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Node.js SDK Reference

This topic describes how to use the Harness Feature Flags Node.js SDK for your Node.js application.

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

Before you begin

You should read and understand the following:


The current version of this SDK is 1.2.12.


To use this SDK, make sure you:  

Install the SDK

Install using npm

npm install @harnessio/ff-nodejs-server-sdk

Install using yarn

yarn add @harnessio/ff-nodejs-server-sdk

Initialize the SDK

To initialize the Node.js SDK, you need to:

  1. Import the package.
  2. Add your Server SDK Key to connect to your Harness Environment.
  3. Add a Target that you want to Evaluate against a Feature Flag.
  4. (Optional) Configure the SDK options. For more details on what features you can configure for this SDK, go to Configure the SDK.

Import the package


To import with CommonJS, use:

const { Client } = require('@harnessio/ff-nodejs-server-sdk');

ES modules

To import with ES modules use:

import { Client } from '@harnessio/ff-nodejs-server-sdk';

Add the Server SDK Key

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

const client = new Client('sdkKey');

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 add a Target, build it and pass in arguments for the following:

identifierUnique ID for the Target.Read Regex requirements for Target names and identifiers below for accepted characters.Requiredidentifier: 'HT_1'
nameName for this Target. This does not have to be unique. Note: If you don’t provide a value, the name will be the same as the identifier.Read Regex requirements for Target names and identifiers below for accepted characters.OptionalNote: If you don't want to send a name, don't send the parameter. Sending an empty argument will cause an 'Harness_Target_1'
attributesAdditional data you can store for a Target, such as email addresses or location.Optionalattributes: {
Regex requirements for Target names and identifiers


Regex: ^[[email protected]_-]*$
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 [email protected]_-]*$

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.

For example:

const target = {  
identifier: 'HT_1',
name: 'Harness_Target_1',
attributes: {
'email': '[email protected]'

Configure the SDK

You can configure the following features of the SDK:

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
pollIntervalThe interval in seconds that we poll for changes when you are using stream mode.60 (seconds)
streamEnabledSet to true to enable streaming mode.Set to false to disable streaming mode.true
analyticsEnabledSet to true to enable analytics.Set to false to disable analytics.Note: When enabled, analytics data is posted every 60 seconds.true

For example:

// Create Options  
const options = {
baseUrl: "http://localhost:7000",
eventsUrl: "http://localhost:7000"

Complete the initialization

To complete the initialization:

  1. Create an instance of the Feature Flag client and pass in the Server SDK Key and configuration options:
// Create client with options  
const client = new Client(apiKey, options);
  1. Wait for the SDK to complete initialization and to fetch the Flag data:
await client.waitForInitialization();

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 Target object you want to evaluate against
  • The default Variation

For example:

Evaluate a boolean Variation

function boolVariation(  
identifier: string,
target: Target,
defaultValue: boolean = true,
): Promise<boolean>;

Evaluate a string Variation

function stringVariation(  
identifier: string,
target: Target,
defaultValue: boolean = '',
): Promise<string>;

Evaluate a number Variation

function numberVariation(  
identifier: string,
target: Target,
defaultValue: boolean = 1.0,
): Promise<number>;

Evaluate a JSON Variation

function jsonVariation(  
identifier: string,
target: Target,
defaultValue: boolean = {},
): Promise<Record<string, unknown>>;

Listen for events

Register the event listener

You can listen for the following events:

  • Event.READY - Indicates the SDK was successfully initialized.
  • Event.FAILED - Indicates the SDK had thrown an error.
  • Event.CHANGED - Indicates a Flag or Segment has been updated.

For example:

on(Event.READY, () => {  

on(Event.FAILED, () => {

on(Event.CHANGED, (identifier) => {
console.log('Changed', identifier);

Close the event listener

To avoid unexpected behavior, when the listener isn't needed, turn it off.

To remove the functionReference listener for Event.READY, use:

off(Event.READY, functionReference);

To remove all listeners, use:


If you call off() without parameters it will close the client.

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.

Close the SDK

To help prevent memory leaks, we recommend closing the SDK when it’s not in use. To do this, us the following function: 

function close(): void;

Additional options

Configure your logger

You can provide your own logger to the SDK by passing it in as a config option. The following example creates an instance of the winston logger, sets the level to DEBUG, and passes it to the client.

const winston = require('winston')  

// Create client with logger
const client = new Client(apiKey, {
logger: new winston.createLogger({
level: 'debug',
transports: [new winston.transports.Console()]

Sample code for a Node.js application

Here is a sample code for using the SDK with a Node.js application:

const pkg = require('ff-nodejs-server-sdk');
const { Client } = pkg;

const client = new Client('1c100d25-4c3f-487b-b198-3b3d01df5794', {
enableStream: true,
pollInterval: 2 * 60 * 1000 // two min pollInterval

setInterval(() => {
const target = {
identifier: 'harness',
const value = client.boolVariation('test', target, false);
console.log('Evaluation for flag test and target: ', value, target);
}, 10000);