javascript 57 lines · 10 steps

Batching analytics events in the browser

An event buffer that batches tracking calls and reliably flushes them on size, time, or page exit.

Explained by highlit
Intermediate batching buffering sendbeacon event-listeners reliability
1const MAX_BATCH_SIZE = 20;
2const FLUSH_INTERVAL = 5000;
3const ENDPOINT = '/api/analytics';
4 
5class AnalyticsBuffer {
6 constructor() {
7 this.queue = [];
8 this.timer = null;
9 this.handleUnload = this.flush.bind(this, true);
10 window.addEventListener('visibilitychange', () => {
11 if (document.visibilityState === 'hidden') this.flush(true);
12 });
13 window.addEventListener('pagehide', this.handleUnload);
14 }
15 
16 track(name, props = {}) {
17 this.queue.push({
18 name,
19 props,
20 ts: Date.now(),
21 url: location.pathname,
22 });
23 
24 if (this.queue.length >= MAX_BATCH_SIZE) {
25 this.flush();
26 } else if (!this.timer) {
27 this.timer = setTimeout(() => this.flush(), FLUSH_INTERVAL);
28 }
29 }
30 
31 flush(isUnloading = false) {
32 if (this.timer) {
33 clearTimeout(this.timer);
34 this.timer = null;
35 }
36 if (this.queue.length === 0) return;
37 
38 const events = this.queue.splice(0, this.queue.length);
39 const payload = JSON.stringify({ events, sentAt: Date.now() });
40 
41 if (navigator.sendBeacon) {
42 const blob = new Blob([payload], { type: 'application/json' });
43 const ok = navigator.sendBeacon(ENDPOINT, blob);
44 if (!ok) this.queue.unshift(...events);
45 return;
46 }
47 
48 fetch(ENDPOINT, {
49 method: 'POST',
50 body: payload,
51 headers: { 'Content-Type': 'application/json' },
52 keepalive: isUnloading,
53 }).catch(() => this.queue.unshift(...events));
54 }
55}
56 
57export const analytics = new AnalyticsBuffer();
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Batching many small events into one request cuts network overhead and server load.
  2. 2Page-exit events like visibilitychange and pagehide are the last chance to flush pending data.
  3. 3sendBeacon and keepalive fetch survive navigation where a normal request would be cancelled.

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