Uncaught ReferenceError: Sys is not defined on ASP.NET

I want use ajax on ASP.NET platform. For that I use ScriptManager. Simply I add this script with jQuery after when "document is ready".

$(document).ready(function () {

    // sync
    {{scopeName}}_init();

    // async

    Sys.WebForms.PageRequestManager.getInstance().add_pageLoaded(function (sender, args) {
        {{scopeName}}_init();
    });

});

And then such a mysterious javascript error happened.

Uncaught ReferenceError: Sys is not defined

Q what I have wrong if javascript stop working after first request?

Solution is not skip "Sys.WebForms.PageRequestManager" using undefined condition

if (typeof(Sys) !== 'undefined') {
    Sys.WebForms.PageRequestManager.getInstance().add_pageLoaded(function (sender, args) {
        {{scopeName}}_init();
    });
}

For the postback we init function just if document is ready - sync section.

For ajax request in ScriptManager block we need to register init function on "add_pageLoaded". After that everytime script works fine at first request - async section.

Most important neglected step is register also "add_endRequest", and that fix the problem.

Whole example code:

$(document).ready(function () {

    // sync
    console.log("sync");
    {{scopeName}}_init();

    // async

    pageRequestManager = Sys.WebForms.PageRequestManager.getInstance();

    pageRequestManager.add_endRequest({{scopeName}}_onAsyncEndRequest);
    pageRequestManager.add_pageLoaded({{scopeName}}_onAsyncPageLoaded);


});

function {{scopeName}}_onAsyncEndRequest(sender, args) {
    console.log("async end");
    console.log(args);
}
function {{scopeName}}_onAsyncPageLoaded(sender, args) {
    console.log("async start");
    {{scopeName}}_init();
}