The url parameter of the open() method, is an address to a file on a server:
xhttp.open("GET", "ajax_test.asp", true);
The file can be any kind of file, like .txt and .xml, or server scripting files like .asp and .php (which can perform actions on the server before sending the response back).
Synchronous XMLHttpRequest (async = false) is not recommended because the JavaScript will stop executing until the server response is ready. If the server is busy or slow, the application will hang or stop.
Synchronous XMLHttpRequest is in the process of being removed from the web standard, but this process can take many years.
Modern developer tools are encouraged to warn about using synchronous requests and may throw an InvalidAccessError exception when it occurs.