Sys is undefined in an AJAX enabled SharePoint

I have a SharePoint app that is configured correctly in the web.config for ASP.NET AJAX, but when I try to use the Sys class it says it's undefined. I have installed SP2 and know that AJAX is enabled because update panels work correctly. ScriptManager is being loaded to the page. I have a script link to register the external JS file and have confirmed that the URL is correct. But I am using _spBodyOnLoadFunctionNames.push(functionName()) to call the function that throws the error. Any help on this is greatly appreciated since all I come up with on google is how to integrate AJAX into ShareP开发者_运维百科oint.

JavaScript

_spBodyOnLoadFunctionNames.push(InitializeDynamicLoadingPanel());

function InitializeDynamicLoadingPanel() {
    modalLayerID = '';
    prm = Sys.WebForms.PageRequestManager.getInstance();
    IsAsyncPostBack = prm.get_isInAsyncPostBack();
    if (!IsAsyncPostBack) {
        prm.add_initializeRequest(InitializeRequestHandler);
        prm.add_beginRequest(BeginRequestHandler);
        prm.add_pageLoading(PageLoadingHandler);
        prm.add_pageLoaded(PageLoadedHandler);
        prm.add_endRequest(EndRequestHandler);
    }
}  

C#

ScriptLink.Register(page, "dynamicLoadingPanel.js", false);

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Change the first line to:

_spBodyOnLoadFunctionNames.push("InitializeDynamicLoadingPanel");

Your current code is basically telling it to push the result of the InitializeDynamicLoadingPanel function, which is forcing the function to execute immediately, before the ScriptManager scripts have been embedded.