This specification defines an interface that web developers can use to schedule asynchronous and non-blocking delivery of data that minimizes resource contention with other time-critical operations, while ensuring that such requests are still processed and delivered to destination.

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The Working Group expects to demonstrate 2 implementations of the features listed in this specification by the end of the Candidate Recommendation phase. Please see the Working Group's implementation report.

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This document is governed by the 1 March 2017 W3C Process Document.

1. Introduction

This section is non-normative.

Web applications often need to issue requests that report events, state updates, and analytics to one or more servers. Such requests typically do not require response processing on the client (e.g. result in 204, or 200 HTTP response codes with an empty response body), and should not compete for network and compute resources with other high priority operations such as fetching critical resources, reacting to input, running animations, and so on. However, such one-way requests (beacons), are also responsible for delivering critical application and measurement data, forcing developers to use costly methods to ensure their delivery:

The mismatch between above delivery and processing requirements leaves most developers with a tough choice and widespread adoption of blocking techniques that hurt the user experience. This specification defines an interface that web developers can use to schedule asynchronous and non-blocking delivery of data that minimizes resource contention with other time-critical operations, while ensuring that such requests are still processed and delivered to destination:

The following example shows use of the sendBeacon method to deliver event, click, and analytics data:

Example 1
  // emit non-blocking beacon to record client-side event
  function reportEvent(event) {
    var data = JSON.stringify({
      event: event,
      time: performance.now()
    navigator.sendBeacon('/collector', data);

  // emit non-blocking beacon with session analytics as the page
  // transitions to background state (Page Visibility API)
  document.addEventListener('visibilitychange', function() {
    if (document.visiblityState === 'hidden') {
      var sessionData = buildSessionReport();
      navigator.sendBeacon('/collector', sessionData);

 <a href='http://www.w3.org/' onclick='reportEvent(this)'>
 <button onclick="reportEvent('some event')">Click me</button>

Above example uses visibilitychange event defined in [PAGE-VISIBILITY] to trigger delivery of session data. This event is the only event that is guaranteed to fire on mobile devices when the page transitions to background state (e.g. when user switches to a different application, goes to homescreen, etc), or is being unloaded. Developers should avoid relying on unload event because it will not fire whenever a page in background state (i.e. visiblityState equal to hidden) and the process is terminated by the mobile OS.

The requests initiated via the sendBeacon method do not block or compete with time-critical work, may be prioritized by the user agent to improve network efficiency, and eliminate the need to use blocking operations to ensure delivery of beacon data.

What sendBeacon does not do and is not intended to solve:

2. Conformance requirements

All diagrams, examples, and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.

The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", " RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in [RFC2119]. For readability, these words do not appear in all uppercase letters in this specification.

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.

Some conformance requirements are phrased as requirements on attributes, methods or objects. Such requirements are to be interpreted as requirements on the user agent.

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

2.1 Dependencies


The following terms are defined in the DOM specification: [ DOM]


The following terms are defined in the HTML specification: [ HTML5]


The following terms are defined in the HTML specification: [ FETCH]

File API

The following terms are defined in the File API specification: [ FILEAPI]

Typed Array

The following terms are defined in the Typed Array specification: [ TYPEDARRAY]


The following terms are defined in the URL specification: [ URL]


The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification [WebIDL].

The following terms are defined in the Web IDL specification:


The following term is defined in the XMLHttpRequest specification: [ XMLHttpRequest]

3. Beacon

3.1 sendBeacon Method

partial interface Navigator {
    boolean sendBeacon(USVString url, optional BodyInit? data = null);

The sendBeacon method transmits data provided by the data parameter to the URL provided by the url parameter:

Beacon API does not provide a response callback. The server is encouraged to omit returning a response body for such requests (e.g. respond with 204 No Content).



The url parameter indicates the URL where the data is to be transmitted.


The data parameter is the BodyInit data that is to be transmitted.

Return Value

The sendBeacon method returns true if the user agent is able to successfully queue the data for transfer. Otherwise it returns false.


The user agent imposes limits on the amount of data that can be sent via this API: this helps ensure that such requests are delivered successfully and with minimal impact on other user and browser activity. If the amount of data to be queued exceeds the user agent limit, this method returns false; a return value of true implies the browser has queued the data for transfer. However, since the actual data transfer happens asynchronously, this method does not provide any information whether the data transfer has succeeded or not.

3.2 Processing Model

On calling the sendBeacon method with url and optional data, the following steps must be run:

  1. Set base to the entry settings object's API base URL.

  2. Set origin to the entry settings object's origin.

  3. Set referrer to the entry settings object's' API referrer source's URL if entry settings object's API referrer source is a document, and entry settings object's API referrer source otherwise

  4. Set parsedUrl to the result of the URL parser steps with url and base. If the algorithm returns an error, or if parsedUrl's scheme is not "http" or "https", throw a " TypeError" exception and terminate these steps.

  5. If data is not null:

    • Extract object's byte stream (transmittedData) and MIME type (mimeType).
    • If the amount of data that can be queued to be sent by keepalive enabled requests is exceeded by the size of transmittedData, set the return value to false and terminate these steps.

      Requests initiated via the Beacon API automatically set the keepalive flag, and developers can similarly set the same flag manually when using the Fetch API. All requests with this flag set share the same in-flight quota restrictions that is enforced within the Fetch API.

    • Let corsMode be "cors".
    • Let headerList be null.
  6. If mimeType is not null:
    • If mimeType value is a CORS-safelisted request-header value for the Content-Type header, set corsMode to "no-cors".
    • Append a Content-Type header with value mimeType to headerList.
  7. Set the return value to true, return the sendBeacon call, and continue to run the following steps in parallel:
    1. Let req be a new request, initialized as follows:

      header list
      keep-alive flag
      credentials mode
    2. Fetch req.

3.3 Privacy and Security

This section is non-normative.

The sendBeacon interface provides an asynchronous and non-blocking mechanism for delivery of data. This API can be used to:

The delivered data might contain potentially sensitive information, for example, data about a user's activity on a web page, to a server. While this can have privacy implications for the user, existing methods, such as scripted form-submit, image beacons, and XHR/fetch requests provide similar capabilities, but come with various and costly performance tradeoffs: the requests can be aborted by the user agent unless the developer blocks the user agent from processing other events (e.g. by invoking a synchronous request, or spinning in an empty loop), and the user agent is unable to prioritize and coalesce such requests to optimize use of system resources.

A request initiated by sendBeacon is subject to following properties:

As such, from the security perspective, the Beacon API is subject to all the same security policies as the current methods in use by developers. Similarly, from the privacy perspective, the resulting requests are initiated immediately when the API is called, or upon a page visibility change, which restricts the exposed information (e.g. user's IP address) to existing lifecycle events accessible to the developers. However, user agents might consider alternative methods to surface such requests to provide transparency to users.

Compared to the alternatives, the sendBeacon does apply two restrictions: there is no callback method, and the payload size can be restricted by the user agent. Otherwise, the sendBeacon API is not subject to any additional restrictions. The user agent ought not skip or throttle processing of sendBeacon calls, as they can contain critical application state, events, and analytics data. Similarly, the user agent ought not disable sendBeacon when in "private browsing" or equivalent mode, both to avoid breaking the application and to avoid leaking that the user is in such mode.

3.4 Acknowledgments

Thanks to Alois Reitbauer, Arvind Jain, Anne van Kesteren, Boris Zbarsky, Chase Douglas, Daniel Austin, Jatinder Mann, James Simonsen, Jason Weber, Jonas Sicking, Nick Doty, Philippe Le Hegaret, Todd Reifsteck, Tony Gentilcore, William Chan, and Yoav Weiss for their contributions to this work.

A. References

A.1 Normative references

DOM Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://dom.spec.whatwg.org/
Fetch Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://fetch.spec.whatwg.org/
File API. Arun Ranganathan; Jonas Sicking. W3C. 21 April 2015. W3C Working Draft. URL: https://www.w3.org/TR/FileAPI/
HTML5. Ian Hickson; Robin Berjon; Steve Faulkner; Travis Leithead; Erika Doyle Navara; Theresa O'Connor; Silvia Pfeiffer. W3C. 28 October 2014. W3C Recommendation. URL: https://www.w3.org/TR/html5/
Page Visibility (Second Edition). Jatinder Mann; Arvind Jain. W3C. 29 October 2013. W3C Recommendation. URL: https://www.w3.org/TR/page-visibility/
Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
Typed Array Specification. David Herman; Kenneth Russell. Khronos. 26 June 2013. Khronos Working Draft. URL: https://www.khronos.org/registry/typedarray/specs/latest/
URL Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://url.spec.whatwg.org/
Web IDL. Cameron McCormack; Boris Zbarsky; Tobie Langel. W3C. 15 December 2016. W3C Working Draft. URL: https://www.w3.org/TR/WebIDL-1/
XMLHttpRequest Level 1. Anne van Kesteren; Julian Aubourg; Jungkee Song; Hallvord Steen et al. W3C. 6 October 2016. W3C Note. URL: https://www.w3.org/TR/XMLHttpRequest/

A.2 Informative references

Service Workers 1. Alex Russell; Jungkee Song; Jake Archibald; Marijn Kruisselbrink. W3C. 11 October 2016. W3C Working Draft. URL: https://www.w3.org/TR/service-workers-1/
Web IDL. Cameron McCormack; Boris Zbarsky; Tobie Langel. W3C. 15 December 2016. W3C Working Draft. URL: https://www.w3.org/TR/WebIDL-1/