Skip to main content

JavaScript SDK reference


This SDK is not currently supported on Harness Self-Managed Enterprise Edition (on premises).

This topic describes how to use the Harness Feature Flags Javascript SDK for your JavaScript application.

For getting started quickly, you can use our sample code from the JavaScript SDK README. You can also clone and run a sample application from the JavaScript SDK GitHub Repository.

Before You Begin

Make sure you read and understand:


Latest SDK version can be found on GitHub Release Page


To use this SDK, make sure you:

Install the SDK

You can install the SDK using npm, Yarn, or by importing it directly from unpkg.

Install using npm

Run the following command:

npm i @harnessio/ff-javascript-client-sdk  

Then, import the SDK using the following command:

import { initialize, Event } from '@harnessio/ff-javascript-client-sdk'

Install using Yarn

Run the following command:

yarn add @harnessio/ff-javascript-client-sdk

Then, import the SDK using the following command:

import { initialize, Event } from '@harnessio/ff-javascript-client-sdk'

Install directly from unpkg

Run the following script:

<script type="module">  
import { initialize, Event } from ''

Make sure you install the latest version of the SDK, which you can view in GitHub and in Version.If you need to support browsers that no longer support ES modules, run the following script instead:

<script src=""></script>  
var initialize = HarnessFFSDK.initialize
var Event = HarnessFFSDK.Event

Initialize the SDK

To initialize the JavaScript SDK, you need to:

  1. Add your Client SDK key to connect to your Harness Environment.
  2. Add a target that you want to evaluate against a Feature Flag.
  3. (Optional) Configure the SDK.
  4. Complete the initialization with the SDK using the client SDK Key, target, and Configuration parameters you set.

Add your Client SDK Key

To connect to the correct Environment that you set up on the Harness Platform, you need to add the Client SDK Key from that Environment. Input the Client SDK Key into the FeatureFlagSDKKey parameter.

Add a target


What is a target? Targets are used to control which users see which variation of a Feature Flag, for example, if you want to do internal testing, you can enable the flag for some users and not others. When creating a target, you give it a name and a unique identifier. Often targets are users but you can create a target from anything that can be uniquely identified, such as an app or a machine.

For more information about targets, go to Targeting users with flags.

To create a target, you must enter an identifier but you can optionally add a name and additional attributes.

The below shows the data type for each parameter:

interface Target {  
identifier: string
name?: string
attributes?: object
Regex requirements for target names and identifiers


Regex: ^[A-Za-z0-9.@_-]*$
Must consist of only alphabetical characters, numbers, and the following symbols:
. (period)
@ (at sign)
_ (underscore)

The characters can be lowercase or uppercase but cannot include accented letters, for example Cafe_789.

Name Regex: ^[\\p{L}\\d .@_-]*$

Must consist of only alphabetical characters, numbers, and the following symbols:
. (period)
@ (at sign)
_ (underscore)

The characters can be lowercase or uppercase and can include accented letters, for example Café_123.

Configure the SDK

When initializing the SDK, you also have the option of providing alternative configurations by using the Options interface.

interface Options {
baseUrl?: string
eventUrl?: string
eventsSyncInterval?: number
pollingInterval?: number
pollingEnabled?: boolean
streamEnabled?: boolean
debug?: boolean
NameDescriptionDefault value
baseURLThe URL used to fetch Feature Flag Evaluations. When using the Relay Proxy, change this to: http://localhost:7000
eventUrlThe URL for posting metrics data to the Feature Flag service. When using the Relay Proxy, change this to: http://localhost:7000
eventsSyncIntervalThe interval in milliseconds that we post flag evaluation metrics.60000 (milliseconds)
pollingIntervalThe interval in milliseconds that we poll for changes when the SDK is running in polling mode.60000 (milliseconds)
streamEnabledSet to true to enable streaming mode. Set to false to disable streaming mode.true
pollingEnabledSet to true to enable polling mode. Set to false to disable polling mode.true
debugSet to true to enable SDK debug level logging. Set to false to disable debug level loggingfalse

Complete the initialization

Complete the initialization using the FeatureFlagSDKKey, target, and Options variables:

initialize(FeatureFlagSDKKey: string, target: Target, options?: Options)

For example:

const cf = initialize('00000000-1111-2222-3333-444444444444', {  
identifier: YOUR-TARGET-IDENTIFIER, // Target identifier
name: YOUR-TARGET-NAME, // Optional target name
attributes: { // Optional target attributes
email: ''

Evaluate a flag

Evaluating a flag is when the SDK processes all flag rules and returns the correct variation of that flag for the target you provide. If a matching flag can’t be found, or the SDK can’t remotely fetch flags, the default value is returned.

There are different methods for the different variation types and for each method you need to pass in:

  • Identifier of the flag you want to evaluate
  • The default variation

The flag is evaluated against the target you pass in when initializing the SDK.

const value = cf.variation('Dark_Theme', false) // second argument is the default value when variation does not exist

(Optional) Provide a set of evaluations

In some cases it might be useful to provide the SDK with a set of evaluations that it can serve instantly. You might consider this when you need to:

  • Reduce application startup time by providing default values or a snapshot of evaluations. For example, if your application is server-side generated, then it might make sense to retrieve evaluations on the server and provide them in the HTML of the page to be injected into the SDK.
  • Provide network redundancy by allowing your app to detect network connectivity issues when accessing the service and loading evaluations from another source.

To provide a set of evaluations:

  • Call the setEvaluations method at any time after initializing the client. The setEvaluations method takes an array of Evaluation objects as an argument.


    Where Evaluation is defined as:

    export interface Evaluation {
    flag: string // Feature flag identifier
    identifier: string // variation identifier
    value: boolean | string | number | object | undefined // variation value
    kind: string // boolean | json | string | int
    deleted?: boolean // mark that feature flag is deleted

Streaming and polling mode

By default, Harness Feature Flags SDK has streaming enabled and polling enabled. Both modes can be toggled according to your preference using the SDK's configuration.

Streaming mode

Streaming mode establishes a continuous connection between your application and the Feature Flags service. This allows for real-time updates on feature flags without requiring periodic checks. If an error occurs while streaming and pollingEnabled is set to true, the SDK will automatically fall back to polling mode until streaming can be reestablished. If pollingEnabled is false, streaming will attempt to reconnect without falling back to polling.

Polling mode

In polling mode, the SDK will periodically check with the Feature Flags service to retrieve updates for feature flags. The frequency of these checks can be adjusted using the SDK's configurations.

No streaming or polling

If both streaming and polling modes are disabled (streamEnabled: false and pollingEnabled: false), the SDK will not automatically fetch feature flag updates after the initial fetch. This means that after the initial load, any changes made to the feature flags on the Harness server will not be reflected in the application until the SDK is re-initialized or one of the modes is re-enabled.

This configuration might be useful in specific scenarios where you want to ensure a consistent set of feature flags for a session or when the application operates in an environment where regular updates are not necessary. However, it's essential to be aware that this configuration can lead to outdated flag evaluations if the flags change on the server.

To configure the modes:

const options = {
streamEnabled: true, // Enable or disable streaming - default is enabled
pollingEnabled: true, // Enable or disable polling - default is enabled if stream enabled, or disabled if stream disabled.
pollingInterval: 60000, // Polling interval in ms, default is 60000ms which is the minimum. If set below this, will default to 60000ms.


const client = initialize(
identifier: 'Harness1',
attributes: {
lastUpdated: Date(),
host: location.href

Listen for events

Register the event listener

The cf method allows you to listen for the different events triggered by SDK and deal with them as needed, for example:

client.on(Event.READY, flags => {
// Event happens when connection to server is established
// flags contains all evaluations against SDK key

client.on(Event.FLAGS_LOADED, evaluations => {
// Event happens when flags are loaded from the server

client.on(Event.CACHE_LOADED, evaluations => {
// Event happens when flags are loaded from the cache

client.on(Event.CHANGED, flagInfo => {
// Event happens when a changed event is pushed
// flagInfo contains information about the updated feature flag

client.on(Event.DISCONNECTED, () => {
// Event happens when connection is disconnected

client.on(Event.CONNECTED, () => {
// Event happens when connection established

client.on(Event.POLLING, () => {
// Event happens when polling begins

client.on(Event.POLLING_CHANGED, flagInfo => {
// Event happens when SDK polls for flags
// flagInfo contains the polled feature flags

client.on(Event.POLLING_STOPPED, () => {
// Event happens when polling stops

client.on(Event.ERROR, error => {
// Event happens when connection some error has occurred

client.on(Event.ERROR_AUTH, error => {
// Event happens when unable to authenticate

client.on(Event.ERROR_FETCH_FLAGS, error => {
// Event happens when unable to fetch flags from the service

client.on(Event.ERROR_FETCH_FLAG, error => {
// Event happens when unable to fetch an individual flag from the service

client.on(Event.ERROR_METRICS, error => {
// Event happens when unable to report metrics back to the service

client.on(Event.ERROR_STREAM, error => {
// Event happens when the stream returns an error

Close the event listener

To turn off the listeners, use You can turn off the listener when there is an error, or turn off all listeners on command.

Turn off listener when there is an error:, () => {  
// Do something when an error occurs

Turn off all listeners:

Test your app is connected to Harness

When you receive a response showing the current status of your Feature Flag, go to the Harness Platform and toggle the flag on and off. Then, check your app to verify if the flag variation displayed is updated with the variation you toggled.


The SDK must run for at least 60 seconds before it sends metrics. Please ensure metrics have not been disabled in the SDK.

Close the SDK client

In most applications, you won't need to close the SDK client.

However, you should close the SDK client if:

  • Your application is about to terminate. Closing the client ensures that all associated resources are released.
  • You have determined that you do not need to evaluate flags again in your application lifecycle.

The SDK does not evaluate flags after the client is closed.

To close the SDK client, call the following function:


Sample code for a JavaScript application

Here is a sample code for using Harness Feature Flag SDKs with a JavaScript application:

var initialize = HarnessFFSDK.initialize  
var Event = HarnessFFSDK.Event

var log = msg => {
document.querySelector('#log').innerHTML += `$\{msg}\n`

var cf = initialize(
identifier: 'Harness1',
attributes: {
lastUpdated: Date(),
host: location.href

cf.on(Event.READY, flags => {
log(JSON.stringify(flags, null, 2))

cf.on(Event.CHANGED, flagInfo => {
if (flagInfo.deleted) {
log('Flag is deleted')
log(JSON.stringify(flagInfo, null, 2))
} else {
log('Flag is changed')
log(JSON.stringify(flagInfo, null, 2))