Notice: The WebPlatform project, supported by various stewards between 2012 and 2015, has been discontinued. This site is now available on github.



The user agent is checking for an update, or attempting to download the manifest for the first time. This is always the first event in the sequence.

Property of apis/appcache/ApplicationCacheapis/appcache/ApplicationCache


var result = window.applicationCache.onchecking;
window.applicationCache.onchecking = value;

Return Value

Returns an object of type nullnull


If more than one event is triggered and the checking event is included, the next events may include noupdate, downloading, obsolete, or error. Alternatively, you could use an anonymous delegate function such as

object.onchecking = function (e) { … }

where e is the cached event.

Related specifications

W3C ApplicationCache Specification
W3C Editor’s Draft


  • Microsoft Developer Network: Article