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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 an API key, which can be generated in the Eppo interface:

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

await init({
apiKey: "<API_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. If you are using the SDK for experiment assignments, make sure to pass in an assignment logging callback (see section below).


API Keys used with Client SDKs should have only 'Feature Flagging READ' permissions on, with all other permissions set to 'No Access'.

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"
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" }

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

3. Assign variations

Assigning users to flags or experiments with a single getAssignment function:

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

const eppoClient = EppoSdk.getInstance();
const variation = eppoClient.getAssignment(
// Optional map of subject metadata for targeting.

The getAssignment function takes two required and one optional input to assign a variation:

  • subjectKey - The entity ID that is being experimented on, typically represented by a uuid.
  • flagOrExperimentKey - This key is available on the detail page for both flags and experiments.
  • targetingAttributes - 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.

Handling null

We recommend always handling the null case in your code. Here are some examples illustrating when the SDK returns null:

  1. The Traffic Exposure setting on experiments/allocations determines the percentage of subjects the SDK will assign to that experiment/allocation. For example, if Traffic Exposure is 25%, the SDK will assign a variation for 25% of subjects and null for the remaining 75% (unless the subject is part of an allow list).

  2. If you are using Eppo for experiment assignments, you must start the experiment in the user interface before getAssignment returns variations. It will return null if the experiment is not running, both before and after.

start-experiment 3. If getAssignment is invoked before the SDK has finished initializing, the SDK may not have access to the most recent experiment configurations. In this case, the SDK will assign a variation based on any previously downloaded experiment configurations stored in local storage, or return null if no configurations have been downloaded.


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 getAssignment 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-API-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.getAssignment("<SUBJECT-KEY>", "<EXPERIMENT-KEY>");
}, []);

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 getAssignment function. The configuration data stored contains the experiment key, experiment variation values, traffic allocation, and any allow-list overrides.