VWO Feature Management and Experimentation SDK for Node.js and JavaScript(Browser)

Overview

The VWO Feature Management and Experimentation SDK (VWO FME Node SDK) enables Node.js and JavaScript developers to integrate feature flagging and experimentation into their applications. This SDK provides full control over feature rollout, A/B testing, and event tracking, allowing teams to manage features dynamically and gain insights into user behavior.

Requirements

• Node.js v12 or later

Installation

Install the SDK via npm or yarn:

# via npm
npm install vwo-fme-node-sdk --save

# via yarn
yarn add vwo-fme-node-sdk

Basic Usage Example

The following example demonstrates initializing the SDK with a VWO account ID and SDK key, setting a user context, checking if a feature flag is enabled, and tracking a custom event.

const { init } = require('vwo-fme-node-sdk');

// Initialize VWO client
(async function () {
  const vwoClient = await init({
    accountId: '123456',
    sdkKey: '32-alpha-numeric-sdk-key',
  });

  // Check if feature is enabled for user
  const userContext = { id: 'unique_user_id' };
  const feature = await vwoClient.getFlag('feature_key', userContext);

  if (feature.isEnabled()) {
    console.log('Feature is enabled!');

    // Get feature variable
    const value = feature.getVariable('feature_variable', 'default_value');
    console.log('Variable value:', value);
  }

  // Track an event
  vwoClient.trackEvent('event_name', userContext);
})();

The SDK includes TypeScript type definitions for better type safety and IDE support. Here's how to use the SDK with TypeScript:

import { init, IVWOClient, IVWOOptions, Flag } from 'vwo-fme-node-sdk';
// Example of using IVWOOptions for type-safe configuration
const options: IVWOOptions = {
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
};

// Example of using IVWOClient for type-safe client usage
const vwoClient: IVWOClient = await init(options);

// Example of using Flag interface for type-safe flag handling
const flag: Flag = await vwoClient.getFlag('feature-key', { id: 'user-123' });
const isEnabled: boolean = flag.isEnabled();

const stringVariable: string = flag.getVariable('variable_key', 'default_value');
const booleanVariable: boolean = flag.getVariable('variable_key', true);
const numberVariable: number = flag.getVariable('variable_key', 10);

Advanced Configuration Options

To customize the SDK further, additional parameters can be passed to the init() API. Here's a table describing each option:

Parameter	Description	Required	Type	Example
accountId	VWO Account ID for authentication.	Yes	String	'123456'
sdkKey	SDK key corresponding to the specific environment to initialize the VWO SDK Client. You can get this key from VWO Application.	Yes	String	'32-alpha-numeric-sdk-key'
pollInterval	Time interval for fetching updates from VWO servers (in milliseconds).	No	Number	60000
gatewayService	An object representing configuration for integrating VWO Gateway Service.	No	Object	see Gateway section
storage	Custom storage connector for persisting user decisions and campaign data.	No	Object	See Storage section
clientStorage	Browser storage configuration for persisting user data in browser environments.	No	Object	See Client Storage section
logger	Toggle log levels for more insights or for debugging purposes. You can also customize your own transport in order to have better control over log messages.	No	Object	See Logger section
shouldWaitForTrackingCalls	Ensures tracking calls complete before resolving promises, useful for edge computing environments like Cloudflare Workers	No	Boolean	true
integrations	A callback function that receives data which can be pushed to any external tool that you need to integrate with.	No	Object	See Integrations
batchEventData	Configuration for batch event processing to optimize network requests	No	Object	See Batch Events section

Refer to the official VWO documentation for additional parameter details.

User Context

The context object uniquely identifies users and is crucial for consistent feature rollouts. A typical context includes an id for identifying the user. It can also include other attributes that can be used for targeting and segmentation, such as customVariables, userAgent and ipAddress.

Parameters Table

The following table explains all the parameters in the context object:

Parameter	Description	Required	Type	Example
id	Unique identifier for the user.	Yes	String	'unique_user_id'
customVariables	Custom attributes for targeting.	No	Object	{ age: 25, location: 'US' }
userAgent	User agent string for identifying the user's browser and operating system.	No	String	'Mozilla/5.0 ... Safari/537.36'
ipAddress	IP address of the user.	No	String	'1.1.1.1'

Example

const userContext = {
  id: 'unique_user_id',
  customVariables: { age: 25, location: 'US' },
  userAgent:
    'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/130.0.0.0 Safari/537.36',
  ipAddress: '1.1.1.1',
};

Basic Feature Flagging

Feature Flags serve as the foundation for all testing, personalization, and rollout rules within FME. To implement a feature flag, first use the getFlag API to retrieve the flag configuration. The getFlag API provides a simple way to check if a feature is enabled for a specific user and access its variables. It returns a feature flag object that contains methods for checking the feature's status and retrieving any associated variables.

Parameter	Description	Required	Type	Example
featureKey	Unique identifier of the feature flag	Yes	String	'new_checkout'
context	Object containing user identification and contextual information	Yes	Object	{ id: 'user_123' }

Example usage:

const featureFlag = await vwoClient.getFlag('feature_key', { id: 'unique_user_id' });
const isEnabled = featureFlag.isEnabled();

if (isEnabled) {
  console.log('Feature is enabled!');

  // Get and use feature variable with type safety
  const variableValue = featureFlag.getVariable('feature_variable', 'default_value');
  console.log('Variable value:', variableValue);
} else {
  console.log('Feature is not enabled!');
}

Custom Event Tracking

Feature flags can be enhanced with connected metrics to track key performance indicators (KPIs) for your features. These metrics help measure the effectiveness of your testing rules by comparing control versus variation performance, and evaluate the impact of personalization and rollout campaigns. Use the trackEvent API to track custom events like conversions, user interactions, and other important metrics:

Parameter	Description	Required	Type	Example
eventName	Name of the event you want to track	Yes	String	'purchase_completed'
context	Object containing user identification and other contextual information	Yes	Object	{ id: 'user_123' }
eventProperties	Additional properties/metadata associated with the event	No	Object	{ amount: 49.99 }

Example usage:

vwoClient.trackEvent('event_name', { id: 'unique_user_id' }, { amount: 49.99 });

See Tracking Conversions documentation for more information.

Pushing Attributes

User attributes provide rich contextual information about users, enabling powerful personalization. The setAttribute method provides a simple way to associate these attributes with users in VWO for advanced segmentation. Here's what you need to know about the method parameters:

Parameter	Description	Required	Type	Example
attributeKey	The unique identifier/name of the attribute you want to set	Yes	String	'plan_type'
attributeValue	The value to be assigned to the attribute	Yes	Any	'premium', 25, etc.
context	Object containing user identification and other contextual information	Yes	Object	{ id: 'user_123' }

Example usage:

vwoClient.setAttribute('attribute_name', 'attribute_value', { id: 'unique_user_id' });

See Pushing Attributes documentation for additional information.

Polling Interval Adjustment

The pollInterval is an optional parameter that allows the SDK to automatically fetch and update settings from the VWO server at specified intervals. Setting this parameter ensures your application always uses the latest configuration.

const vwoClient = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  pollInterval: 60000,
});

Gateway

The VWO FME Gateway Service is an optional but powerful component that enhances VWO's Feature Management and Experimentation (FME) SDKs. It acts as a critical intermediary for pre-segmentation capabilities based on user location and user agent (UA). By deploying this service within your infrastructure, you benefit from minimal latency and strengthened security for all FME operations.

Why Use a Gateway?

The Gateway Service is required in the following scenarios:

• When using pre-segmentation features based on user location or user agent.
• For applications requiring advanced targeting capabilities.
• It's mandatory when using any thin-client SDK (e.g., Go).

How to Use the Gateway

The gateway can be customized by passing the gatewayService parameter in the init configuration.

const vwoClient = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  gatewayService: {
    url: 'https://custom.gateway.com',
  },
});

Refer to the Gateway Documentation for further details.

Storage

The SDK operates in a stateless mode by default, meaning each getFlag call triggers a fresh evaluation of the flag against the current user context.

To optimize performance and maintain consistency, you can implement a custom storage mechanism by passing a storage parameter during initialization. This allows you to persist feature flag decisions in your preferred database system (like Redis, MongoDB, or any other data store).

Key benefits of implementing storage:

• Improved performance by caching decisions
• Consistent user experience across sessions
• Reduced load on your application

The storage mechanism ensures that once a decision is made for a user, it remains consistent even if campaign settings are modified in the VWO Application. This is particularly useful for maintaining a stable user experience during A/B tests and feature rollouts.

class StorageConnector extends StorageConnector {
  constructor() {
    super();
  }

  /**
   * Get data from storage
   * @param {string} featureKey
   * @param {string} userId
   * @returns {Promise<Object>}
   */
  async get(featureKey, userId) {
    // return await data (based on featureKey and userId)
  }

  /**
   * Set data in storage
   * @param {object} data
   */
  async set(data) {
    // Set data corresponding to a featureKey and user ID
    // Use data.featureKey and data.userId to store the above data for a specific feature and a user
  }
}

const vwoClient = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  storage: StorageConnector,
});

Client Storage

In browser environments, the SDK automatically uses localStorage to persist user data. You can customize this behavior using the clientStorage option:

const vwoClient = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  clientStorage: {
    key: 'vwo_data', // defaults to vwo_fme_data
    provider: sessionStorage, // defaults to localStorage
    isDisabled: false, // defaults to false, set to true to disable storage
  },
});

Parameter	Description	Required	Type	Default
key	Key used to store data in browser storage	No	String	'vwo_fme_data'
provider	Storage provider (localStorage or sessionStorage)	No	Object	localStorage
isDisabled	Disable browser storage completely	No	Boolean	false

Note: This feature is only applicable in browser environments. In Node.js environments, you should continue using the storage option for custom storage implementations.

Logger

VWO by default logs all ERROR level messages to your server console. To gain more control over VWO's logging behaviour, you can use the logger parameter in the init configuration.

Parameter	Description	Required	Type	Example
level	Log level to control verbosity of logs	Yes	String	DEBUG
prefix	Custom prefix for log messages	No	String	'CUSTOM LOG PREFIX'
transport	Custom logger implementation	No	Object	See example below

Example 1: Set log level to control verbosity of logs

const vwoClient1 = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  logger: {
    level: 'DEBUG',
  },
});

Example 2: Add custom prefix to log messages for easier identification

const vwoClient2 = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  logger: {
    level: 'DEBUG',
    prefix: 'CUSTOM LOG PREFIX',
  },
});

Example 3: Implement custom transport to handle logs your way

The transport parameter allows you to implement custom logging behavior by providing your own logging functions. You can define handlers for different log levels (debug, info, warn, error, trace) to process log messages according to your needs.

For example, you could:

• Send logs to a third-party logging service
• Write logs to a file
• Format log messages differently
• Filter or transform log messages
• Route different log levels to different destinations

The transport object should implement handlers for the log levels you want to customize. Each handler receives the log message as a parameter.

const vwoClient3 = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  logger: {
    transport: {
      level: 'INFO',
      log: (level, message) => {
        // your custom implementation here
      },
    },
  },
});

For multiple transports you can use the transports parameter. For example:

const vwoClient3 = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  logger: {
    transports: [
      {
        level: 'INFO',
        log: (level, message) => {
          // your custom implementation here
        },
      },
      {
        level: 'ERROR',
        log: (level, message) => {
          // your custom implementation here
        },
      },
    ],
  },
});

Integrations

VWO FME SDKs provide seamless integration with third-party tools like analytics platforms, monitoring services, customer data platforms (CDPs), and messaging systems. This is achieved through a simple yet powerful callback mechanism that receives VWO-specific properties and can forward them to any third-party tool of your choice.

const vwoClient = await vwo.init({
  sdkKey: '32-alpha-numeric-sdk-key', //replace with the SDK key for your environment
  accountId: '123456', //replace with your VWO account ID
  integrations: {
    callback(properties) {
      // your custom implementation here
    },
  },
});

Wait for Tracking Calls

The shouldWaitForTrackingCalls is an optional parameter that allows you to control whether the SDK should wait for tracking calls to complete before resolving promises. When enabled, tracking API calls will wait for the server response before resolving.

This is particularly useful for edge computing environments like Cloudflare Workers, where it ensures that tracking calls complete before resolving the promise.

const vwoClient = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  shouldWaitForTrackingCalls: true,
});

Batch Events

The batchEventData configuration allows you to optimize network requests by batching multiple events together. This is particularly useful for high-traffic applications where you want to reduce the number of API calls.

Parameter	Description	Required	Type	Default
requestTimeInterval	Time interval (in seconds) after which events are flushed to the server	No	Number	600
eventsPerRequest	Maximum number of events to batch together before sending to the server	No	Number	100
flushCallback	Callback function to be executed after events are flushed	No	Function	See example

Example usage:

const vwoClient = await init({
  accountId: '123456',
  sdkKey: '32-alpha-numeric-sdk-key',
  batchEventData: {
    requestTimeInterval: 60, // Flush events every 60 seconds
    eventsPerRequest: 100, // Send up to 100 events per request
    flushCallback: (error, events) => {
      console.log('Events flushed successfully');
      // custom implementation here
    },
  },
});

You can also manually flush events using the flushEvents() method:

vwoClient.flushEvents();

Version History

The version history tracks changes, improvements, and bug fixes in each version. For a full history, see the CHANGELOG.md.

Development and Testing

Install Dependencies and Bootstrap Git Hooks

yarn install

Compile TypeScript to JavaScript

yarn build

Run Tests

# production tests
yarn run test:prod

# development tests
yarn run test:dev

Contributing

We welcome contributions to improve this SDK! Please read our contributing guidelines before submitting a PR.

Code of Conduct

Our Code of Conduct outlines expectations for all contributors and maintainers.

License

Apache License, Version 2.0

Copyright 2024-2025 Wingify Software Pvt. Ltd.