Referring to the synchronous mode of operation: "Because the JavaScript engine is blocked until the request completes, the browser will be frozen. The user cannot cancel the request, cannot click away, cannot go to another tab. This is extremely bad behavior. Fortunately, XMLHttpRequest provides an option for asynchronous operation. When you set the asyncFlag flag to true, the JavaScript engine does not block. Instead the request returns immediately, with a potential action that will be triggered later when the result on the request is known."