navigator.sendBeacon

navigator.sendBeacon sends a small HTTP POST request reliably during page unload. The browser guarantees delivery without blocking navigation — making it ideal for fire-and-forget data like analytics pings.

Browser support: Widely available across all modern browsers.

Use cases

• Analytics pings (page views, click tracking)
• Diagnostics and error reporting
• Session-end data (time on page, scroll depth)

Example

The modern best practice is to send beacons on visibilitychange instead of unload (which is unreliable on mobile):

document.addEventListener("visibilitychange", () => {
  if (document.visibilityState === "hidden") {
    const data = JSON.stringify({ event: "session_end", duration: 42000 });
    navigator.sendBeacon("/analytics", new Blob([data], { type: "application/json" }));
  }
});

Comparison with fetch + keepalive

You can achieve similar reliability with fetch, plus gain more control:

document.addEventListener("visibilitychange", () => {
  if (document.visibilityState === "hidden") {
    fetch("/analytics", {
      method: "POST",
      body: JSON.stringify({ event: "session_end", duration: 42000 }),
      headers: { "Content-Type": "application/json", "X-Custom": "value" },
      keepalive: true,
      priority: "low",
    });
  }
});

Feature	sendBeacon	fetch + keepalive
Custom headers	No (Content-Type inferred from body)	Yes
Response access	No	Yes
Priority control	No	Yes (priority option)
Simplicity	One-liner	More verbose
HTTP methods	POST only	Any

When to use which

Use sendBeacon for simple fire-and-forget POST requests — typically analytics or diagnostics. Prefer fetch with keepalive: true and priority: "low" when you need custom headers, response access, priority control, or a method other than POST.

Limitations

• 64 KiB payload cap — shared quota across all in-flight keepalive requests (both sendBeacon and fetch + keepalive)
• POST only — no other HTTP methods
• No access to the response body
• No custom headers beyond what the body type implies for Content-Type