feat: support persistent UTM targeting

packages/experiment-browser/src/types/user.ts

@@ -126,6 +126,11 @@ export type ExperimentUser = {
    */
   url_param?: Record<string, string | string[]>;
 
+  /**
+   * Persisted parameters parsed from the URL and stored in local storage.
+   */
+  persisted_url_param?: Record<string, string | string[]>;
+
   /**
    * The user agent string.
    */

packages/experiment-tag/src/experiment.ts

@@ -33,6 +33,7 @@ import {
   RevertVariantsOptions,
 } from './types';
 import { applyAntiFlickerCss } from './util/anti-flicker';
+import { enrichUserWithCampaignData } from './util/campaign';
 import { setMarketingCookie } from './util/cookie';
 import { getInjectUtils } from './util/inject-utils';
 import { VISUAL_EDITOR_SESSION_KEY, WindowMessenger } from './util/messenger';
@@ -247,6 +248,8 @@ export class DefaultWebExperimentClient implements WebExperimentClient {
       }
     }
 
+    const enrichedUser = await enrichUserWithCampaignData(this.apiKey, user);
+
     // If no integration has been set, use an Amplitude integration.
     if (!this.globalScope.experimentIntegration) {
       const connector = AnalyticsConnector.getInstance('$default_instance');
@@ -258,7 +261,7 @@ export class DefaultWebExperimentClient implements WebExperimentClient {
     }
     this.globalScope.experimentIntegration.type = 'integration';
     this.experimentClient.addPlugin(this.globalScope.experimentIntegration);
-    this.experimentClient.setUser(user);
+    this.experimentClient.setUser(enrichedUser);
 
     if (!this.isRemoteBlocking) {
       // Remove anti-flicker css if remote flags are not blocking

packages/experiment-tag/src/types.ts

@@ -62,7 +62,7 @@ export const Defaults: WebExperimentConfig = {
  */
 
 export interface WebExperimentClient {
-  start(): void;
+  start(): Promise<void>;
 
   getExperimentClient(): ExperimentClient;
 

packages/experiment-tag/src/util/campaign.ts

@@ -0,0 +1,79 @@
+import {
+  type Campaign,
+  CampaignParser,
+  CookieStorage,
+  getStorageKey,
+  MKTG,
+} from '@amplitude/analytics-core';
+import { UTMParameters } from '@amplitude/analytics-core/lib/esm/types/campaign';
+import { type ExperimentUser } from '@amplitude/experiment-js-client';
+
+import { getStorageItem, setStorageItem } from './storage';
+
+/**
+ * Enriches the user object's userProperties with UTM parameters based on priority:
+ * 1. URL params (highest priority)
+ * 2. experiment-tag persisted props (medium priority)
+ * 3. analytics-browser persisted props (lowest priority, if using default Amplitude Analytics integration)
+ */
+export async function enrichUserWithCampaignData(
+  apiKey: string,
+  user: ExperimentUser,
+): Promise<ExperimentUser> {
+  const experimentStorageKey = `EXP_${MKTG}_${apiKey.substring(0, 10)}`;
+  const [currentCampaign, persistedAmplitudeCampaign] = await fetchCampaignData(
+    apiKey,
+  );
+  const persistedExperimentCampaign = getStorageItem<UTMParameters>(
+    'localStorage',
+    experimentStorageKey,
+  );
+
+  // Filter out undefined values and non-UTM parameters
+  const utmParams: Partial<UTMParameters> = {};
+  const allCampaigns = [
+    persistedAmplitudeCampaign, // lowest priority
+    persistedExperimentCampaign, // medium prioirty
+    currentCampaign, // highest priority
+  ];
+
+  for (const campaign of allCampaigns) {
+    if (campaign) {
+      for (const [key, value] of Object.entries(campaign)) {
+        if (key.startsWith('utm_') && value !== undefined) {
+          utmParams[key] = value;
+        }
+      }
+    }
+  }
+
+  if (Object.keys(utmParams).length > 0) {
+    persistUrlParams(apiKey, utmParams);
+    return {
+      ...user,
+      persisted_url_param: utmParams,
+    };
+  }
+  return user;
+}
+
+/**
+ * Persists UTM parameters from the current URL to experiment-tag storage
+ */
+export function persistUrlParams(
+  apiKey: string,
+  campaign: Record<string, string>,
+): void {
+  const experimentStorageKey = `EXP_${MKTG}_${apiKey.substring(0, 10)}`;
+  setStorageItem('localStorage', experimentStorageKey, campaign);
+}
+
+async function fetchCampaignData(
+  apiKey: string,
+): Promise<[Campaign, Campaign | undefined]> {
+  const storage = new CookieStorage<Campaign>();
+  const storageKey = getStorageKey(apiKey, MKTG);
+  const currentCampaign = await new CampaignParser().parse();
+  const previousCampaign = await storage.get(storageKey);
+  return [currentCampaign, previousCampaign];
+}