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This documentation is for our client-side SDK intended for use by browser applications. For use in server applications, refer to our Node JS SDK.

Eppo's open source JavaScript SDK can be used for both feature flagging and experiment assignment:

1. Install the SDK

You can install the SDK with Yarn or NPM:

yarn add @eppo/js-client-sdk

2. Initialize the SDK

Initialize the SDK with a SDK key, which can be generated in the Eppo interface:

import { init } from "@eppo/js-client-sdk";

await init({
apiKey: "<SDK_KEY>",

During initialization, the SDK sends an API request to Eppo to retrieve the most recent experiment configurations such as variation values and traffic allocation. The SDK stores these configurations in memory so that assignments are effectively instant. For more information, see the architecture overview page.

If you are using the SDK for experiment assignments, make sure to pass in an assignment logging callback (see section below).

Initialization options

How the SDK fetches, serves, and caches experiment configurations is configurable via additional optional initialization options:

requestTimeoutMs (number)Timeout in milliseconds for HTTPS requests for the experiment configurations.5000
numInitialRequestRetries (number)Number of additional times the initial configurations request will be attempted if it fails. This is the request typically synchronously waited (via await) for completion. A small wait will be done between requests.1
pollAfterSuccessfulInitialization (boolean)Poll for new configurations (every 30 seconds) after successfully requesting the initial configurations.false
pollAfterFailedInitialization (boolean)Poll for new configurations even if the initial configurations request failed.false
throwOnFailedInitialization (boolean)Throw an error (reject the promise) if unable to fetch initial configurations during initialization.true
numPollRequestRetries (number)If polling for updated configurations after initialization, the number of additional times a request will be attempted before giving up. Subsequent attempts are done using an exponential backoff.7
skipInitialRequest (boolean)Skip the initial request for a new configuration during initialization (if polling is enabled, this will still take place later)false
persistentStore (IAsyncStore)An asynchronous, persistent storage for caching fetched flag configurations for use in subsequent sessionsEppo provided
maxCacheAgeSeconds (number)Maximum age, in seconds, that a previously cached configuration is considered valid and the wait-time before fetching a fresh configuration0
useExpiredCache (boolean)Consider initialization successfully complete without fetching updates, even if the configuration loaded from the cache is expiredfalse
updateOnFetch (always|expired|empty)Sets how the configuration is updated after a successful fetch:
• always - immediately start using the new configuration
• expired - immediately start using the new configuration only if the current one has expired
• empty - only use the new configuration if the current one is both expired and uninitialized/empty

Configuration cache

The SDK can cache previously loaded configurations for use in subsequent sessions, speeding up the time for the SDK to initialize. By default, a Local Storage-based cache is used in a browser environment and a Chrome Storage-based cache is used in a Chrome extension environment. A custom cache can be provided as the persistentStore initialization option.

Caches have a concept of being "expired", dictated by the maxCacheAgeSeconds initialization option, and also overridable by the implementation of isExpired() for a custom persistent store. When the cached configuration is not expired, it is considered valid and no fetches will be made for updated configurations.

When fetches are made--either from initialization or regular polling--you can control when the changes take effect with the updateOnFetch initialization option. The default of 'always' will start serving values from the most up-to-date configuration immediately. 'expired' will only update if the cached configuration--now being used to serve assignments--has expired. This is useful in combination with polling to prevent configurations in long-lived sessions from getting too stale. 'empty' will only update if there is no pre-existing configuration (expired or not) and is useful in combination with useExpiredCache for quickly initializing using a cached configuration and maintaining consistent assignments throughout the session even when the configuration is old. It is essentially prefetching an updated configuration for the next session.

Example configuration options

For example, if you want to optimize for always using the latest flag values, you would consider anything cached expired, leave the default of not using expired cached configurations, configuring the SDK to poll after successful and failed initializations, and leave the default of 'always' updating after a fetch.

// Default settings left alone
maxCacheAgeSeconds: 0, // Always consider cached configurations expired
useExpiredCache: false, // Wait for an updated configuration to be fetched before initialization is considered successful
updateOnFetch: 'always', // Immediately start using the new configuration after it is fetched
requestTimeoutMs: 5000, // Don't time out the initial configuration request until five seconds has passed
numInitialRequestRetries: 1, // If the initial configuration request fails, try one more time
// Settings set to non-default values
pollAfterSuccessfulInitialization: true, // Check for updated configurations every 30 seconds
pollAfterFailedInitialization: false, // Check for updated configurations even if initialization wasn't able to load one

However, if you want to optimize for the quickest time to initialization and serving assignments--even if those assignments are old, you would allow initializing with older configurations. You also could reduce the initial fetch timeout and retries to quickly fall back to default values.

// Default settings left alone
updateOnFetch: 'always', // Immediately start using the new configuration once it is fetched
// Settings set to non-default values
maxCacheAgeSeconds: 300, // Don't even bother fetching updated configurations unless the last one is more than five minutes old
useExpiredCache: true, // If the cached configuration is expired, use it to serve assignments until an updated one is fetched
requestTimeoutMs: 500, // Give up on fetching updated configurations after half a second and--if this is the first-ever initialization--just serve default values
numInitialRequestRetries: 0, // Don't retry a failed initialization fetch

Note that when new configurations are loaded, the same flag may start getting a different assignment for the same session. If you want to avoid this, and have consistent assignments until the next initialization, change updateOnFetch to empty. You could consider caches always expired so that it non-blocking loads an updated configuration to be used in the next session.

// Default settings left alone
maxCacheAgeSeconds: 0, // Always consider cached configurations expired
// Settings set to non-default values
updateOnFetch: 'empty', // Immediately start using the new configuration once it is fetched
useExpiredCache: true, // Always used the previously cached assignments
requestTimeoutMs: 500, // Give up on fetching updated configurations after half a second and--if this is the first-ever initialization--just serve default values
numInitialRequestRetries: 0, // Don't retry a failed initialization fetch

Define an assignment logger (experiment assignment only)

If you are using the Eppo SDK for experiment assignment (i.e randomization), pass in a callback logging function to the init function on SDK initialization. The SDK invokes the callback to capture assignment data whenever a variation is assigned.

The code below illustrates an example implementation of a logging callback using Segment. You could also use your own logging system, the only requirement is that the SDK receives a logAssignment function. Here we define an implementation of the Eppo AssignmentLogger interface containing a single function named logAssignment:

import { IAssignmentLogger } from "@eppo/js-client-sdk";
import { AnalyticsBrowser } from "@segment/analytics-next";

// Connect to Segment (or your own event-tracking system)
const analytics = AnalyticsBrowser.load({ writeKey: "<SEGMENT_WRITE_KEY>" });

const assignmentLogger: IAssignmentLogger = {
logAssignment(assignment) {
userId: assignment.subject,
event: "Eppo Randomized Assignment",
type: "track",
properties: { ...assignment },

The SDK will invoke the logAssignment function with an assignment object that contains the following fields:

timestamp (string)The time when the subject was assigned to the variation2021-06-22T17:35:12.000Z
featureFlag (string)An Eppo feature flag key"recommendation-algo"
allocation (string)An Eppo allocation key"allocation-17"
experiment (string)An Eppo experiment key"recommendation-algo-allocation-17"
subject (string)An identifier of the subject or user assigned to the experiment variationUUID
subjectAttributes (map)A free-form map of metadata about the subject. These attributes are only logged if passed to the SDK assignment function{ "country": "US" }
variation (string)The experiment variation the subject was assigned to"control"
metaData (Record<string,string>)Metadata around the assignment, such as the version of the SDK{ "obfuscated: "true", "sdkLanguage": "javascript", "sdkLibVersion": "3.2.1" }

More details about logging and examples (with Segment, Rudderstack, mParticle, and Snowplow) can be found in the event logging page.

Avoiding duplicated assignment logs

Eppo's SDK uses an internal cache to ensure that duplicate assignment events are not logged to the data warehouse. While Eppo's analytic engine will automatically deduplicate assignment records, this internal cache prevents firing unnecessary events and can help minimize costs associated with event logging.

3. Assign variations

Assign users to flags or experiments using get<Type>Assignment, depending on the type of the flag. For example, for a String-valued flag, use getStringAssignment:

import * as EppoSdk from "@eppo/js-client-sdk";

const eppoClient = EppoSdk.getInstance();
const variation = eppoClient.getStringAssignment(
<SUBJECT-ATTRIBUTES>, // Metadata used for targeting

The getStringAssignment function takes three required and one optional input to assign a variation:

  • subjectKey - The entity ID that is being experimented on, typically represented by a uuid.
  • flagKey - This key is available on the detail page for both flags and experiments. Can also be an experiment key.
  • defaultValue - The value that will be returned if no allocation matches the subject, if the flag is not enabled, if getStringAssignment is invoked before the SDK has finished initializing, or if the SDK was not able to retrieve the flag configuration. Its type must match the get<Type>Assignment call.
  • subjectAttributes - An optional map of metadata about the subject used for targeting. If you create rules based on attributes on a flag/experiment, those attributes should be passed in on every assignment call.

Typed assignments

The following typed functions are available:


It may take up to 10 seconds for changes to Eppo experiments to be reflected by the SDK assignments.


Usage in React

For usage in React, we recommend using the below EppoRandomizationProvider at the root of your component tree. By default, this component waits for initialization of the SDK before rendering its children. If waitForInitialization is set to false, the SDK getStringAssignment function will return null assignments while initializing and will only start assigning subjects when a new browser session is started.

import { useEffect, useState } from "react";

import { init } from "@eppo/js-client-sdk";

interface IEppoRandomizationProvider {
waitForInitialization?: boolean;
children: JSX.Element;
loadingComponent?: JSX.Element;

export default function EppoRandomizationProvider({
waitForInitialization = true,
loadingComponent = <div>Loading...</div>,
}: IEppoRandomizationProvider): JSX.Element {
const [isInitialized, setIsInitialized] = useState(false);
useEffect(() => {
apiKey: "<YOUR-SDK-KEY>",
assignmentLogger: {
logAssignment(assignment) {
// logging implementation
}).then(() => {
return setIsInitialized(true);
}, []);

if (!waitForInitialization || isInitialized) {
return children;
return loadingComponent;

After the SDK is initialized, you may assign variations from any child component of EppoRandomizationProvider. We recommend wrapping the SDK code in a useMemo hook to avoid invoking the assignment logic on every render:

<MyComponent />
function MyComponent(): JSX.Element {
const assignedVariation = useMemo(() => {
const eppoClient = getInstance();
return eppoClient.getStringAssignment("<FLAG-KEY>", "<SUBJECT-KEY>", <SUBJECT-ATTRIBUTES>, "<DEFAULT-VALUE>");
}, []);

return (
{assignedVariation === "<VARIATION-KEY>" && <p>Assigned control</p>}

Browser Support

The SDK is supported on all modern browsers. It relies on JavaScript promises, which may not be supported on older browsers including IE. If you need to run the SDK on a browser that does not support promises, it is possible to use a polyfill library.

Local Storage

The SDK uses browser local storage to store experiment configurations downloaded from Eppo. This allows for quick lookup by the getStringAssignment function. The configuration data stored contains the experiment key, experiment variation values, traffic allocation, and any allow-list overrides.


You may encounter a situation where a flag assignment produces a value that you did not expect. There are functions detailed here to help you understand how flags are assigned, which will allow you to take corrective action on potential configuration issues.