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Also asked, is Ajax synchronous or asynchronous?
AJAX: Synchronous or Asynchronous Synchronously, in which the script stops and waits for the server to send back a reply before continuing. Asynchronously, in which the script allows the page to continue to be processed and handles the reply if and when it arrives.
Similarly, is XMLHttpRequest deprecated?
You're probably getting a message that the synchronous use of XMLHttpRequest is deprecated (because of its harmful effect on the user experience; it freezes the page while waiting for a response). I can assure you that proper asynchronous use of that API is not deprecated whatsoever.
HTTP is a synchronous protocol: the client issues a request and waits for a response. In contrast to HTTP, message passing (e.g. over AMQP, or between Akka actors) is asynchronous. As a sender, you usually don't wait for a response.