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Unleash Client SDK for Node.js

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Unleash Client SDK for Node.js. It is compatible with the SaaS offering and Unleash Open-Source.

Getting started

1. Install the unleash-client into your project

$ npm install unleash-client --save

2. Initialize unleash-client

You should as early as possible in your node (web) app initialize the unleash-client. The unleash-client will set-up a in-memory repository, and poll updates from the unleash-server at regular intervals.

const { initialize } = require('unleash-client');
const instance = initialize({
    url: '',
    appName: 'my-app-name',
    instanceId: 'my-unique-instance-id',

// optional events
instance.on('error', console.error);
instance.on('warn', console.warn);
instance.on('ready', console.log);

// metrics hooks
instance.on('registered', clientData => console.log('registered', clientData));
instance.on('sent', payload => console.log('metrics bucket/payload sent', payload));
instance.on('count', (name, enabled) => console.log(`isEnabled(${name}) returned ${enabled}`));

3. Use unleash

After you have initialized the unleash-client you can easily check if a feature toggle is enabled or not.

const {
} = require('unleash-client');


const { enabled, name, payload } = getVariant('app.ToggleY', { userId: '1234' });

const featureToogleX = getFeatureToggleDefinition('app.ToggleX');
const featureToggles = getFeatureToggleDefinitions();

4. Stop unleash

To shut down the client (turn off the polling) you can simply call the destroy-method. This is typically not required.

const { destroy } = require('unleash-client');

Built in activation strategies

The client comes with implementations for the built-in activation strategies provided by unleash.

  • DefaultStrategy
  • UserIdStrategy
  • GradualRolloutUserIdStrategy
  • GradualRolloutSessionIdStrategy
  • GradualRolloutRandomStrategy
  • RemoteAddressStrategy
  • ApplicationHostnameStrategy

Read more about the strategies in

Unleash context

In order to use some of the common activation strategies you must provide a unleash-context. This client SDK allows you to send in the unleash context as part of the isEnabled call:

const context = {
    userId: '123',
    sessionId: 'some-session-id',
    remoteAddress: '',
unleash.isEnabled('someToggle', unleashContext);

Advanced usage

The initialize method takes the following arguments:

  • url - the url to fetch toggles from. (required)
  • appName - the application name / codebase name (required)
  • instanceId - an unique identifier, should/could be somewhat unique
  • refreshInterval - The poll-intervall to check for updates. Defaults to 15000ms.
  • metricsInterval - How often the client should send metrics to Unleash API. Defaults to 60000ms.
  • strategies - Custom activation strategies to be used.
  • disableMetrics - disable metrics
  • customHeaders - Provide a map(object) of custom headers to be sent to the unleash-server
  • customHeadersFunction - Provide a function that return a Promise resolving as custom headers to be sent to unleash-server. When options are set, this will take precedence over customHeaders option.
  • timeout - specify a timeout in milliseconds for outgoing HTTP requests. Defaults to 10000ms.
  • repository - Provide a custom repository implementation to manage the underlying data

Custom strategies

1. implement the custom strategy:

const { Strategy, initialize } = require('unleash-client');
class ActiveForUserWithEmailStrategy extends Strategy {
    constructor() {

    isEnabled(parameters, context) {
        return parameters.emails.indexOf( !== -1;

2. register your custom strategy:

    url: '',
    strategies: [new ActiveForUserWithEmailStrategy()],

Alternative usage

Its also possible to ship the unleash instance around yourself, instead of using on the default require.cache to have share one instance.

const { Unleash } = require('unleash-client');

const instance = new Unleash({
    appName: 'my-app-name',
    url: '',

instance.on('ready', console.log.bind(console, 'ready'));
// required error handling when using instance directly
instance.on('error', console.error);


The unleash instance object implements the EventEmitter class and emits the following events:

event payload description
ready - is emitted once the fs-cache is ready. if no cache file exists it will still be emitted.
registered - is emitted after the app has been registered at the api server
sent object data key/value pair of delivered metrics
count string name, boolean enabled is emitted when a feature is evaluated
warn string msg is emitted on a warning
error Error err is emitted on a error
unchanged - is emitted each time the client gets new toggle state from server, but nothing has changed
changed object data is emitted each time the client gets new toggle state from server and changes has been made

Example usage:

const { Unleash, isEnabled } = require('unleash-client');

const instance = new Unleash({
    appName: 'my-app-name',
    url: '',

await instance.once('ready');
// do something

Custom repository

You can manage the underlying data layer yourself if you want to. This enables you to use unleash offline, from a browser environment or implement your own caching layer. See example.

Unleash depends on a ready event of the repository you pass in. Be sure that you emit the event after you've initialized unleash.

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