XMLHttpRequest is an API that provides client functionality for transferring data between a client and a server. It provides an easy way to retrieve data from a URL without having to do a full page refresh. This enables a Web page to update just a part of the page without disrupting what the user is doing.XMLHttpRequest is used heavily in AJAX programming.
In the following code snippet, we make a request to open a database, and include handlers for the success and error cases. For a full working example, see our To-do Notifications app (view example live.)
The PerformanceTiming.fetchStart read-only property returns an unsigned long long representing the moment, in miliseconds since the UNIX epoch, the browser is ready to fetch the document using an HTTP request. This moment is before the check to any application cache.