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3.1.4 • Public • Published

Eppo Javascript SDK

Test and lint SDK

Eppo is a modular flagging and experimentation analysis tool. Eppo's Javascript SDK is built to make assignments for single user client applications that run in a web browser. Before proceeding you'll need an Eppo account.


  • Feature gates
  • Kill switches
  • Progressive rollouts
  • A/B/n experiments
  • Mutually exclusive experiments (Layers)
  • Global holdouts
  • Dynamic configuration


npm install @eppo/js-client-sdk

Quick start

Begin by initializing a singleton instance of Eppo's client. Once initialized, the client can be used to make assignments anywhere in your app.

Initialize once

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

await init({ apiKey: "<SDK-KEY-FROM-DASHBOARD>" });

Assign anywhere

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

const eppoClient = EppoSdk.getInstance();
const user = getCurrentUser();

const variation = eppoClient.getBooleanAssignment('show-new-feature', user.id, { 
  'country': user.country,
  'device': user.device,
}, false);

Assignment functions

Every Eppo flag has a return type that is set once on creation in the dashboard. Once a flag is created, assignments in code should be made using the corresponding typed function:


Each function has the same signature, but returns the type in the function name. For booleans use getBooleanAssignment, which has the following signature:

getBooleanAssignment: (
  flagKey: string,
  subjectKey: string,
  subjectAttributes: Record<string, any>,
  defaultValue: string,
) => boolean

Initialization options

The init function accepts the following optional configuration arguments.

Option Type Description Default
assignmentLogger IAssignmentLogger A callback that sends each assignment to your data warehouse. Required only for experiment analysis. See example below. null
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

Assignment logger

To use the Eppo SDK for experiments that require analysis, 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 assignment data is needed in the warehouse to perform analysis.

The code below illustrates an example implementation of a logging callback using Segment, but you can use any system you'd like. The only requirement is that the SDK receives a logAssignment callback function. Here we define an implementation of the Eppo IAssignmentLogger 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 },


Eppo's SDKs are built for simplicity, speed and reliability. Flag configurations are compressed and distributed over a global CDN (Fastly), typically reaching end users in under 15ms. Those configurations are then cached locally, ensuring that each assignment is made instantly. Each SDK is as light as possible, with evaluation logic at around 25 simple lines of code. The simple typed functions listed above are all developers need to know about, abstracting away the complexity of the underlying set of features.


Visit the Eppo docs for best practices when using this SDK within a React context.




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