Using Wingify Web Testing Visitor UUID as userId in FE SDK

Overview

Wingify Web Testing and Feature Experimentation (FE) are two separate products that can coexist on the same page. When both are running, they independently identify users — Web Testing via a browser cookie (_vwo_uuid), and FE via a userId passed in userContext. By default, these two identifiers are different, which means a visitor bucketed into a Web Testing campaign will be tracked as a different user in FE.

This guide explains how to bridge the two systems by reading the Web Testing UUID from the browser cookie and passing it as the userId in FE's userContext, ensuring consistent user identity across both products.

Important to note, that this will not work for mobile SDKs, due to the dependence on cookies. The flow described here, is best suited for client (JS, React) and serverSide SDKs (Node, Java, Python etc), used in tandem with the Web Testing product.

How Wingify Web Testing Identifies Visitors

Wingify Web Testing automatically sets a first-party cookie named vwo_uuid in the visitor's browser when the Wingify SmartCode loadscalled vwo_uuid in the visitor's browser when the VWO SmartCode is loaded.

Cookie details:

• Name: _vwo_uuid
• Format: 33-character string — a prefix letter (D for desktop/browser or J for JavaScript environments) followed by 32 hexadecimal characters
  Example: D3F2504E04F8911D39A0C0305E82C330
• Scope: Set on your domain (first-party)

This UUID is the primary visitor identifier used by Wingify Web Testing for bucketing, analytics, and reporting.

How FE SDK Handles userId

When you call getFlag(), you pass a userContext object containing an id (the userId). Normally, FE converts this userId into an internal UUID using UUIDv5 hashing — deriving a deterministic hash from the userId combined with your accountId. This is done to protect your privacy and ensure that Wingify servers never store any PII, even accidentally.

Standard UUID Conversion Flow:

userId (string)  →  UUIDv5 hash(userId + accountId namespace)  →  internal UUID

This internal UUID is what FE uses for bucketing, feature rollouts, and reporting.

The Exception: Web Testing UUID Passthrough

FE's SDKs include a special detection mechanism: if the userId you provide matches the Web Testing UUID format — a D or J prefix followed by exactly 32 hexadecimal characters — the SDK skips the hashing step entirely and uses the provided userId directly as the internal UUID.

This is the key to cross-product identity alignment.

Step-by-Step: Unifying User Identity

Step 1 — Read the Web Testing UUID from the Cookie

The _vwo_uuid cookie is accessible from client-side JavaScript running on the same domain:

function getWingifyUuid() {
  const match = document.cookie.match(/(?:^|;\s*)_vwo_uuid=([^;]+)/);
  return match ? decodeURIComponent(match[1]) : null;
}

const wingifyUuid = getWingifyUuid();
// Example value: "D3F2504E04F8911D39A0C0305E82C330"

Step 2 — Pass the UUID as userId in FE userContext

import { init } from 'wingify-fme-node-sdk';

const wingifyClient = await init({
  sdkKey: 'WINGIFYYOUR_SDK_KEY',
  accountId: 'WINGIFYYOUR_ACCOUNT_ID',
});

const wingifyUuid = getWingifyUuid();

const userContext = {
  id: wingifyUuid || 'fallback_user_id',
};

const flag = await wingifyClient.getFlag('your_feature_key', userContext);

if (flag.isEnabled()) {
  // serve the feature
}

Because the value of id matches the Web Testing UUID format, FE uses it as-is — no hashing. The same UUID that Wingify Web Testing uses for this visitor is now the identifier in FE as well.

Complete Example

// wingify-helper.js

function getWingifyUuid() {
  const match = document.cookie.match(/(?:^|;\s*)_vwo_uuid=([^;]+)/);
  return match ? decodeURIComponent(match[1]) : null;
}

async function evaluateFeatureFlag(wingifyClient, featureKey, fallbackUserId) {
  const wingifyUuid = getWingifyUuid();
  const userId = wingifyUuid || fallbackUserId;

  const userContext = { id: userId };
  const flag = await wingifyClient.getFlag(featureKey, userContext);

  return {
    isEnabled: flag.isEnabled(),
    variables: flag.getVariables(),
    userId,
    usingWebUuid: !!wingifyUuid,
  };
}

Prerequisites

1. Wingify SmartCode must be installed and running on your website (generates the _vwo_uuid cookie)

How it all fits together

Important Considerations