Add LUX.on() to subscribe to events

src/events.ts

@@ -0,0 +1,30 @@
+type Callback = (data?: EventData) => void;
+type EventData = unknown;
+
+export type Event = "beacon" | "new_page_id";
+
+const subscribers: Partial<Record<Event, Callback[]>> = {};
+const eventData: Partial<Record<Event, EventData>> = {};
+
+export function subscribe(event: Event, callback: Callback): void {
+  if (!subscribers[event]) {
+    subscribers[event] = [];
+  }
+
+  subscribers[event].push(callback);
+
+  // Ensure previous event data is available to new subscribers
+  if (eventData[event] !== undefined) {
+    callback(eventData[event]);
+  }
+}
+
+export function emit(event: Event, data?: EventData): void {
+  eventData[event] = data;
+
+  if (!subscribers[event]) {
+    return;
+  }
+
+  subscribers[event].forEach((callback) => callback(data));
+}

src/global.ts

@@ -1,8 +1,9 @@
-import { UserConfig } from "./config";
-import { LogEventRecord } from "./logger";
+import type { UserConfig } from "./config";
+import type { Event } from "./events";
+import type { LogEventRecord } from "./logger";
 
 export type Command = [CommandFunction, ...CommandArg[]];
-type CommandFunction = "addData" | "init" | "mark" | "markLoadTime" | "measure" | "send";
+type CommandFunction = "addData" | "init" | "mark" | "markLoadTime" | "measure" | "on" | "send";
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 type CommandArg = any;
 type PerfMarkFn = typeof performance.mark;
@@ -22,6 +23,7 @@ export interface LuxGlobal extends UserConfig {
   mark: (...args: Parameters<PerfMarkFn>) => ReturnType<PerfMarkFn> | void;
   markLoadTime?: (time?: number) => void;
   measure: (...args: Parameters<PerfMeasureFn>) => ReturnType<PerfMeasureFn> | void;
+  on: (event: Event, callback: (data?: unknown) => void) => void;
   /** Timestamp representing when the LUX snippet was evaluated */
   ns?: number;
   send: () => void;

src/lux.ts

@@ -12,6 +12,7 @@ import { SESSION_COOKIE_NAME } from "./cookie";
 import * as CustomData from "./custom-data";
 import { onVisible, isVisible, wasPrerendered, wasRedirected } from "./document";
 import { getNodeSelector } from "./dom";
+import * as Events from "./events";
 import Flags, { addFlag } from "./flags";
 import { Command, LuxGlobal } from "./global";
 import { getTrackingParams } from "./integrations/tracking";
@@ -1689,6 +1690,8 @@ LUX = (function () {
 
   function _sendBeacon(url: string) {
     new Image().src = url;
+
+    Events.emit("beacon", url);
   }
 
   // INTERACTION METRICS
@@ -1826,12 +1829,18 @@ LUX = (function () {
   // (because they get sent at different times). Each "page view" (including SPA) should have a
   // unique gSyncId.
   function createSyncId(inSampleBucket = false): string {
+    let syncId: string;
+
     if (inSampleBucket) {
       // "00" matches all sample rates
-      return Number(new Date()) + "00000";
+      syncId = Number(new Date()) + "00000";
+    } else {
+      syncId = Number(new Date()) + padStart(String(round(100000 * Math.random())), 5, "0");
     }
 
-    return Number(new Date()) + padStart(String(round(100000 * Math.random())), 5, "0");
+    Events.emit("new_page_id", syncId);
+
+    return syncId;
   }
 
   // Unique ID (also known as Session ID)
@@ -2031,6 +2040,7 @@ LUX = (function () {
   globalLux.measure = _measure;
   globalLux.init = _init;
   globalLux.markLoadTime = _markLoadTime;
+  globalLux.on = Events.subscribe;
   globalLux.send = () => {
     logger.logEvent(LogEvent.SendCalled);
     beacon.send();

src/snippet.ts

@@ -1,4 +1,4 @@
-import { Command, LuxGlobal } from "./global";
+import type { Command, LuxGlobal } from "./global";
 import { performance } from "./performance";
 import scriptStartTime from "./start-marker";
 import { msSinceNavigationStart } from "./timing";
@@ -20,6 +20,7 @@ LUX.init = () => LUX.cmd(["init"]);
 LUX.mark = _mark;
 LUX.markLoadTime = () => LUX.cmd(["markLoadTime", msSinceNavigationStart()]);
 LUX.measure = _measure;
+LUX.on = (event, callback) => LUX.cmd(["on", event, callback]);
 LUX.send = () => LUX.cmd(["send"]);
 LUX.ns = scriptStartTime;
 

tests/integration/events.spec.ts

@@ -0,0 +1,48 @@
+import { test, expect } from "@playwright/test";
+import { getSearchParam } from "../helpers/lux";
+import RequestInterceptor from "../request-interceptor";
+
+test.describe("LUX events", () => {
+  test("new_page_id", async ({ page }) => {
+    const luxRequests = new RequestInterceptor(page).createRequestMatcher("/beacon/");
+    await page.goto(
+      "/default.html?injectScript=LUX.auto=false;LUX.on('new_page_id', (id) => window.page_id = id);",
+      { waitUntil: "networkidle" },
+    );
+    await luxRequests.waitForMatchingRequest(() => page.evaluate(() => LUX.send()));
+
+    const firstPageId = await page.evaluate(() => window.page_id);
+    const firstBeacon = luxRequests.getUrl(0)!;
+    expect(firstPageId).toEqual(getSearchParam(firstBeacon, "sid"));
+
+    await luxRequests.waitForMatchingRequest(() =>
+      page.evaluate(() => {
+        LUX.init();
+        LUX.send();
+      }),
+    );
+
+    const secondBeacon = luxRequests.getUrl(1)!;
+    const secondPageId = await page.evaluate(() => window.page_id);
+    expect(secondPageId).toEqual(getSearchParam(secondBeacon, "sid"));
+  });
+
+  test("beacon", async ({ page }) => {
+    const luxRequests = new RequestInterceptor(page).createRequestMatcher("/beacon/");
+    await page.goto(
+      "/default.html?injectScript=LUX.auto=false;LUX.on('beacon', (url) => window.beacon_url = url);",
+      { waitUntil: "networkidle" },
+    );
+    await luxRequests.waitForMatchingRequest(() => page.evaluate(() => LUX.send()));
+
+    const beacon = luxRequests.getUrl(0)!;
+    let beaconUrl = await page.evaluate(() => window.beacon_url);
+
+    // We don't encode the Delivery Type parameter before sending the beacon, but Chromium seems to
+    // check that everything is encoded before making the actual request. This is a small hack to
+    // allow us to compare the strings.
+    beaconUrl = beaconUrl.replace("dt(empty string)_", "dt(empty%20string)_");
+
+    expect(beaconUrl).toEqual(beacon.href);
+  });
+});

tests/integration/unload.spec.ts

@@ -1,8 +1,8 @@
 import { test, expect } from "@playwright/test";
 import Flags from "../../src/flags";
+import { setPageHidden } from "../helpers/browsers";
 import { hasFlag } from "../helpers/lux";
 import RequestInterceptor from "../request-interceptor";
-import { setPageHidden } from "../helpers/browsers";
 
 test.describe("LUX unload behaviour", () => {
   test("not automatically sending a beacon when the user navigates away from a page with LUX.auto = false", async ({