetch without inputting the user.
await experiment.fetch();
Timeout & Retries
If fetch()
times out (default 10 seconds) or fails for any reason, the SDK client will return and retry in the background with back-off. You may configure the timeout or disable retries in the configuration options when the SDK client is initialized.
Variant¶
Access a variant for a flag or experiment from the SDK client's local store.
Automatic Exposure Tracking
When an integration is used or a custom exposure tracking provider is set, variant()
will automatically track an exposure event through the tracking provider. To disable this functionality, configure automaticExposureTracking
to be false
, and track exposures manually using exposure()
.
variant(key: string, fallback?: string | Variant): Variant
Parameter | Requirement | Description |
---|---|---|
key |
required | The flag key to identify the flag or experiment to access the variant for. |
fallback |
optional | The value to return if no variant was found for the given flagKey . |
When determining which variant a user has been bucketed into, you'll want to compare the variant value
to a well-known string.
const variant = experiment.variant('<FLAG_KEY>');
if (variant.value === 'on') {
// Flag is on
} else {
// Flag is off
}
Accessing the variant's payload
A variant may also be configured with a dynamic payload of arbitrary data. Access the payload
field from the variant object after checking the variant's value
.
const variant = experiment.variant('<FLAG_KEY>');
if (variant.value === 'on') {
const payload = variant.payload;
}
A null
variant value
means that the user hasn't been bucketed into a variant. You may use the built in fallback parameter to provide a variant to return if the store doesn't contain a variant for the given flag key.
const variant = experiment.variant('<FLAG_KEY>', { value: 'control' });
if (variant === 'control') {
// Control
} else if (variant === 'treatment') {
// Treatment
}
All¶
Access all variants stored by the SDK client.
all(): Variants
Clear¶
Clear all variants in the cache and storage.
clear(): void
You can call clear
after user logout to clear the variants in cache and storage.
experiment.clear();
Exposure¶
Manually track an exposure event for the current variant of the given flag key through configured integration or custom exposure tracking provider. Generally used in conjunction with setting the automaticExposureTracking
configuration optional to false
.
exposure(key: string): void
Parameter | Requirement | Description |
---|---|---|
key |
required | The flag key to identify the flag or experiment variant to track an exposure event for. |
const variant = experiment.variant('<FLAG_KEY>');
// Do other things...
experiment.exposure('<FLAG_KEY>');
if (variant === 'control') {
// Control
} else if (variant === 'treatment') {
// Treatment
}
Providers¶
Integrations
If you use Amplitude or Segment analytics SDKs along side the Experiment Client SDK, Amplitude recommends you use an integration instead of implementing custom providers.
Provider implementations enable a more streamlined developer experience by making it easier to manage user identity and track exposures events.
User provider¶
The user provider is used by the SDK client to access the most up-to-date user information only when it's needed (for example, when fetch()
is called). This provider is optional, but helps if you have a user information store already set up in your application. This way, you don't need to manage two separate user info stores in parallel, which may result in a divergent user state if the application user store is updated and experiment isn't (or via versa).
interface ExperimentUserProvider {
getUser(): ExperimentUser;
}
To use your custom user provider, set the userProvider
configuration option with an instance of your custom implementation on SDK initialization.
const experiment = Experiment.initialize('<DEPLOYMENT_KEY>', {
userProvider: new CustomUserProvider(),
});
Exposure tracking provider¶
Implementing an exposure tracking provider is highly recommended. Exposure tracking increases the accuracy and reliability of experiment results and improves visibility into which flags and experiments a user is exposed to.
export interface ExposureTrackingProvider {
track(exposure: Exposure): void;
}
The implementation of track()
should track an event of type $exposure
(a.k.a name) with two event properties, flag_key
and variant
, corresponding to the two fields on the Exposure
object argument. Finally, the event tracked must eventually end up in Amplitude Analytics for the same project that the [deployment] used to initialize the SDK client lives within, and for the same user that variants were fetched for.
To use your custom user provider, set the exposureTrackingProvider
configuration option with an instance of your custom implementation on SDK initialization.
const experiment = Experiment.initialize('<DEPLOYMENT_KEY>', {
exposureTrackingProvider: new CustomExposureTrackingProvider(),
});
Bootstrapping¶
You may want to bootstrap the experiment client with an initial set of flags and variants when variants are obtained from an external source (for example, not from calling fetch()
on the SDK client). Use cases include local evaluation, server-side rendering, or integration testing on specific variants.
To bootstrap the client, set the flags and variants in the initialVariants
configuration object, then set the source
to Source.InitialVariants
so that the SDK client prefers the bootstrapped variants over any previously fetched & stored variants for the same flags.
const experiment = Experiment.initialize('<DEPLOYMENT_KEY>', {
initialVariants: { /* Flags and variants */ },
source: Source.InitialVariants,
});