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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 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

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:

experiment (string)An Eppo experiment key"recommendation-algo-allocation-17"
subject (string)An identifier of the subject or user assigned to the experiment variationUUID
variation (string)The experiment variation the subject was assigned to"control"
timestamp (string)The time when the subject was assigned to the variation2021-06-22T17:35:12.000Z
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" }
featureFlag (string)An Eppo feature flag key"recommendation-algo"
allocation (string)An Eppo allocation key"allocation-17"

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.